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Overview of the Development of 3D-Printing Concrete: A Review

Overview of the Development of 3D-Printing Concrete: A Review applied sciences Review Overview of the Development of 3D-Printing Concrete: A Review Fuyan Lyu, Dongliang Zhao , Xiaohui Hou, Li Sun and Qiang Zhang * School of Mechanical and Electronic Engineering, Shandong University of Science and Technology, Qingdao 266590, China; skdlfy@sdust.edu.cn (F.L.); skdzdl@sdust.edu.cn (D.Z.); skdhxh@sdust.edu.cn (X.H.); skdsl@sdust.edu.cn (L.S.) * Correspondence: zhangqiangskd@sdust.edu.cn Abstract: 3D-printing concrete technology has attracted more and more attention for smart con- struction due to its advantages of digitization, automation, and high degree of intelligence. This article introduces the basic principles and related processes of concrete 3D-printing technology, and reviews the development from the following four fields: the material properties, preparation technology, printing parameters, and evaluation criteria of 3D-printing concrete technology. Then the existing difficulties, development direction and key technologies of 3D-printing concrete are described. Finally, we look forward to the development prospects of 3D-printing concrete from the aspects of printing materials, software and hardware cooperation, printing technology, etc. All the researches will provide the useful references for the later development and research. Keywords: 3D-printing; concrete; material properties; preparation technology; printing parameters 1. Introduction Citation: Lyu, F.; Zhao, D.; Hou, X.; Sun, L.; Zhang, Q. Overview of the Concrete material is a mixture made by mixing cementitious materials, water, coarse Development of 3D-Printing and fine aggregates in appropriate proportions. It is widely used in construction, water Concrete: A Review. Appl. Sci. 2021, conservancy, bridges, highways, railways and urban infrastructure construction which is 11, 9822. https://doi.org/10.3390/ one of the important civil engineering materials. As the demand for concrete increases, the app11219822 problems of high pollution and high energy consumption in production and application process have become increasingly prominent, restricting the green, healthy and sustainable Academic Editor: Tiago Pinto Ribeiro development of concrete materials. At the same time, the increasingly complex concrete structures have put forward higher requirements on the strength and durability of concrete Received: 14 August 2021 materials due to the environment and stress characteristics. Accepted: 12 October 2021 For the past few years 3D-printing technology has been widely used in architectural Published: 20 October 2021 design, industrial manufacturing, aerospace, biological engineering, cultural relics protec- tion and other industries with its advantages of low cost, high efficiency, strong design, and Publisher’s Note: MDPI stays neutral reliable quality [1,2]. Especially the 3D-printing technology combined with concrete tech- with regard to jurisdictional claims in nology that provides new ideas for the development and application of concrete materials. published maps and institutional affil- However, 3D-printing concrete technology has strict requirements in different performance iations. domains, and further, deeper research is still needed. This article introduces the research status of 3D-printing concrete technology, summa- rizes the impact of concrete material properties, key preparation technologies, 3D-printing control parameters and other factors on 3D-printing concrete technology. Based on the Copyright: © 2021 by the authors. current status and problems, looks forward to the research direction of 3D-printing concrete Licensee MDPI, Basel, Switzerland. technology and development trends. This article is an open access article distributed under the terms and 2. 3D-Printing Concrete Technology conditions of the Creative Commons 3D-printing concrete technology is a new technology developed on the basis of 3D- Attribution (CC BY) license (https:// printing technology and applied to concrete construction. Its main working principle is creativecommons.org/licenses/by/ to pass the configured concrete slurry through the extrusion device, under the control of 4.0/). Appl. Sci. 2021, 11, 9822. https://doi.org/10.3390/app11219822 https://www.mdpi.com/journal/applsci Appl. Sci. 2021, 11, 9822 2 of 25 the 3D software, according to the preset settings A good printing program is extruded by a nozzle to print, and finally the designed concrete component is obtained. As a new type of concrete moldless molding technology [3], it can apply the computer-aided design tools to the construction process. Compared with the traditional construction forms, 3D-printing concrete technology consumes less energy when applied to complex structures, and 3D-printing concrete technology can optimize the structure according to the working conditions. With the large-scale application and promotion of 3D-printing concrete technology, it can effectively reduce the input of materials, personnel, and machinery in the building construction process, and promote the development of digital and intelligent building construction technology [4]. 3D-printing concrete technology originated from Rensselaer Polytechnic Institute in New York, USA. Pegna [5] first explored 3D-printing construction technology in 1997, which proved the feasibility and prospects of 3D-printing technology in the field of construc- tion. In 1998, at the University of Southern California, Khoshnevis et al. [6,7] developed contour crafting (CC), which adopts computer precise control to automatically complete the pouring process and realizes the smooth contour surface and complex feature model pouring by controlling the nozzle. In 2007, Dini demonstrated its D-shape technology [8]. D-shape technology uses hundreds of nozzles at the bottom of the printing device, which can spray magnesia binders. Fine sand containing magnesia powder is sprayed on the binders to gradually solidify to form a stone solid. The sand layers are glued together to form a stone building in the end. In 2009, Buswell et al. [8,9] of Loughborough University in the United Kingdom developed concrete printing technology (concrete printing, Con- print 3D ). Compared with the contour process, the preparation of the equipment and computer control program used in the concrete printing technology is simpler. In 2012, ETH Zurich [10,11] launched the mesh mold project to explore the possibility of digitally manufacturing concrete structures with high geometric complexity. This technology uses 3D-printing to build a polymer network. The mold selects the appropriate particle size aggregate according to the size of the mesh to design the concrete mix ratio, so as to realize the retention of the concrete by blocking the mesh. In 2015, ETH Zurich Lloret et al. [12] developed smart dynamic casting (SDC ) based on the fast sliding mold process. Com- pared with other 3D-printing building technologies, its significant advantage lies in the ability to form a sliding mold around the steel bar, which realizes the enhancement of the printing structure. In 2019, China Construction Industrial Technology Research Institute Co., Ltd. and China Construction Second Bureau South China Company jointly established a project to print “In-situ 3D-printing double-layer demonstration buildings”, marking a breakthrough in In-situ 3D-printing technology in the field of construction [13]. Figure 1 shows the development history of 3D-printing construction technology based on cement-based materials [14]. Since the concept of 3D-printing concrete was put forward for more than 20 years, the research and application of 3D-printing concrete technology has developed exponentially, relying on the advantages of short construction period, topology optimization, high mechanization, non-modeling, meeting personalized customization, complex structure building construction and shape Unique architectural construction and other advantages have entered a period of rapid development. The development of 3D- printing concrete technology presents the following characteristics: (1) The scale of the industry continues to grow; (2) New materials and new technologies continue to emerge; (3) New engineering applications continue to emerge. Appl. Appl. Sci. Sci. 2021 2021 , 11 , 11 , x, FOR 9822 PEER REVIEW  3  of 3 of 2425  Figure 1. Development history of 3D-printing construction technology. Reprinted with permission Figure 1. Development history of 3D‐printing construction technology, [14].  from ref. [14]. Copyright 1969 Elsevier. 3.3. Pe Performance rformance Re Requirements quirements ofof 3D 3D-printing ‐printing Concre Concrete te Mater Materials ials  3.1. Printability of 3D-printing Concrete Materials 3.1. Printability of 3D‐printing Concrete Materials  3D-printing concrete technology is different from traditional molding concrete tech- 3D‐printing concrete technology is different from traditional molding concrete tech‐ nology, and 3D-printing concrete technology has stricter requirements for materials. The nology, and 3D‐printing concrete technology has stricter requirements for materials. The  printing material not only needs to have enough fluidity to ensure the smooth pumping of printing material not only needs to have enough fluidity to ensure the smooth pumping  the material and continuous extrusion from the nozzle. It also needs to have good water of the material and continuous extrusion from the nozzle. It also needs to have good water  retention to avoid the clogging of the pumping tube due to material segregation, and it also retention to avoid the clogging of the pumping tube due to material segregation, and it  needs to have enough hardening speed to maintain the stable accumulation of subsequent also needs to have enough hardening speed to maintain the stable accumulation of sub‐ layers to build [14]. Therefore, the printability of the material mainly includes fluidity, sequent layers to build [14]. Therefore, the printability of the material mainly includes  extrudability, buildability, and setting time. fluidity, extrudability, buildability, and setting time.  3.1.1. Fluidity 3.1.1. Fluidity  Fluidity refers to the ability of concrete materials to be easily pumped, transported, and Fluidity refers to the ability of concrete materials to be easily pumped, transported,  smoothly extruded from the discharge port of the print head. It is an important parameter and smoothly extruded from the discharge port of the print head. It is an important pa‐ for evaluating printability. If the fluidity is small, it is likely to cause a high mechanical rameter for evaluating printability. If the fluidity is small, it is likely to cause a high me‐ wear rate and equipment blockage. If the fluidity is large, the printed components are easy chanical wear rate and equipment blockage. If the fluidity is large, the printed components  to collapse. Therefore, it is necessary to make reasonable adjustments to fluidity to meet are easy to collapse. Therefore, it is necessary to make reasonable adjustments to fluidity  printing needs. to meet printing needs.  The most important factor affecting fluidity is water content. If the water content is The most important factor affecting fluidity is water content. If the water content is  too small, the mixture will become dry and hard and cannot pass the conveying pipeline too small, the mixture will become dry and hard and cannot pass the conveying pipeline  smoothly. If the water content is too large, it will cause a large number of harmful pores in smoothly. If the water content is too large, it will cause a large number of harmful pores  the printed sample, which will affect the later strength. The water content of cement slurry in the printed sample, which will affect the later strength. The water content of cement  can be effectively controlled by adding high-performance water-reducing agent to improve slurry can be effectively controlled by adding high‐performance water‐reducing agent to  the fluidity of cement slurry. Perrot et al. [15] obtained a mixture that satisfies 3D-printing improve the fluidity of cement slurry. Perrot et al. [15] obtained a mixture that satisfies  which contains water-cement ratio of 0.41 and polycarboxylate polymer powder of 0.3% 3D‐printing which contains water‐cement ratio of 0.41 and polycarboxylate polymer pow‐ cement mass. Results showed the best construction speed was 1.1m/h. Le et al. [16] der of 0.3% cement mass. Results showed the best construction speed was 1.1m/h. Le et  obtained 3D printed mixture could be smoothly extruded through the nozzle and achieved al. [16] obtained 3D printed mixture could be smoothly extruded through the nozzle and  the construction of a specimen with the highest 61 layers. The gelling material is a water-to- achieved the construction of a specimen with the highest 61 layers. The gelling material is  binder ratio of 0.26 and a water reducing agent of 1%. The fluidity can also be improved by a water‐to‐binder ratio of 0.26 and a water reducing agent of 1%. The fluidity can also be  adding mineral admixtures to optimize the particle size gradation. The more continuous improved by adding mineral admixtures to optimize the particle size gradation. The more  the particle gradation, the more conducive to the formation of a densely packed state of the continuous  the  particle  gradation,  the  more  conducive  to  the  formation  of  a  densely  mixture, resulting in better fluidity. Güneyisi et al. [17] replaced the mixture produced by packed state of the mixture, resulting in better fluidity. Güneyisi et al. [17] replaced the  Portland cement with 50% fly ash, and the flow time reduced by 43.2%. Zhang et al. [18] mixture produced by Portland cement with 50% fly ash, and the flow time reduced by  studied the relationship between fluidity and aggregate content and aggregate fineness, 43 and .2%.the  Zha results ng [18] showed  et al. stu thatdied ther the e is relat a linear ionship relationship  between between  fluidityfluidity  and aggreg and aggr ate content egate, as  Appl. Sci. 2021, 11Appl. , x FOR  Sci. P EER 2021 ,RE  11V , xIE FOR W   PEER REVIEW  4  of  24  4  of  24  Appl. Sci. 2021, 11, 9822 4 of 25 and aggregateand  finen aggr ess,egat  ande  the finen results ess, an showed d the results  that there  showed  is a  tha lineta there r relationsh  is a line ipa rbetween  relationsh   ip between  fluidity and aggregate, as shown in Figure 2. Ting [19] and others used recycled glass as  fluidity and aggregate, as shown in Figure 2. Ting [19] and others used recycled glass as  fine aggregate and added it to concrete, which significantly improved the fluidity of con‐ fine aggregate and added it to concrete, which significantly improved the fluidity of con‐ shown in Figure 2. Ting [19] and others used recycled glass as fine aggregate and added it crete.  crete.  to concrete, which significantly improved the fluidity of concrete.     Figure 2. Relationship between the flowability of cement paste and the optimum amount of natural Figure 2. Relationship Figure  2. betwee  Relatinonship  the flowabi  betwee lityn  of the cem  flowabi ent pa litste y of and  cem the ent opt  paimum ste and amo  theu opt nt of imum  natu amo ‐ unt of natu‐ ral sand, [18].  ral sand.  sand Reprinted , [18].  with permission from ref. [18]. Copyright 2019 Elsevier Ltd. The fluidity test is generally determined by the jumping table test, and its expanded The fluidity test is generally determined by the jumping table test, and its expanded  The fluidity test is generally determined by the jumping table test, and its expanded  diameter is used as the fluidity index of the material, as shown in Figure 3. Zhang [20] and diameter is used as the fluidity index of the material, as shown in Figure 3. Zhang [20] and  diameter is used as the fluidity index of the material, as shown in Figure 3. Zhang [20] and  Ma [21] etc. used the jumping table test to measure the fluidity range of 170~226 mm and Ma [21] etc. used the jumping table test to measure the fluidity range of 170 ~ 226 mm and  Ma [21] etc. used the jumping table test to measure the fluidity range of 170 ~ 226 mm and  174~210 mm, respectively. Tay et al. [22] used slump test and slump flow test to study the 174 ~ 210 mm, respectively. Tay et al. [22] used slump test and slump flow test to study  174 ~ 210 mm, respectively. Tay et al. [22] used slump test and slump flow test to study  3D printable range of concrete. The results show that a mixture with a slump of 4–8 mm the 3D printable range of concrete. The results show that a mixture with a slump of 4‐8  the 3D printable range of concrete. The results show that a mixture with a slump of 4‐8  and a fluidity of 150–190 mm can have a better printing characteristic. mm and a fluidity of 150–190 mm can have a better printing characteristic.  mm and a fluidity of 150–190 mm can have a better printing characteristic.      Figure 3. Jump Figure table test  3.  Jump employed  table for  test fl empl owaboyed ility  meas for flurement, owability [21].  meas  urement, [21].  Figure 3. Jump table test employed for flowability measurement. Reprinted with permission from ref. [21]. Copyright 2017 Elsevier Ltd. 3.1.2. Extrudability  3.1.2. Extrudability  3.1.2. Extrudability Extrudability refers to the difficulty of 3D‐printing concrete in the extrusion process  Extrudability refers to the difficulty of 3D‐printing concrete in the extrusion process  Extrudability refers to the difficulty of 3D-printing concrete in the extrusion process and the continuity and surface quality after extrusion. The study of extrudability can en‐ and the continuity and surface quality after extrusion. The study of extrudability can en‐ and the continuity and surface quality after extrusion. The study of extrudability can sure that the slurry sure tha  cant  the be  cont slurry inuou  cans be ly  tra cont nsported inuously th tra roungsported h the feed  thr opipe ugh  an thed  feed depos pipe ited an  d deposited  ensure that the slurry can be continuously transported through the feed pipe and deposited smoothly through the nozzle of the print head. It is a guarantee for continuous printing  smoothly through the nozzle of the print head. It is a guarantee for continuous printing  smoothly through the nozzle of the print head. It is a guarantee for continuous printing construction and can ensure the integrity of the printing building.  construction and can ensure the integrity of the printing building.  construction and can ensure the integrity of the printing building. An important factor affecting extrudability is the ratio of aggregate particle size to  An important factor affecting extrudability is the ratio of aggregate particle size to  An important factor affecting extrudability is the ratio of aggregate particle size to the the diameter of the extrusion nozzle. If the aggregate particle size is too large, the extru‐ the diameter of the extrusion nozzle. If the aggregate particle size is too large, the extru‐ diameter of the extrusion nozzle. If the aggregate particle size is too large, the extrusion sion nozzle will be blocked. Conversely, if the aggregate particle size is too small, its sur‐ sion nozzle will be blocked. Conversely, if the aggregate particle size is too small, its sur‐ nozzle will be blocked. Conversely, if the aggregate particle size is too small, its surface face area will increase, and the amount of slurry that required to wrap the surface of the  face area will increase, and the amount of slurry that required to wrap the surface of the  area will increase, and the amount of slurry that required to wrap the surface of the aggregate will increase, causing the concrete to easily crack. Liu [23] found that the use of Appl. Sci. 2021, 11, x FOR PEER REVIEW  5  of  24  Appl. Sci. 2021, 11, 9822 5 of 25 aggregate will increase, causing the concrete to easily crack. Liu [23] found that the use of  fine aggregate materials can ensure good extrudability and effectively prevent concrete  fine aggregate materials can ensure good extrudability and effectively prevent concrete materials from blocking the printing pipes and nozzles. Noura et al. [24] found that when  materials from blocking the printing pipes and nozzles. Noura et al. [24] found that when the maximum particle size of the sand in the mixture is 2 mm, and the weight ratio of the  the maximum particle size of the sand in the mixture is 2 mm, and the weight ratio of cement is 2, and when the ratio of the nozzle diameter to the maximum aggregate particle  the cement is 2, and when the ratio of the nozzle diameter to the maximum aggregate size particle   is  grsize eater is  tha greater n  5, than the  printing 5, the printing   matermaterial ial  will  will not  be not  extrude be extruded. d.  Bloc Blockage kage  occurred occurred. .  Malaeb Malaeb etet alal. . [2[5] 25 fou ] found nd tha that t while while red reducing ucing sa sand, nd, in incr creas easing ing the the am amount ount of of ce cement ment will will  ma make ke it itha have ve better better ext extr rudabil udability ity. When . When the the ratio ratio ofof fine fine agg aggr reg egate ate toto cement cement isis 1.1.28, 28, the the  ratio ratio ofof fin fine e aggreg aggreate gate toto sand sand isis 2.2. The The cecement ment ratio ratio rerquired equired for for proper proper ext extr ruda udability bility wi with th  minimum water is 0.48, the best nozzle diameter is 2 cm, and the extrudability is the best. minimum water is 0.48, the best nozzle diameter is 2 cm, and the extrudability is the best.  Hambach et al. [26] found that when the content of fiber mixed in cement-based materials Hambach et al. [26] found that when the content of fiber mixed in cement‐based materials  exceeds about 1.5%, it will cause the printing nozzle to block. exceeds about 1.5%, it will cause the printing nozzle to block.  The extrudability test is mainly evaluated by the apparent quality of the strip or the The extrudability test is mainly evaluated by the apparent quality of the strip or the  extrusion pressure. Le et al. [16] extruded a band with a total length of 4500 mm from a extrusion pressure. Le et al. [16] extruded a band with a total length of 4500 mm from a 9  9 mm wide nozzle without clogging or breaking, as a criterion for meeting extrudability, mm wide nozzle without clogging or breaking, as a criterion for meeting extrudability, as  as shown in Figure 4a. Lafhaj et al. [27] used the process parameters with a print nozzle shown in Figure 4a. Lafhaj et al. [27] used the process parameters with a print nozzle di‐ diameter of 15 mm and a printing speed of 100 mm/s, as shown in Figure 4b, and evaluated ameter of 15 mm and a printing speed of 100 mm/s, as shown in Figure 4b, and evaluated  the extrudability by observing whether the mixture was blocking the pipe during the the extrudability by observing whether the mixture was blocking the pipe during the pro‐ process of stacking 20 layers of strips with a length of 500 mm and a width of 350 mm. cess of stacking 20 layers of strips with a length of 500 mm and a width of 350 mm. Chen  Chen et al. [28] used a stamping extruder to quantify the extrusion pressure of different et al. [28] used a stamping extruder to quantify the extrusion pressure of different blends  blends to characterize the extrudability. to characterize the extrudability.  (a)  (b)  Figure 4. Test sample to evaluate extrudability. (a) Long strip evaluation chart. (b) Stacking evaluation chart, [16,27].  Figure 4. Test sample to evaluate extrudability. (a) Long strip evaluation chart. (b) Stacking evaluation chart. Reprinted with permission from ref. [16]. Copyright 2012, RILEM. Reprinted with 3.1.3. Buildability  permission from ref. [27]. Copyright 2019 MDPI. Buildability refers to the degree of deformation and overall stability of the 3D printed  3.1.3. Buildability cement‐based material after extrusion under its own weight and subsequent extrusion  Buildability refers to the degree of deformation and overall stability of the 3D printed and gravity of the printed layer. Because the 3D‐printing concrete technology does not  cement-based material after extrusion under its own weight and subsequent extrusion have templates, it is easy to have interlayer defects or settlement of the printing layer. The  and gravity of the printed layer. Because the 3D-printing concrete technology does not previous printing layer must support the subsequent printing layer with better bonding  have templates, it is easy to have interlayer defects or settlement of the printing layer. The ability with the subsequent printing layer, and the components do not collapse or deform.  previous printing layer must support the subsequent printing layer with better bonding Thus, the printing material needs to have interlayer support and shape retention. One of  ability with the subsequent printing layer, and the components do not collapse or deform. the basic requirements of 3D printed concrete is good buildability. The buildability is an  Thus, the printing material needs to have interlayer support and shape retention. One of index to measure the early stiffness of the structure, the foundation of the integrity of the  the basic requirements of 3D printed concrete is good buildability. The buildability is an printed components, and the key to the stability of the layers. Research on the buildability  index to measure the early stiffness of the structure, the foundation of the integrity of the can ensure the printed building stability.  printed components, and the key to the stability of the layers. Research on the buildability Buildability can be improved by increasing the amount of aggregate and adding min‐ can ensure the printed building stability. eral admixtures and additives. Zhang et al. [20] prepared the slurry by mixing 2% cement  Buildability can be improved by increasing the amount of aggregate and adding to  replace  silica  fume  and  nanoclay,  which  greatly  improved  the  construction  perfor‐ mineral admixtures and additives. Zhang et al. [20] prepared the slurry by mixing 2% mance of the slurry, and the construction height was increased from the original 72 mm  cement to replace silica fume and nanoclay, which greatly improved the construction Appl. Sci. 2021, 11, 9822 6 of 25 Appl. Sci. 2021, 11, x FOR PEER REVIEW  6  of  24  performance of the slurry, and the construction height was increased from the original to 260 mm. Shakor et al. [29] added 4.5% lithium carbonate to the mixture to promote rapid  72 mm to 260 mm. Shakor et al. [29] added 4.5% lithium carbonate to the mixture to promote rapid coagulation of the printing mixture, thereby achieving the purpose of coagulation of the printing mixture, thereby achieving the purpose of improving buildabil‐ improving buildability. Zhang et al. [18] found that the buildability of 3D printed concrete ity. Zhang et al. [18] found that the buildability of 3D printed concrete is determined by its  is determined by its yield stress, while the yield stress of concrete depends on the ratio of yield stress, while the yield stress of concrete depends on the ratio of cement paste and ag‐ cement paste and aggregate in the mixture. With the same aggregate content, the finer the gregate in the mixture. With the same aggregate content, the finer the concrete, the higher  concrete, the higher the concrete yield stress, which is easier to buildability. Long et al. [30] the concrete yield stress, which is easier to buildability. Long et al. [30] controlled the pro‐ controlled the proportion of cement-based materials by controlling a single variable of microcrystalline cellulose (MCC). The detailed parameters are shown in Table 1. The study portion of cement‐based materials by controlling a single variable of microcrystalline cellu‐ found that, as the addition of microcrystalline cellulose increases, the slump rate decreases. lose (MCC). The detailed parameters are shown in Table 1. The study found that, as the  The buildable height is increased, and the buildability is significantly improved, as shown addition  of  microcrystalline  cellulose  increases,  the  slump  rate  decreases.  The  buildable  in Figure 5. height is increased, and the buildability is significantly improved, as shown in Figure 5.  Table 1. Mix proportions of the cement mortars used in this study. Adapted with permission from ref. [30]. Copyright 2019 Elsevier Ltd. Table 1. Mix proportions of the cement mortars used in this study, [30].  Mix Cement Sand MCC HRWRA Li CO 2 3 Mix No.  Cement (g)  SF (g)  FA (g)  SFSand (g)  (g FA) (g) w/b  MCC w/b  (g)  HRWRA (g)  Li2CO3 (g)  No. (g) (g) (g) (g) (g) M‐1  780  130  390  1300  0.35  0  4.55  13  M-1 780 130 390 1300 0.35 0 4.55 13 M‐2  780  130  390  1300  0.35  6.5  4.55  13  M-2 780 130 390 1300 0.35 6.5 4.55 13 M‐3  780  130  390  1300  0.35  13  4.55  13  M-3 780 130 390 1300 0.35 13 4.55 13 M‐4  780  130  390  1300  0.35  19.5  4.55  13  M-4 780 130 390 1300 0.35 19.5 4.55 13 R‐2  780  130  390  1300  0.3  0  4.55  13  R-2 780 130 390 1300 0.3 0 4.55 13 Figure 5. Buildability of different mix proportions: heights of the built layers. Reprinted with Figure 5. Buildability of different mix proportions: heights of the built layers, [30].  permission from ref. [30]. Copyright 2019 Elsevier Ltd. The buildability test can directly explore the non-deformation of the stacking and The buildability test can directly explore the non‐deformation of the stacking and  accumulation of concrete through traditional construction test methods. Long et al. [30] accumulation of concrete through traditional construction test methods. Long et al. [30]  used a print nozzle with a diameter of 20 mm and a printing speed of 80 mm/s to prepare used a print nozzle with a diameter of 20 mm and a printing speed of 80 mm/s to prepare  a 5-layer hollow structure with a size of 400 mm  150 mm  100 mm, as shown in a 5‐layer hollow structure with a size of 400 mm × 150 mm × 100 mm, as shown in Figure 6.  Figure 6. The structure slump was used to characterize the buildability of the mixture. Yuan et al. [31] used a loading device to detect the deformation of the 20-layer printing The structure slump was used to characterize the buildability of the mixture. Yuan et al.  structure. The loading rate was set according to the printing construction plan. When the [31] used a loading device to detect the deformation of the 20‐layer printing structure. The  deformation was less than 0.2%, it was deemed to meet the construction requirements. loading rate was set according to the printing construction plan. When the deformation  Bhattacherjee et al. [32] printed a 300 mm  300 mm rectangular hollow thin-walled was less than 0.2%, it was deemed to meet the construction requirements. Bhattacherjee  et al. [32] printed a 300 mm × 300 mm rectangular hollow thin‐walled structure, stacked  20 layers vertically. Then the difference between the actual build height and the design  height was calculated. The above difference value and the thickness of first printed layer  were used to evaluate the buildability. Le et al. [33] proposed quantifying the buildability  as the number of filament layers that can be established without significant deformation  of the lower layer. The shear strength is measured by the shear blade strain gauge, and  the shear strength is in the range of 0.3 ~ 0.9 kPa, which can meet the construction require‐ ments, and the best shear strength is 0.55 kPa. Ma et al. [21] printed 20 layers of extruded  filaments with a vertical stacking length of 250 mm and a width of 30 mm, with a layer  Appl. Sci. 2021, 11, 9822 7 of 25 structure, stacked 20 layers vertically. Then the difference between the actual build height and the design height was calculated. The above difference value and the thickness of first printed layer were used to evaluate the buildability. Le et al. [33] proposed quantifying the buildability as the number of filament layers that can be established without significant Appl. Sci. 2021, 11, x FOR PEER REVIEW  7  of  24  deformation of the lower layer. The shear strength is measured by the shear blade strain gauge, and the shear strength is in the range of 0.3~0.9 kPa, which can meet the construction requirements, and the best shear strength is 0.55 kPa. Ma et al. [21] printed 20 layers of extruded filaments with a vertical stacking length of 250 mm and a width of 30 mm, with height of 8 mm, and meeting the construction requirements without collapsing within 10  a layer height of 8 mm, and meeting the construction requirements without collapsing min of standing time. Ogura et al. [34] proposed to build a wall of 1000 mm long, 30 mm  within 10 min of standing time. Ogura et al. [34] proposed to build a wall of 1000 mm wide, and 120 mm high long, 30 comm mpo wide, sed of and 7  120 layers mm high of fila composed ments.  of If 7the layers  wall of ca filaments. n maint Ifathe in it wall s  can maintain its geometric shape and no obvious deformation is observed, it is considered to geometric shape and no obvious deformation is observed, it is considered to be buildable.  be buildable. (a)  (b)  Figure 6. Print shape adopted for evaluating the buildability. (a) Shape map. (b) Evaluation Chart. Reprinted with Figure 6. Print shape adopted for evaluating the buildability. (a) Shape map. (b) Evaluation Chart, [30].  permission from ref. [30]. Copyright 2019 Elsevier Ltd. 3.1.4. Setting Time  3.1.4. Setting Time Setting time refers to the time that the 3D printed concrete mixture is mixed with Setting time refers to the time that the 3D printed concrete mixture is mixed with  water to maintain the printing performance. Setting time is an important parameter of the water to maintain the printing performance. Setting time is an important parameter of the  performance indicators of 3D-printing materials. A longer setting time can obtain good performance indicators of 3D‐printing materials. A longer setting time can obtain good  fluidity and extrudability, and a shorter setting time can obtain sufficient early strength. fluidity and extrudability, and a shorter setting time can obtain sufficient early strength.  Due to the different scales of structures, the setting time of materials should be adjusted Due to the different accor scales ding of to structur the scalees, of the the printed  setting str  uctur time e to ofmeet  materia differlent s shou needs. ld  Ther be adju efore,st the edsetting   time of 3D printed concrete should be adjustable within a certain range. according to the scale of the printed structure to meet different needs. Therefore, the set‐ Setting time can be adjusted by coagulant, retarder, or by changing the gelling material. ting time of 3D printed concrete should be adjustable within a certain range.  Le et al. [16] found that stirring the cementitious material can increase the setting time; by Setting time can be adjusted by coagulant, retarder, or by changing the gelling material.  adding a retarder and a superplasticizer, respectively, the influence of the two on the setting Le et al. [16] found that stirring the cementitious material can increase the setting time; by  time was obtained, as shown in Figure 7. The setting time of the superplasticizer and retarder with the dosage of 1% and 0.5% is 68 min and 100 min, respectively. Zareiyan et al. [35] mixed adding a retarder and a superplasticizer, respectively, the influence of the two on the setting  60% calcium sulfoaluminate and 40% type I ordinary Portland cement to shorten the setting time was obtained, as shown in Figure 7. The setting time of the superplasticizer and re‐ time of the cementitious material to 45 min. Khalil et al. [24] mixed 93% ordinary Portland tarder with the dosage of 1% and 0.5% is 68 min and 100 min, respectively. Zareiyan et al.  cement and 7% sulphoaluminate cement, and the obtained mixtures had initial setting and [35] mixed 60% calcium sulfoaluminate and 40% type I ordinary Portland cement to shorten  final setting times of 110 min and 150 min, respectively. Kazemian et al. [36] obtained an the setting time of the in ce itiamenti l settint gious time ma of 1t6e 3ria miln to by 45 ad min. ding 3 Khalil % calciu et m al. chl o[24] ride .mixed 93% ordinary  Portland cement and 7% sulphoaluminate cement, and the obtained mixtures had initial  setting and final setting times of 110 min and 150 min, respectively. Kazemian et al. [36]  obtained an initial setting time of 163 min by adding 3% calcium chloride.      (a)  (b)  Figure 7. (a) Retarder dosage versus open time. (b) Superplasticiser dosage versus open time, [16].  The setting time test is based on different standards, and the instruments and meth‐ ods used by the researchers are not the same. Chen et al. [37] started printing a strip of  Appl. Sci. 2021, 11, x FOR PEER REVIEW  7  of  24  height of 8 mm, and meeting the construction requirements without collapsing within 10  min of standing time. Ogura et al. [34] proposed to build a wall of 1000 mm long, 30 mm  wide, and 120 mm high composed of 7 layers of filaments. If the wall can maintain its  geometric shape and no obvious deformation is observed, it is considered to be buildable.  (a)  (b)  Figure 6. Print shape adopted for evaluating the buildability. (a) Shape map. (b) Evaluation Chart, [30].  3.1.4. Setting Time  Setting time refers to the time that the 3D printed concrete mixture is mixed with  water to maintain the printing performance. Setting time is an important parameter of the  performance indicators of 3D‐printing materials. A longer setting time can obtain good  fluidity and extrudability, and a shorter setting time can obtain sufficient early strength.  Due to the different scales of structures, the setting time of materials should be adjusted  according to the scale of the printed structure to meet different needs. Therefore, the set‐ ting time of 3D printed concrete should be adjustable within a certain range.  Setting time can be adjusted by coagulant, retarder, or by changing the gelling material.  Le et al. [16] found that stirring the cementitious material can increase the setting time; by  adding a retarder and a superplasticizer, respectively, the influence of the two on the setting  time was obtained, as shown in Figure 7. The setting time of the superplasticizer and re‐ tarder with the dosage of 1% and 0.5% is 68 min and 100 min, respectively. Zareiyan et al.  [35] mixed 60% calcium sulfoaluminate and 40% type I ordinary Portland cement to shorten  the setting time of the cementitious material to 45 min. Khalil et al. [24] mixed 93% ordinary  Portland cement and 7% sulphoaluminate cement, and the obtained mixtures had initial  Appl. Sci. 2021, 11, 9822 8 of 25 setting and final setting times of 110 min and 150 min, respectively. Kazemian et al. [36]  obtained an initial setting time of 163 min by adding 3% calcium chloride.  (a)  (b)  Figure 7. (a) Retarder dosage versus open time. (b) Superplasticiser dosage versus open time. Reprinted with permission Figure 7. (a) Retarder dosage versus open time. (b) Superplasticiser dosage versus open time, [16].  from ref. [16]. Copyright 2012, RILEM. The setting time test is based on different standards, and the instruments and meth‐ The setting time test is based on different standards, and the instruments and methods ods used by the researchers are not the same. Chen et al. [37] started printing a strip of  used by the researchers are not the same. Chen et al. [37] started printing a strip of 800 mm 40 mm every 10 min after a static time of 30 min and recorded the time of rupture as the setting time. Kazemian et al. [36] used a penetration resistance meter to test the initial setting time of the mixture based on the ASTM C403 standard, and found that the initial setting time of the mixture with 3% calcium chloride was 163 min. Aqel et al. [38] used a Vicat tester to measure the initial setting time of cement slurry containing limestone fillers according to ASTMC191-08, and found that with the increase of limestone content and its fineness, the initial setting time of the mixture was shortened. Khalil et al. [35] obtained the initial setting and final setting time of 99 min for the mixture to meet the requirements of 3D-printing through the Vicat tester. Ma et al. [39] used the penetration resistance method to determine the stiffness development of the cementitious material during the coagulation process in accordance with the Chinese national testing standard GB/T50080-2002 and determined the setting time of the mixture based on the penetration resistance measured at different times. However, now the setting time of printing materials is still relatively long, which makes the advantages of 3D-printing rapid manufacturing unable to be fully utilized. 3.2. Mechanical Properties of 3D-Printing Concrete During the construction of 3D-printing concrete, slurry is piled up layer by layer. Slurry should not only support its own weight, but also be able to withstand the weight of the slurry accumulated on the upper layer. Thus, the slurry needs certain mechanical properties, strong early strength, and higher late strength, otherwise it will collapse and deform due to insufficient strength. Compressive strength and flexural strength are used to characterize the mechanical properties of 3D printed concrete, which can be improved by adding mineral admixtures and optimizing the gradation. Paul et al. [40] used ordinary cement, fly ash, silica fume, fine sand, glass fiber and other materials to develop new 3D-printing materials. The 28 d compressive strength is in the range of 36 to 57 MPa, and the flexural strength is about 10 MPa. Le et al. [33] developed a high-performance fiber concrete suitable for 3D- printing by reducing the water-binder ratio and optimizing the material gradation method, which has high compressive strength (about 100 MPa) and flexural strength (11 MPa). Hambach et al. [26] found that glass, basalt, and carbon fiber can effectively improve the bending strength of concrete. Shakor et al. [29] studied the 3D-printing calcium aluminate cement water-based adhesive and found that it can effectively enhance the strength of the design structure, and studied the impact of saturation and other compressive strength, at the saturation level of 170% When, the maximum compressive strength of 3DP cementitious material is 8.26MPa, as shown in Figure 8. Appl. Sci. 2021, 11, x FOR PEER REVIEW  8  of  24  800 mm × 40 mm every 10 min after a static time of 30 min and recorded the time of rup‐ ture as the setting time. Kazemian et al. [36] used a penetration resistance meter to test the  initial setting time of the mixture based on the ASTM C403 standard, and found that the  initial setting time of the mixture with 3% calcium chloride was 163 min. Aqel et al. [38]  used a Vicat tester to measure the initial setting time of cement slurry containing limestone  fillers according to ASTMC191‐08, and found that with the increase of limestone content  and its fineness, the initial setting time of the mixture was shortened. Khalil et al. [35]  obtained the initial setting and final setting time of 99 min for the mixture to meet the  requirements of 3D‐printing through the Vicat tester. Ma et al. [39] used the penetration  resistance method to determine the stiffness development of the cementitious material  during the coagulation process in accordance with the Chinese national testing standard  GB/T50080‐2002 and determined the setting time of the mixture based on the penetration  resistance measured at different times. However, now the setting time of printing materi‐ als is still relatively long, which makes the advantages of 3D‐printing rapid manufactur‐ ing unable to be fully utilized.  3.2. Mechanical Properties of 3D‐Printing Concrete  During the construction of 3D‐printing concrete, slurry is piled up layer by layer.  Slurry should not only support its own weight, but also be able to withstand the weight  of the slurry accumulated on the upper layer. Thus, the slurry needs certain mechanical  properties, strong early strength, and higher late strength, otherwise it will collapse and  deform due to insufficient strength.  Compressive strength and flexural strength are used to characterize the mechanical  properties of 3D printed concrete, which can be improved by adding mineral admixtures  and optimizing the gradation. Paul et al. [40] used ordinary cement, fly ash, silica fume,  fine sand, glass fiber and other materials to develop new 3D‐printing materials. The 28 d  compressive strength is in the range of 36 to 57 MPa, and the flexural strength is about 10  MPa. Le et al. [33] developed a high‐performance fiber concrete suitable for 3D‐printing  by reducing the water‐binder ratio and optimizing the material gradation method, which  has high compressive strength (about 100 MPa) and flexural strength (11 MPa). Hambach  et al. [26] found that glass, basalt, and carbon fiber can effectively improve the bending  strength of concrete. Shakor et al. [29] studied the 3D‐printing calcium aluminate cement  water‐based adhesive and found that it can effectively enhance the strength of the design  structure, and studied the impact of saturation and other compressive strength, at the sat‐ uration level of 170% When, the maximum compressive strength of 3DP cementitious ma‐ terial is 8.26MPa, as shown in Figure 8.  Appl. Sci. 2021, 11, 9822 9 of 25 Appl. Sci. 2021, 11, x FOR PEER REVIEW  9  of  24  Figure 8. Compressive strength of 3DP cubic samples with lithium carbonate. Reprinted with Figure 8. Compressive strength of 3DP cubic samples with lithium carbonate, [29].  permission from ref. [29]. Copyright 2017 Elsevier Ltd. Fiber reinforcement is also an effective means to improve mechanical properties such  Fiber reinforcement is also an effective means to improve mechanical properties such as compressive strength and flexural strength, but too high fiber content will reduce flu‐ as compressive strength and flexural strength, but too high fiber content will reduce fluidity, idity, increase the risk of nozzle clogging, and reduce bonding strength. Christ et al. [41]  increase the risk of nozzle clogging, and reduce bonding strength. Christ et al. [41] studied studied the fiber reinforcement effect of different fibers, as shown in Figure 9. The me‐ the fiber reinforcement effect of different fibers, as shown in Figure 9. The mechanical test chanical test of the printed sample showed that the bending strength was increased by  of the printed sample showed that the bending strength was increased by 180%. Le [33] 180%. Le [33] and Paul [40] etc. respectively selected 12 mm/8 mm (length/diameter) pol‐ and Paul [40] etc. respectively selected 12 mm/8 mm (length/diameter) polypropylene ypropylene fiber and 0.5% alkali‐resistant glass fiber (6 mm) to improve the surface prop‐ fiber and 0.5% alkali-resistant glass fiber (6 mm) to improve the surface properties of the erties of the sample. Hambach et al. [26] found that the compressive strength of glass fiber  sample. Hambach et al. [26] found that the compressive strength of glass fiber and basalt and basalt fiber sample with 1% volume content is only 13 MPa, while the compressive  fiber sample with 1% volume content is only 13 MPa, while the compressive strength of strength of carbon fiber sample with the same content can reach 30 MPa.  carbon fiber sample with the same content can reach 30 MPa. Figure 9. Flexural strength of fiber-reinforced samples printed in x and y direction. Reprinted with Figure 9. Flexural strength of fiber‐reinforced samples printed in x and y direction, [41].  permission from ref. [41]. Copyright 2014 Elsevier B.V. 3D‐printing concrete has high requirements for compressive strength as conventional  3D-printing concrete has high requirements for compressive strength as conventional concrete, the interlayer bond strength is also a necessary condition for maintaining struc‐ concrete, the interlayer bond strength is also a necessary condition for maintaining struc- tural stability. Studies showed the 3D print concrete component is easily destroyed at the  tural stability. Studies showed the 3D print concrete component is easily destroyed at interlayer bonding when the load is received [42]. The weakness of the 3D‐printing con‐ the interlayer bonding when the load is received [42]. The weakness of the 3D-printing crete structure is an inter‐interlayer adhesion, and the interlayer bonding performance  significantly affects structural mechanical properties.  Interlayer print interval is a key factor affecting bond strength [33]. Panda et al. [43]  found that the methods that increasing the interval, print speed and printhead height can  reduce the interlayer bond strength of 3D‐printing concrete structure. Nerella et al. [44]  observed that the interlayer bond strength decreases over time intervals. When the inter‐ val time interval is 1min, the bond strength is reduced by 50%, and when the interval time  interval is 1d, the bond strength is reduced by 90%.  Adding a mortar between the interlayer can effectively improve the interlayer bond  strength. Hosseini et al. [45] added resin mortar to the interlayer, which was composed of  black charcoal particles, sulfur, and sand. Results showed that the use of epoxy resins and  Kefla fibers can increase the interlayer bond strength 20%. Ma et al. [46] proposed to add  the cellulose fiber mortar between the print layers, which can effectively improve the in‐ terlayer bonding performance. When the interval is 60 min, the interlayer bond strength  is still higher than 1.91 MPa.  Later maintenance can also improve the mechanical properties of the components.  Because there is no template that constraints the components, a long‐range evaporation of  the surface of the member generate surface cracking. The cracking not only reduces the  Appl. Sci. 2021, 11, 9822 10 of 25 concrete structure is an inter-interlayer adhesion, and the interlayer bonding performance significantly affects structural mechanical properties. Interlayer print interval is a key factor affecting bond strength [33]. Panda et al. [43] found that the methods that increasing the interval, print speed and printhead height can reduce the interlayer bond strength of 3D-printing concrete structure. Nerella et al. [44] observed that the interlayer bond strength decreases over time intervals. When the interval time interval is 1min, the bond strength is reduced by 50%, and when the interval time interval is 1d, the bond strength is reduced by 90%. Adding a mortar between the interlayer can effectively improve the interlayer bond strength. Hosseini et al. [45] added resin mortar to the interlayer, which was composed of black charcoal particles, sulfur, and sand. Results showed that the use of epoxy resins and Kefla fibers can increase the interlayer bond strength 20%. Ma et al. [46] proposed to add the cellulose fiber mortar between the print layers, which can effectively improve the interlayer bonding performance. When the interval is 60 min, the interlayer bond strength is still higher than 1.91 MPa. Later maintenance can also improve the mechanical properties of the components. Because there is no template that constraints the components, a long-range evaporation of the surface of the member generate surface cracking. The cracking not only reduces the strength of the member, but also affects durability. Li et al. [47] obtained that under standard curing and water care conditions, the anti-bending strength of the sample is substantially equivalent (3.5 MPa), and under steam conservation conditions it is 12.93 Mpa. Xia et al. [48] studied the effect of maintenance temperature (25 C, 40 C, 60 C and 80 C) on the strength of the fly ash geological polymer, and found that temperature can increase the strength of 3D-printing geological polymer samples. However, due to large structural dimensions, in a full-size 3D-printing structure, it is more difficult to improve their intensity by high temperature conservation. 3.3. The Durable Performance of 3D-Printing Concrete There are fewer reports on the durability of 3D-printing concrete materials, but this is a very important performance indicator, which directly affects the safety of 3D-printing concrete buildings and building structures. Once a problem occurs, it will not only waste a lot of manpower and material resources, but also threaten life safety. Thus, it is necessary to study the durability of 3D-printing concrete materials. In 3D-printing buildings, there are many factors that affect the durability of materials, such as temperature, external forces, and chemical effects. Developing materials with good anti-seepage and anti-freezing properties can ensure the durability of the structure. Weng et al. [49] used peeling test, four-point bending test, and compressive strength test to study the fire resistance and high temperature resistance of 3D-printing fiber-reinforced cement-based composites, and found that PVA fiber can effectively prevent cement-based materials from flaking and bursting when exposed to high temperatures. The bending and compressive strength of 3DPFRCC at different temperatures are higher than those of 3D printed plain concrete, as shown in Figure 10. Weger et al. [50] tested the freeze-thaw resistance and carbonization resistance of 3D printed cement-based materials formed by the powder bonding process. Results showed that the freeze-thaw resistance properties of 3D printed cement-based materials meet the standard requirements. With normal or increasing CO concentration, the specimen did not undergo carbonization. 2 Appl. Sci. 2021, 11, x FOR PEER REVIEW  10  of  24  strength of the member, but also affects durability. Li et al. [47] obtained that under stand‐ ard curing and water care conditions, the anti‐bending strength of the sample is substan‐ tially equivalent (3.5 MPa), and under steam conservation conditions it is 12.93 Mpa. Xia  et al. [48] studied the effect of maintenance temperature (25 °C, 40 °C, 60 °C and 80 °C) on  the strength of the fly ash geological polymer, and found that temperature can increase  the strength of 3D‐printing geological polymer samples. However, due to large structural  dimensions, in a full‐size 3D‐printing structure, it is more difficult to improve their inten‐ sity by high temperature conservation.  3.3. The Durable Performance of 3D‐Printing Concrete  There are fewer reports on the durability of 3D‐printing concrete materials, but this  is a very important performance indicator, which directly affects the safety of 3D‐printing  concrete buildings and building structures. Once a problem occurs, it will not only waste  a lot of manpower and material resources, but also threaten life safety. Thus, it is necessary  to study the durability of 3D‐printing concrete materials.  In 3D‐printing buildings, there are many factors that affect the durability of materi‐ als, such as temperature, external forces, and chemical effects. Developing materials with  good anti‐seepage and anti‐freezing properties can ensure the durability of the structure.  Weng et al. [49] used peeling test, four‐point bending test, and compressive strength test  to study the fire resistance and high temperature resistance of 3D‐printing fiber‐reinforced  cement‐based composites, and found that PVA fiber can effectively prevent cement‐based  materials from flaking and bursting when exposed to high temperatures. The bending and  compressive strength of 3DPFRCC at different temperatures are higher than those of 3D  printed plain concrete, as shown in Figure 10. Weger et al. [50] tested the freeze‐thaw re‐ sistance and carbonization resistance of 3D printed cement‐based materials formed by the  powder bonding process. Results showed that the freeze‐thaw resistance properties of 3D  printed cement‐based materials meet the standard requirements. With normal or increas‐ Appl. Sci. 2021, 11, 9822 11 of 25 ing CO2 concentration, the specimen did not undergo carbonization.  Figure 10. Mechanical properties at elevated temperature. Reprinted with permission from ref. [49]. Copyright 2018 Elsevier. Figure 10. Mechanical properties at elevated temperature, [49].  4. Key Preparation Technology The preparation of printing materials is the key of 3D-printing concrete technology. 4. Key Preparation Technology  Now concrete is used as the base material of the printing materials, and cementing materials, The preparation of printing materials is the key of 3D‐printing concrete technology.  aggregates, admixtures, admixtures, special fibers, etc., are added to it to meet the required of actual project 3D printed concrete. Now concrete is used as the base material of the printing materials, and cementing mate‐ rials, aggregates, admixtures, 4.1. Cementitious  admiMaterials xtures, special fibers, etc., are added to it to meet the  The types of 3D-printing concrete cementing materials are wide, mainly including required of actual project 3D printed concrete.  Portland cement, sulphoaluminate cement, resin, geopolymer, and so on. Cementitious ma- terials have a certain adjustment effect on the setting time, strength, bonding performance, 4.1. Cementitious Materials and stability of 3D printed concrete structures. Portland cement is the most common coagulation material in the construction industry, The types of 3D‐printing concrete cementing materials are wide, mainly including  but studies have found that ordinary Portland cement cannot bend moments, and it has disadvantageous shear forces, bonding ability, and setting time between layers, which Portland cement, sulphoaluminate cement, resin, geopolymer, and so on. Cementitious  makes it difficult to meet the needs of 3D-printing [33,51]. It is necessary to improve the materials have a certain adjustment effect on the setting time, strength, bonding perfor‐ fineness and mineral composition of cement. Sun Jianzhi et al. [52] modified the properties of Portland cement by adding polymer and other emulsions to reduce shrinkage and mance, and stability of 3D printed concrete structures.  improve adhesion. It is also possible to consider mixing ordinary Portland cement and sulfoaluminate cement into 3D printed concrete to adjust the early hydration speed and early strength of the concrete. Sulphoaluminate cement has the characteristics of early strength, and the initial setting time is about 6 min. Its early strength is relatively high, applying it to the proportion of printed concrete can get a significant quick setting effect [53]. Li et al. [54] found that magnesium oxysulfide cement does not harden in water, and has the advantages of light weight, fast setting speed, early strength, and strong adhesion. The compressive strength and flexural strength of magnesium phosphate cement are higher than other types of cement. The setting and hardening time is 1~10 min, which can meet the requirements of the bearing capacity of 3D printed concrete structures. Resin-based cementitious materials can greatly improve the bearing capacity of com- ponents. Issa et al. [55] used epoxy resin to repair concrete cracks. Due to the existence of cracks, the compressive strength of the cube decreased by 40.9%, and only 8% after repair. Ahmad et al. [56] injected epoxy resin into the flexural-shear cracks to strengthen the cracked reinforced concrete members, which increased the bearing capacity by 49%. Geopolymer cementitious materials can repair cracks, improving the mechanical properties of components. The strength of components can also be improved by increasing the ratio of geopolymers. Porto et al. [57] used polymer-modified cement mortar to repair the reinforced concrete column surface layer, and the axial stiffness of the repaired column Appl. Sci. 2021, 11, 9822 12 of 25 was only reduced by 13%. Pellegrino et al. [58] used geopolymer modified cement mortar to repair the tension zone of the beam, and the load value of the first crack increased by 30%. Sakka et al. [59] added 7.5% styrene-butadiene rubber polymer to cement-based materials, forming polymer membranes to better combine the cement hydration products which effectively reduced the adverse effects caused by the interlayer tensile strength of the printed specimens. Hosseini et al. [45] mixed sand aggregate and carbon-sulfur polymer in a ratio of 3:2 to prepare carbon-sulfur polymer modified cement mortar which can enhance the inter-layer tensile properties of the cement mortar. The tensile strength is 100% higher than that of non-adhesive bonded specimens. Yang et al. [60] mixed cement and styrene-butadiene emulsion in a ratio of 3:2 to prepare a styrene-butadiene emulsion modified cement paste to enhance the interlayer tensile properties of the concrete. The stretched strength of styrene-butadiene emulsion bonded specimens 7 d and 28 d interlayer increased by 144% and 96% respectively. Bong et al. [61] optimized the ratio of printable geopolymer materials, and the direct tensile strength of the 28d layers of the printed specimen was 2.7 MPa, which was 3 times the direct tensile strength of the 7d layers, as Appl. Sci. 2021, 11, x FOR PEER REVIEW  12  of  24  shown in Figure 11. Komljenovic ´ et al. [51] found that the types of catalysts HS and SS are the factors that determine the rate of increase in compressive strength, because the increase in the mass ratio of SS/HS increases the soluble silicate content in the geopolymerization system, which increases the compressive strength. Figure 11. Interlayer bond strength of Na-N-2.5 (the optimum 3D printable mixture). Reprinted with Figure 11. Interlayer bond strength of Na‐N‐2.5 (the optimum 3D printable mixture), [61].  permission from ref. [61]. Copyright 2019 MDPI. In order to facilitate the comparison of the effects of different cementing materials on Table 2. The effect of different cementitious materials on components.  the components, they are summarized, as shown in Table 2. Cementitious Material Name  Effect  Reference  Table 2. The effect of different cementitious materials on components. Portland cement  The most commonly used condensing material, with high early strength  [33,51]  Cementitious Material Name Effect Reference High early strength, good quick‐setting effect, high compressive and  Sulphoaluminate cement  [53,54]  The most commonly used condensing flexural strength  Portland cement [33,51] material, with high early strength Resin  Significantly improve component bearing capacity  [55,56]  High early strength, good quick-setting Sulphoaluminate cement [53,54] It can repair cracks with good repair effect and improve the strength of  effect, high compressive and flexural strength Geopolymer  [45,53,57–61]  components  Significantly improve component bearing Resin [55,56] capacity It can repair cracks with good repair effect 4.2. Aggregate  Geopolymer [45,53,57–61] and improve the strength of components Aggregate affects the rheology, viscosity, stress characteristics before and after hard‐ ening and durability of concrete directly or indirectly from the internal structure [62]. The  aggregate particle size has a great influence on the 3D printed concrete. When the aggre‐ gate particle size is too large, it will block the extrusion nozzle. On the contrary, if the  aggregate particle size is too small, its specific surface area will increase, and the amount  of paste increases that used to wrap the surface of the aggregate which will make the con‐ crete easy to crack. When the ratio of the nozzle diameter to the maximum aggregate par‐ ticle size is greater than 5, the printing material will not be blocked during the extrusion  process. When the maximum particle size of the aggregate is less than 1/10 of the nozzle  diameter, the cementitious material can be smoothly extruded through the nozzle [24,25].  Lin et al. [63] measured the fluidity, compressive strength, and flexural strength of mortar  influenced by sand of different fineness. Within a certain range, as the fineness modulus  of sand increases, the fluidity of mortar, 7 d compressive strength and flexural strength  will be improved to varying degrees, as shown in Figure 12.  Appl. Sci. 2021, 11, 9822 13 of 25 4.2. Aggregate Aggregate affects the rheology, viscosity, stress characteristics before and after harden- ing and durability of concrete directly or indirectly from the internal structure [62]. The aggregate particle size has a great influence on the 3D printed concrete. When the aggregate particle size is too large, it will block the extrusion nozzle. On the contrary, if the aggregate particle size is too small, its specific surface area will increase, and the amount of paste increases that used to wrap the surface of the aggregate which will make the concrete easy to crack. When the ratio of the nozzle diameter to the maximum aggregate particle size is greater than 5, the printing material will not be blocked during the extrusion process. When the maximum particle size of the aggregate is less than 1/10 of the nozzle diameter, the cementitious material can be smoothly extruded through the nozzle [24,25]. Lin et al. [63] measured the fluidity, compressive strength, and flexural strength of mortar influenced by Appl. Sci. 2021, 11, x FOR PEER REVIEW  13  of  24  sand of different fineness. Within a certain range, as the fineness modulus of sand increases, the fluidity of mortar, 7 d compressive strength and flexural strength will be improved to varying degrees, as shown in Figure 12. (a)  (b)  Figure 12. (a) Effect of fineness modulus on Fluidity. (b) The Mechanical Properties of Mortar. Reprinted with permission Figure 12. (a) Effect of fineness modulus on Fluidity. (b) The Mechanical Properties of Mortar, [63].  from ref. [63]. Copyright 2017 IPO. 4.3. Mineral Admixture 4.3. Mineral Admixture  The active ingredients of mineral admixtures can greatly increase the strength of 3D The active ingredients of mineral admixtures can greatly increase the strength of 3D  printed concrete components, the density of the structure, and improve the durability of the material and the service life of the structure. printed concrete components, the density of the structure, and improve the durability of  The incorporation of fly ash can effectively improve the working performance, me- the material and the service life of the structure.  chanical properties, and durability of printed concrete. Fly ash is the main admixture for The incorporation of fly ash can effectively improve the working performance, me‐ preparing high-performance 3D printed concrete. However, the incorporation of a large amount of fly ash causes the slow development of the early strength of the printed concrete, chanical properties, and durability of printed concrete. Fly ash is the main admixture for  and the obvious reduction of carbonization resistance and frost resistance. Therefore, fly preparing high‐performance 3D printed concrete. However, the incorporation of a large  ash is generally mixed with other admixtures. Kang Jingfu et al. [64] mixed fly ash and GH ore powder to prepare high-strength and high-performance concrete. Voigt et al. [65] amount of fly ash causes the slow development of the early strength of the printed con‐ observed that the addition of fly ash improved the fluidity of cement-based materials, but crete, and the obvious reduction of carbonization resistance and frost resistance. There‐ reduced the shape stability of the material. The cement-based material obtained by mixing fore, fly ash is generally mixed with other admixtures. Kang Jingfu et al. [64] mixed fly  sintered clay and fly ash has good fluidity and shape stability. Silica fume has high activity. As an admixture for concrete, it not only increases the ash and GH ore powder to prepare high‐strength and high‐performance concrete. Voigt  initial strength of the printed concrete, but also improves the later flexural strength of the et al. [65] observed that the addition of fly ash improved the fluidity of cement‐based ma‐ printed concrete. The silica fume particles are very small, which can be used to fill the pores terials,  but  reduced  the ar  ound shape the st aggr abi egate, lity  impr of  othe ving  ma the t bonding erial.  perf The ormance   cemeof ntthe‐ba concr sedete,   ma and teria enhancing l  ob‐ the transition zone between the cement slurry and the aggregate interface [66]. March- tained by mixing sintered clay and fly ash has good fluidity and shape stability.  ment et al. [67] added silica fume, which accounts for 10% of the cementitious material, to Silica fume has high activity. As an admixture for concrete, it not only increases the  initial strength of the printed concrete, but also improves the later flexural strength of the  printed concrete. The silica fume particles are very small, which can be used to fill the  pores around the aggregate, improving the bonding performance of the concrete, and en‐ hancing the transition zone between the cement slurry and the aggregate interface [66].  Marchment et al. [67] added silica fume, which accounts for 10% of the cementitious ma‐ terial, to improve the adhesion and extrusion properties of the printing material. Rahul et  al. [68] found that the addition of silica fume can optimize the particle size distribution of  the printing material. After adding silicon powder, the actual and ideal particle size dis‐ tribution can be more closely matched, as shown in Figure 13, increasing its yield stress,  and improving the buildability of the printing material. However, it will reduce the fluid‐ ity of the material. Zhang et al. [69] added 2% of the cement mass of nano‐clay and silica  fume into concrete to prepare a concrete material with high thixotropy.  Appl. Sci. 2021, 11, 9822 14 of 25 improve the adhesion and extrusion properties of the printing material. Rahul et al. [68] found that the addition of silica fume can optimize the particle size distribution of the printing material. After adding silicon powder, the actual and ideal particle size distribu- tion can be more closely matched, as shown in Figure 13, increasing its yield stress, and Appl. Sci. 2021, 11, x FOR PEER REVIEW  14  of  24  improving the buildability of the printing material. However, it will reduce the fluidity of the material. Zhang et al. [69] added 2% of the cement mass of nano-clay and silica fume into concrete to prepare a concrete material with high thixotropy. Figure 13. Comparison of the combined particle size distribution of all dry ingredients after adding Figure 13. Comparison of the combined particle size distribution of all dry ingredients after add‐ silica fume and the optimum distribution suggested by the Andreassen model. Reprinted with ing silica fume and the optimum distribution suggested by the Andreassen model, [68].  permission from ref. [68]. Copyright 2018 Elsevier Ltd. 4.4. Fiber Material 4.4. Fiber Material  Fibrous materials can significantly improve the crack resistance of the printed con- Fibrous materials can significantly improve the crack resistance of the printed con‐ crete and the elastic modulus of the maximum load. It can also significantly improve crete and the elastic modulus of the maximum load. It can also significantly improve the  the toughness, ductility of the printed concrete, delay the surface deterioration of the toughness, ductility of the printed concrete, delay the surface deterioration of the concrete  concrete product, and improve its durability. Tohamy et al. [70] showed that the addi- tion of polypropylene fiber can prevent the printed concrete samples from peeling off, product, and improve its durability. Tohamy et al. [70] showed that the addition of poly‐ optimize the extrusion process of the concrete in the printer output port to a certain ex- propylene fiber can prevent the printed concrete samples from peeling off, optimize the  tent, and obtain a uniform and continuous printed sample structure. However, when the extrusion process of the concrete in the printer output port to a certain extent, and obtain  content of polypropylene fiber is too much, it will reduce the impermeability of concrete. a uniform and continuous printed sample structure. However, when the content of poly‐ Feng et al. [71] used gypsum as the main printing material and tested it with GFRP (Glass propylene fiber is too much, it will reduce the impermeability of concrete. Feng et al. [71]  Reinforced Plastics) materials, and found that the printing column has a significant in- crease in axial load-bearing capacity and ductility. Mechtcherine et al. [72] found that the used gypsum as the main printing material and tested it with GFRP (Glass Reinforced  continuous embedded carbon fiber during the printing process can effectively improve Plastics) materials, and found that the printing column has a significant increase in axial  the bending strength and deformation ability of the specimen. Ding et al. [73] used PE load‐bearing capacity and ductility. Mechtcherine et al. [72] found that the continuous  (polyethylene) fiber to reinforce the printing matrix material and found that the addition of embedded carbon fiber during the printing process can effectively improve the bending  PE fiber significantly improved the bending strength of the specimen in different directions, as shown in Figure 14. Arunothayan et al. [74] added steel fibers to concrete materials, strength and deformation ability of the specimen. Ding et al. [73] used PE (polyethylene)  which greatly improved the bending resistance of the printed specimens. Ma et al. [75] fiber to reinforce the printing matrix material and found that the addition of PE fiber sig‐ added 0.5% basalt fiber to concrete materials, and the basic mechanical properties and nificantly improved the bending strength of the specimen in different directions, as shown  anisotropy of printable cement-based materials were significantly improved. in Figure 14. Arunothayan et al. [74] added steel fibers to concrete materials, which greatly  improved the bending resistance of the printed specimens. Ma et al. [75] added 0.5% basalt  fiber to concrete materials, and the basic mechanical properties and anisotropy of printa‐ ble cement‐based materials were significantly improved.  In order to facilitate the comparison of the effects of different fiber materials on the  components, they are summarized, as shown in Table 3.  Figure 14. Tensile strength of specimens with various fiber ratios, [73].  Appl. Sci. 2021, 11, x FOR PEER REVIEW  14  of  24  Figure 13. Comparison of the combined particle size distribution of all dry ingredients after add‐ ing silica fume and the optimum distribution suggested by the Andreassen model, [68].  4.4. Fiber Material  Fibrous materials can significantly improve the crack resistance of the printed con‐ crete and the elastic modulus of the maximum load. It can also significantly improve the  toughness, ductility of the printed concrete, delay the surface deterioration of the concrete  product, and improve its durability. Tohamy et al. [70] showed that the addition of poly‐ propylene fiber can prevent the printed concrete samples from peeling off, optimize the  extrusion process of the concrete in the printer output port to a certain extent, and obtain  a uniform and continuous printed sample structure. However, when the content of poly‐ propylene fiber is too much, it will reduce the impermeability of concrete. Feng et al. [71]  used gypsum as the main printing material and tested it with GFRP (Glass Reinforced  Plastics) materials, and found that the printing column has a significant increase in axial  load‐bearing capacity and ductility. Mechtcherine et al. [72] found that the continuous  embedded carbon fiber during the printing process can effectively improve the bending  strength and deformation ability of the specimen. Ding et al. [73] used PE (polyethylene)  fiber to reinforce the printing matrix material and found that the addition of PE fiber sig‐ nificantly improved the bending strength of the specimen in different directions, as shown  in Figure 14. Arunothayan et al. [74] added steel fibers to concrete materials, which greatly  improved the bending resistance of the printed specimens. Ma et al. [75] added 0.5% basalt  fiber to concrete materials, and the basic mechanical properties and anisotropy of printa‐ ble cement‐based materials were significantly improved.  Appl. Sci. 2021, 11, 9822 15 of 25 In order to facilitate the comparison of the effects of different fiber materials on the  components, they are summarized, as shown in Table 3.  Figure 14. Tensile strength of specimens with various fiber ratios. Reprinted with permission from Figure 14. Tensile strength of specimens with various fiber ratios, [73].  ref. [73]. Copyright 2020 Elsevier Ltd. In order to facilitate the comparison of the effects of different fiber materials on the components, they are summarized, as shown in Table 3. Table 3. The effect of different fiber materials on components. Fiber Material Name Effect Reference Anti-flaking, make the extrusion more uniform Polypropylene fibers [70] and continuous GFRP (Glass fiber reinforced Improve axial load carrying capacity and [71] plastic) fiber ductility Improve bending strength and deformation Carbon fiber [72] ability PE (Polyethylene) fiber Significantly improve the bending strength in [73,74] Steel fiber different directions Improve anisotropy and basic mechanical Basalt fiber [75] properties 4.5. Admixture Admixtures are an important part of 3D printed concrete materials. They can signifi- cantly adjust and improve the fluidity, setting time, and mechanical properties of concrete. Adding admixtures to 3D printed concrete appropriately will get a small amount, but have a big effect. Slavcheva et al. [76] found that by increasing the amount of water re- ducing agent, the total pore volume of the printed sample was reduced, and the content of nanopores was increased. It also caused an increase in the compactness of the sample, and the drying and shrinkage reduced. Perrot et al. [15] found that xanthan gum and Brunei gum thickeners can significantly improve the moldability of the accumulated layer of printed concrete test strips. Le et al. [16] used 1% water-reducing agent gelling materials to improve fluidity and realized the construction of specimens with the highest 61 layers. 5. Printing Parameters In addition to the material composition, the final performance of 3D printed concrete is also affected by the printing parameters to a large extent. A reasonable choice of printing parameters will greatly improve the printing quality of the components. The printing parameters of 3D-printing include the nozzle size selected by the printer, the height of the nozzle from the printing platform, the movement speed of the print head, and the interval time between layers. Printing process control and printing parameter selection are the key links in the formation of concrete 3D-printing structures, and they are also important factors that affect the final printing quality of concrete. Appl. Sci. 2021, 11, 9822 16 of 25 5.1. Design of the Print Head The key technology of 3D-printing concrete lies in whether the prepared concrete can pass through the nozzle smoothly, and realize layer-by-layer bonding and solidification to form a whole, so the design of the print head is very important. Khoshnevis et al. [77,78] used a large three-dimensional extrusion device and a nozzle with a spatula to achieve layered printing of concrete, which can solve the problem of uneven 3D-printing surface and can be used for printing large buildings. The print head is shown in Figure 15. Buswell and Lim et al. [8,9,79] used the main components of concrete container, conveying pipe, printing pump, output pipe, nozzle to form the printing head. The printing of solid components is realized through the cross operation of spraying and extruding material layered printing and implanting transverse steel mesh. It has higher precision and freedom, Appl. Sci. 2021, 11, x FOR PEER REVIEW  16  of  24  and has great advantages in printing small and heterogeneous building components, but it has a slight disadvantage compared with printing in large buildings and whole houses.    (a)  (b)  Figure 15. Installation of the print head: (a) extruder sits on an x‐y‐zgantry robot. (b) details of nozzle assembly, [77].  Figure 15. Installation of the print head: (a) extruder sits on an x-y-zgantry robot. (b) details of nozzle assembly. Reprinted with permission from ref. [77]. Copyright 2003 Elsevier B.V. 5.2. Nozzle size.  5.2. Nozzle size The nozzle size and shape in the printing parameters will have different effects on  The nozzle size and shape in the printing parameters will have different effects on the printing effect, and different nozzle shapes and sizes should be adopted for different  the printing effect, and different nozzle shapes and sizes should be adopted for different printing conditions. Paul et al. [40] found that the strength of specimens printed with rec‐ printing conditions. Paul et al. [40] found that the strength of specimens printed with tangular nozzle holes is higher than that of specimens printed with circular nozzle holes,  rectangular nozzle holes is higher than that of specimens printed with circular nozzle and circular nozzles are more suitable for printing complex structures. Because the rec‐ holes, and circular nozzles are more suitable for printing complex structures. Because the tangular nozzle has fewer interlayer gaps than the circular nozzle,   the strength is higher.  rectangular nozzle has fewer interlayer gaps than the circular nozzle, the strength is higher. Sun Xiaoyan [80] and others found that under the same extrusion flow conditions, the  Sun Xiaoyan [80] and others found that under the same extrusion flow conditions, the printed specimens with a triangular extruded shape have the fewest number of interlayer  printed specimens with a triangular extruded shape have the fewest number of interlayer defects and the best mechanical properties. The defects are reduced, and the mechanical  defects and the best mechanical properties. The defects are reduced, and the mechanical properties are improved with the extrusion size larger for the printed specimens of the  properties are improved with the extrusion size larger for the printed specimens of the same shape. When the extrusion shape and size are same, the staggered arrangement of  same shape. When the extrusion shape and size are same, the staggered arrangement of the the printing ports can improve the mechanical properties of the printed specimen by 13%  printing ports can improve the mechanical properties of the printed specimen by 13% to to 47%. Tay et al. [81] found that when the print nozzle area is equal to the cross‐sectional  47%. Tay et al. [81] found that when the print nozzle area is equal to the cross-sectional area area of the actual mortar strip, the printed mortar strip is better. The setting of the print  of the actual mortar strip, the printed mortar strip is better. The setting of the print nozzle nozzle aperture must be consistent with the content and size of the aggregate to avoid  aperture must be consistent with the content and size of the aggregate to avoid clogging. clogging.  5.3. Print Height 5.3. Print Height  Printing height is the height of the nozzle from the printing platform. Printing height Printing height is the height of the nozzle from the printing platform. Printing height  is a parameter that affects the bonding strength and surface quality between layers. Printing is a parameter that affects the bonding strength and surface quality between layers. Print‐ height will also affect the settlement of the printed mortar strips. When the printing height ing height will also affect the settlement of the printed mortar strips. When the printing  is too high, the interlayer bonding strength of the printed mortar strip will be greatly height  is  too  high,  the  interlayer  bonding  strength  of  the  printed  mortar  strip  will  be  greatly reduced, and the shape of the printed mortar strip will be unstable. When the  printing height is too low, the printing nozzle will generate additional pressure on the  printed mortar strip, resulting in more Great settlement. Wolfs et al. [82] believe that the  print height should be equal to the width of the nozzle section, because it can smoothly  deposit the extruded mortar strip and avoid the interaction between the print head and  the mortar strip. Panda et al. [43] based their tests on the direct tensile test method and  found that when the height of the printing nozzle is smaller than the height of the printing  mortar strip, the bond strength between layers increases as the height decreases, as shown  in Figure 16. Panda adjusted the print height of the nozzle to increase the tensile strength  between layers. Results show that the direct interlayer tensile strength of the sample pre‐ pared with a nozzle height of 0 mm is 53.3% higher than that of a sample prepared with a  nozzle height of 4 mm. Zareiyan et al. [83] reduced the height of the print bar to increase  the interlayer cleavage strength, and found that the 28 d interlayer cleavage strength of  the 25.4 mm layer thickness sample was 11.4% higher than when the layer thickness was  Appl. Sci. 2021, 11, 9822 17 of 25 reduced, and the shape of the printed mortar strip will be unstable. When the printing height is too low, the printing nozzle will generate additional pressure on the printed mortar strip, resulting in more Great settlement. Wolfs et al. [82] believe that the print height should be equal to the width of the nozzle section, because it can smoothly deposit the extruded mortar strip and avoid the interaction between the print head and the mortar strip. Panda et al. [43] based their tests on the direct tensile test method and found that when the height of the printing nozzle is smaller than the height of the printing mortar strip, the bond strength between layers increases as the height decreases, as shown in Figure 16. Panda adjusted the print height of the nozzle to increase the tensile strength between layers. Results show that the direct interlayer tensile strength of the sample prepared with a nozzle Appl. Sci. 2021, 11, x FOR PEER REVIEW  17  of  24  height of 0 mm is 53.3% higher than that of a sample prepared with a nozzle height of 4 mm. Zareiyan et al. [83] reduced the height of the print bar to increase the interlayer cleavage strength, and found that the 28 d interlayer cleavage strength of the 25.4 mm layer thickness sample was 11.4% higher than when the layer thickness was 50.8 mm, and the 50.8 mm, and the 25.4 mm layer thickness sample shows a higher splitting crack growth  25.4 mm layer thickness sample shows a higher splitting crack growth rate. rate.  Figure 16. Effect of nozzle standoff distance on tensile bond strength, [43].  Figure 16. Effect of nozzle standoff distance on tensile bond strength. Reprinted with permission from ref. [43]. Copyright 2017 Elsevier Ltd. 5.4. Speed of Print Head Movement  5.4. Speed of Print Head Movement The moving speed of the print head is an important printing parameter of 3D‐print‐ The moving speed of the print head is an important printing parameter of 3D-printing ing technology, and the selection of the moving speed of the print head has a great influ‐ technology, and the selection of the moving speed of the print head has a great influence ence on the bonding strength between layers and the surface quality. Tay et al. [84] chose  on the bonding strength between layers and the surface quality. Tay et al. [84] chose a a 30 mm × 15 mm rectangular nozzle for printing, and found that the nozzle’s moving  30 mm  15 mm rectangular nozzle for printing, and found that the nozzle’s moving speed speed has a great influence on the cross‐sectional shape of the printed mortar strip. Rahul  has a great influence on the cross-sectional shape of the printed mortar strip. Rahul et al. [68] et al. [68] believe that the extrusion speed of the printer is the same as the nozzle move‐ believe that the extrusion speed of the printer is the same as the nozzle movement speed, ment speed, which can maintain the shape stability of the printed mortar. Kruger et al.  which can maintain the shape stability of the printed mortar. Kruger et al. [85] developed [85] developed a design model for 3D concrete printing and predicted the printing speed  a design model for 3D concrete printing and predicted the printing speed to prevent to prevent structural failure under given conditions. Panda et al. [43] studied the effect of  structural failure under given conditions. Panda et al. [43] studied the effect of different different nozzle speeds on the performance of fresh concrete for 3D‐printing, and found  nozzle speeds on the performance of fresh concrete for 3D-printing, and found that the that the adhesive strength of the sample decreased slightly as the nozzle speed increased,  adhesive strength of the sample decreased slightly as the nozzle speed increased, as shown as shown in Figure 17. Panda adjusted the running speed of the nozzle to increase the  in Figure 17. Panda adjusted the running speed of the nozzle to increase the tensile tensile strength between layers. The results show that the direct interlayer tensile strength  strength between layers. The results show that the direct interlayer tensile strength of of the samples made with the 70 mm/s nozzle operating speed is 10.5% higher than that  the samples made with the 70 mm/s nozzle operating speed is 10.5% higher than that of of the samples made with the 110 mm/s nozzle operating speed. The printing speed used  the samples made with the 110 mm/s nozzle operating speed. The printing speed used by Nerella et al. [86] is about 75 mm/s. The initial setting time of concrete materials is  by Nerella et al. [86] is about 75 mm/s. The initial setting time of concrete materials is controlled within 3 min through the accelerator, and the interval between layers is 30 s,  controlled within 3 min through the accelerator, and the interval between layers is 30 s, which greatly optimizes the rheological properties of concrete.  which greatly optimizes the rheological properties of concrete. Figure 17. Effect of nozzle speed on tensile bond strength, [43].  5.5. Interlayer Interval Time  Appl. Sci. 2021, 11, x FOR PEER REVIEW  17  of  24  50.8 mm, and the 25.4 mm layer thickness sample shows a higher splitting crack growth  rate.  Figure 16. Effect of nozzle standoff distance on tensile bond strength, [43].  5.4. Speed of Print Head Movement  The moving speed of the print head is an important printing parameter of 3D‐print‐ ing technology, and the selection of the moving speed of the print head has a great influ‐ ence on the bonding strength between layers and the surface quality. Tay et al. [84] chose  a 30 mm × 15 mm rectangular nozzle for printing, and found that the nozzle’s moving  speed has a great influence on the cross‐sectional shape of the printed mortar strip. Rahul  et al. [68] believe that the extrusion speed of the printer is the same as the nozzle move‐ ment speed, which can maintain the shape stability of the printed mortar. Kruger et al.  [85] developed a design model for 3D concrete printing and predicted the printing speed  to prevent structural failure under given conditions. Panda et al. [43] studied the effect of  different nozzle speeds on the performance of fresh concrete for 3D‐printing, and found  that the adhesive strength of the sample decreased slightly as the nozzle speed increased,  as shown in Figure 17. Panda adjusted the running speed of the nozzle to increase the  tensile strength between layers. The results show that the direct interlayer tensile strength  of the samples made with the 70 mm/s nozzle operating speed is 10.5% higher than that  of the samples made with the 110 mm/s nozzle operating speed. The printing speed used  by Nerella et al. [86] is about 75 mm/s. The initial setting time of concrete materials is  Appl. Sci. 2021, 11, 9822 18 of 25 controlled within 3 min through the accelerator, and the interval between layers is 30 s,  which greatly optimizes the rheological properties of concrete.  Figure 17. Effect of nozzle speed on tensile bond strength. Reprinted with permission from ref. [43]. Figure 17. Effect of nozzle speed on tensile bond strength, [43].  Appl. Sci. 2021, 11, x FOR PEER REVIEW  18  of  24  Copyright 2017 Elsevier Ltd. 5.5. Interlayer Interval Time  5.5. Interlayer Interval Time The interlayer interval time is a key factor affecting the bond strength. Optimize the  The interlayer interval time is a key factor affecting the bond strength. Optimize the int interlayer erlayer int interval erval time time to to pr prevent event significant  significantdeformation  deformation or or collapse  collapse and  and ensur  ensure e acceptable  accepta‐ ble bonding  bonding str ength strengbetween th between printed  printed layers.  layers. Panda  Panda et etal. al.[ 43 [43 ]]showed  showed that that incr increasing easing the the  time interval between layers, printing speed and print head height reduces the interlayer time interval between layers, printing speed and print head height reduces the interlayer  bonding bonding str strength ength of of3D  3Dprinted  printed concr  concret ete e str structur uctures,es, as as shown  shown in Figur in Fig eu 18 re. 18 Ner . Ne ellarell eta al. et [ 44 al].  showed that the bonding strength between layers decreases with the extension of the time [44] showed that the bonding strength between layers decreases with the extension of the  interval. When the time interval between layers is 1 min, the bonding strength decreases time interval. When the time interval between layers is 1 min, the bonding strength de‐ by 50%, and when the time interval between layers is 1 d, the bonding strength decreases creases by 50%, and when the time interval between layers is 1 d, the bonding strength  by 90%. Wolfs et al. [87] optimized the interlayer interval and surface dehydration to decreases by 90%. Wolfs et al. [87] optimized the interlayer interval and surface dehydra‐ increase the interlayer tensile strength. Results show that reducing the interlayer interval tion to increase the interlayer tensile strength. Results show that reducing the interlayer  can increase the interlayer tensile strength. The interlayer tensile strength and splitting of interval can increase the interlayer tensile strength. The interlayer tensile strength and  the specimens with an interval of 15 s is 19% and 27% higher than that of the 24 h specimen, splitting of the specimens with an interval of 15 s is 19% and 27% higher than that of the  respectively. 24 h specimen, respectively.  Figure Figure 18. 18. Effe . Effect ct ofof time time gay gay bebetween tween layers layers on on tensile tensile bond bond streng strength. th, [43] Reprinted .  with permission from ref. [43]. Copyright 2017 Elsevier Ltd. 6. Print Quality Evaluation Index  6. Print Quality Evaluation Index The forming quality of printed components is also a key issue of 3D‐printing concrete  The forming quality of printed components is also a key issue of 3D-printing concrete technology. It mainly includes geometric quality problems such as the shape and size of  technology. It mainly includes geometric quality problems such as the shape and size of the printed components and surface quality problems such as surface roughness. Basic  the printed components and surface quality problems such as surface roughness. Basic requirements of the construction process are to print components with good forming qual‐ requirements of the construction process are to print components with good forming ity. Therefore, it is very important to systematically evaluate the forming quality of 3D  quality. Therefore, it is very important to systematically evaluate the forming quality of 3D printed concrete components.  printed concrete components. The quality of concrete 3D‐printing is affected by many factors such as printing ma‐ The quality of concrete 3D-printing is affected by many factors such as printing terials, printer systems, pumps, control systems, print heads, and the shape of the dis‐ materials, printer systems, pumps, control systems, print heads, and the shape of the charge nozzle [88]. Lim et al. [9] proposed that the surface quality of 3D‐printing cement‐ based  materials  can  be  judged  by  the  three  criteria  of  no  defects  on  the  surface,  clear  square  boundaries  of  the  printed  layer,  and  meeting  the  requirements  of  surface  con‐ sistency and homogeneity.  Printing parameters are the most critical factor affecting the surface quality of the  parts; thus, scholars focus mainly on adaptive layering and optimization of various print‐ ing parameters. Nadiyapara et al. [89] believe that, compared with equal layer thickness  layering, adaptive layering can set a smaller layer thickness in the feature‐intensive area  of the model, so that the dense features are not easy to lose, thereby improving the surface  quality of the part. The algorithm is complex; the quality of the algorithm determines the  accuracy of the layering. If the algorithm is unreasonable, it will reduce the surface quality  of the model. Kim et al. [90] believe that the fused deposition printing parameters have  strong coupling and influence each other during the printing process, which ultimately  determines the surface quality of the parts. Optimizing the molding parameters can sig‐ nificantly improve the poor surface finish of the molded parts. Li Yanru et al. [91] effec‐ tively optimized the surface quality of the part by selecting the optimal forming angle,  optimal layer thickness, and optimal extrusion magnification.  Appl. Sci. 2021, 11, 9822 19 of 25 discharge nozzle [88]. Lim et al. [9] proposed that the surface quality of 3D-printing cement-based materials can be judged by the three criteria of no defects on the surface, clear square boundaries of the printed layer, and meeting the requirements of surface consistency and homogeneity. Printing parameters are the most critical factor affecting the surface quality of the parts; thus, scholars focus mainly on adaptive layering and optimization of various printing parameters. Nadiyapara et al. [89] believe that, compared with equal layer thickness layering, adaptive layering can set a smaller layer thickness in the feature-intensive area of the model, so that the dense features are not easy to lose, thereby improving the surface quality of the part. The algorithm is complex; the quality of the algorithm determines the accuracy of the layering. If the algorithm is unreasonable, it will reduce the surface quality of the model. Kim et al. [90] believe that the fused deposition printing parameters have strong coupling and influence each other during the printing process, which ultimately determines the surface quality of the parts. Optimizing the molding parameters can significantly improve the poor surface finish of the molded parts. Li Yanru et al. [91] effectively optimized the surface quality of the part by selecting the optimal forming angle, optimal layer thickness, and optimal extrusion magnification. 7. The Problems and Prospects of 3D-Printing Concrete 7.1. Existing Problems Many challenges and technical problems have always existed in the field of 3D- printing concrete, although much research has been undertaken. A brief list is as follows: (1) The problem of accurate conversion between software design and real architecture. In contrast to the traditional building structure, 3D-printing construction technology requires the design of the building model on the computer, and then converts it into a physical object through an automated program. The design software has become an important part of the preparation phase of the 3D-printing construction technology, but there are certain errors between the software 3D design and the entity in architectural printing, which lead to construction quality problems and limit the development of 3D-printing concrete technology. How to realize the conversion between software and reality is an indispensable step in the development of printing concrete. (2) The feasibility of complex architectural printing. The mechanical properties of 3D- printing construction technology are still at the stage of research and development. Although it can be applied to the construction of low-rise and large-area buildings, one-time 3D-printing cannot be done for common high-rise buildings. Thus, the feasible method is to print prefabricated ones first and re-assembly of parts, which is similar to prefabricated buildings. Thus, the advantage of rapid prototyping is lost. Difficulties lie in the high-rise buildings of tens of meters or even hundreds of meters. First of all, it requires a printer compatible with the height of the building. Secondly, to obtain the structural strength of the high-rise building and the problem of the steel bar structure in the building structure, a reasonable solution is needed. (3) The requirements of building materials is another problem. Most 3D printed build- ings are made of high-strength special concrete materials. Ordinary cement cannot meet the requirements of building performance and printing process. It can be seen that the requirements of 3D-printing for material performance must be considered. The 3D-printing building mainly uses printer nozzles to spray materials to build walls. This spraying method not only requires certain stress and mechanical molding characteristics, but also has certain requirements for the quality of coarse and fine aggregates. The requirements for the quality of aggregates are very high, and even new crushing processes are needed. The effect of admixtures in ordinary concrete may be due to the change of materials, which makes it play a role in the specific system of 3D-printing buildings, and even its mechanism of action also changes. Thus, to Appl. Sci. 2021, 11, 9822 20 of 25 realize the printing of three-dimensional buildings, materials are the first problem to be solved. (4) The problem of molding height. The current 3D-printing concrete technology is still in the stage of plane expansion. It can be applied to the construction of low-rise and large-area buildings. However, it is not possible to print the widely used high-rise buildings. It can only be printed by first printing prefabs and then assembling them. To print dozens of floors of buildings, it is necessary to design a giant 3D printer to solve the problem of structural strength of large buildings and the printing of steel bars in buildings. (5) The problem of anisotropy. This is one of the main challenges to limit the development of 3D printed concrete. The main reason for the anisotropy is that the internal structure of the layers or the internal structure of the material is different due to the layer-by- layer printing, which causes the difference of mechanical properties when the printed components loaded in different directions. It is also related to various factors such as material properties, nozzle types, and steel bar layout besides the impact of the layer-by-layer printing construction process. (6) Although the 3D printed concrete over traditional concrete pouring methods can adapt to complex designs and free-style buildings, the true connotations of opti- mization, sustainability and effective weight reduction have not been fully explored. Topology optimization is a mathematical method that has been widely used in opti- mizing the appearance and layout of materials. Now a topology optimization method has been introduced in the numerical simulation stage of 3D printed concrete, with the purpose of optimizing the printed structure. However, it is difficult to apply this method directly to the structural design of concrete because it fails to consider the nonlinear behavior of the material (such as the significant difference between the compressive and tensile strength of concrete). (7) The adhesion between different layers of 3D printed concrete is mainly characterized by tensile and shear strength. Most of the current research focuses on tensile strength, and the research on shear strength is relatively insufficient. In addition, 3D printed concrete will have major changes in the production process, safety measures, etc., which will cause more or less problems. These all require further research. 7.2. Development Direction and Key Technology Now 3D-printing concrete technology is in the initial stage of development. In order to realize the universalization of the application of 3D-printing concrete technology, it is necessary to solve its existing problems with key technologies. The future research and development of 3D-printing concrete technology can be carried out from the following aspects: (1) Research on concrete materials. Although a lot of research on 3D-printing concrete ma- terials has been carried out, and some properties that 3D-printing concrete needs have been proposed, it is mainly based on the preliminary understanding of 3D-printing concrete technology, and there is a lack of systematic, theoretical, and in-depth re- search. It is necessary to conduct systematic and in-depth research on concrete raw materials and compounding theory for 3D-printing concrete technology to develop rapidly. Thus, the new concrete mix theory and new raw material requirements need to be improved according to performance requirements. This will be the main research topic in the development of 3D-printing concrete technology. (2) Software and hardware collaboration. The 3D printer needs to recognize the given three-dimensional model to print the building based on the model. The modeling and design software of the architectural field and the CNC software of the 3D printer are effectively docked to realize the seamless recognition of information which is the future software development direction. In addition, using the CNC software to control the hardware device precisely also needs further development. Appl. Sci. 2021, 11, 9822 21 of 25 (3) Research on printing technology. How to reasonably arrange the printing direction and order, control the all-round climb of the print head, etc., are important factors that determine the printing efficiency, and it is also an aspect that needs attention in future research. The achievable printing height of the current 3D architectural printing process is greatly restricted, but the development of the society has gradually made the concrete buildings develop toward the towering direction. It requires a new printing process to achieve high-rise printing and development to the high-rise to solve the problem including the structural strength, reinforcement, and other issues. (4) Application of micro characterization technology. The current research still focuses on analysis of macroscopic mechanical properties, while the research on micro-scale can provide theoretical support at the specific material level for the macro-performance. The analysis at the micro level will further deepen and improve the understanding of the mechanical properties of 3D printed concrete. (5) Durability of printing materials. The main focus of study is rheology, buildability, anisotropy of mechanical properties, and interlayer adhesion for extrusion-type 3D- printing concrete. It is necessary to conduct a detailed study on the durability of its material structure considering the service time limit and environment of the printing structure and the building. Quantitatively evaluating the durability of 3D printed concrete is necessary in order to compare the durability of different 3D printed concretes, and finally realize the prediction and improvement of the durability of 3D printed concrete. (6) Hardening performance is one of the important indicators of 3D-printing concrete materials. Due to the existence of weak surfaces between layers, 3D-printing concrete materials have obvious anisotropy. Although researchers have conducted a lot of re- search on the factors that influence the interlayer bonding performance of 3D-printing concrete materials, and also have proposed some effective interface enhancement methods, the current methods lack flexibility and universality. It is necessary to further study flexible and effective interlayer performance enhancement methods, and optimize the design of the printing structure and path. (7) Strengthening and toughening methods are an important guarantee for the safety of 3D printed concrete structures. Adding chopped fibers can effectively improve the tensile strength and toughness of 3D printed concrete materials, but the increase in tensile strength is limited and it is difficult to achieve the enough reinforcement of steel bars. Although the continuous rib co-printing method is effective, its enhancement effect on the structure is limited. Other co-printing methods are in the conceptual design stage and need to be studied further. As a new type of construction technology in the construction industry, concrete 3D- printing has the characteristics of moldless construction, high efficiency, and high precision, showing a good application prospect. Although there are still some problems with the development of materials and equipment, and the development of construction technology, ideas such as rapid in-situ printing of large building structures on site, simultaneous and coordinated printing of multiple printers, and fine printing of factory components and assembly construction will gradually be realized. This technology will surely lead to great vitality in the field of construction and become an important supplement to traditional construction methods. Author Contributions: F.L., D.Z., X.H., L.S. and Q.Z., all contributed to the collection of data and preparation of the paper. All authors have read and agreed to the published version of the manuscript. Funding: This work is supported by National Science Foundation under Grant No. 5210041604, 52174144, 52174120, China Postdoctoral Science Foundation under Grant No. 2021M691967, and Natural Science Foundation of Shandong Province, China under Grant No. ZR202103010529. 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Overview of the Development of 3D-Printing Concrete: A Review

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applied sciences Review Overview of the Development of 3D-Printing Concrete: A Review Fuyan Lyu, Dongliang Zhao , Xiaohui Hou, Li Sun and Qiang Zhang * School of Mechanical and Electronic Engineering, Shandong University of Science and Technology, Qingdao 266590, China; skdlfy@sdust.edu.cn (F.L.); skdzdl@sdust.edu.cn (D.Z.); skdhxh@sdust.edu.cn (X.H.); skdsl@sdust.edu.cn (L.S.) * Correspondence: zhangqiangskd@sdust.edu.cn Abstract: 3D-printing concrete technology has attracted more and more attention for smart con- struction due to its advantages of digitization, automation, and high degree of intelligence. This article introduces the basic principles and related processes of concrete 3D-printing technology, and reviews the development from the following four fields: the material properties, preparation technology, printing parameters, and evaluation criteria of 3D-printing concrete technology. Then the existing difficulties, development direction and key technologies of 3D-printing concrete are described. Finally, we look forward to the development prospects of 3D-printing concrete from the aspects of printing materials, software and hardware cooperation, printing technology, etc. All the researches will provide the useful references for the later development and research. Keywords: 3D-printing; concrete; material properties; preparation technology; printing parameters 1. Introduction Citation: Lyu, F.; Zhao, D.; Hou, X.; Sun, L.; Zhang, Q. Overview of the Concrete material is a mixture made by mixing cementitious materials, water, coarse Development of 3D-Printing and fine aggregates in appropriate proportions. It is widely used in construction, water Concrete: A Review. Appl. Sci. 2021, conservancy, bridges, highways, railways and urban infrastructure construction which is 11, 9822. https://doi.org/10.3390/ one of the important civil engineering materials. As the demand for concrete increases, the app11219822 problems of high pollution and high energy consumption in production and application process have become increasingly prominent, restricting the green, healthy and sustainable Academic Editor: Tiago Pinto Ribeiro development of concrete materials. At the same time, the increasingly complex concrete structures have put forward higher requirements on the strength and durability of concrete Received: 14 August 2021 materials due to the environment and stress characteristics. Accepted: 12 October 2021 For the past few years 3D-printing technology has been widely used in architectural Published: 20 October 2021 design, industrial manufacturing, aerospace, biological engineering, cultural relics protec- tion and other industries with its advantages of low cost, high efficiency, strong design, and Publisher’s Note: MDPI stays neutral reliable quality [1,2]. Especially the 3D-printing technology combined with concrete tech- with regard to jurisdictional claims in nology that provides new ideas for the development and application of concrete materials. published maps and institutional affil- However, 3D-printing concrete technology has strict requirements in different performance iations. domains, and further, deeper research is still needed. This article introduces the research status of 3D-printing concrete technology, summa- rizes the impact of concrete material properties, key preparation technologies, 3D-printing control parameters and other factors on 3D-printing concrete technology. Based on the Copyright: © 2021 by the authors. current status and problems, looks forward to the research direction of 3D-printing concrete Licensee MDPI, Basel, Switzerland. technology and development trends. This article is an open access article distributed under the terms and 2. 3D-Printing Concrete Technology conditions of the Creative Commons 3D-printing concrete technology is a new technology developed on the basis of 3D- Attribution (CC BY) license (https:// printing technology and applied to concrete construction. Its main working principle is creativecommons.org/licenses/by/ to pass the configured concrete slurry through the extrusion device, under the control of 4.0/). Appl. Sci. 2021, 11, 9822. https://doi.org/10.3390/app11219822 https://www.mdpi.com/journal/applsci Appl. Sci. 2021, 11, 9822 2 of 25 the 3D software, according to the preset settings A good printing program is extruded by a nozzle to print, and finally the designed concrete component is obtained. As a new type of concrete moldless molding technology [3], it can apply the computer-aided design tools to the construction process. Compared with the traditional construction forms, 3D-printing concrete technology consumes less energy when applied to complex structures, and 3D-printing concrete technology can optimize the structure according to the working conditions. With the large-scale application and promotion of 3D-printing concrete technology, it can effectively reduce the input of materials, personnel, and machinery in the building construction process, and promote the development of digital and intelligent building construction technology [4]. 3D-printing concrete technology originated from Rensselaer Polytechnic Institute in New York, USA. Pegna [5] first explored 3D-printing construction technology in 1997, which proved the feasibility and prospects of 3D-printing technology in the field of construc- tion. In 1998, at the University of Southern California, Khoshnevis et al. [6,7] developed contour crafting (CC), which adopts computer precise control to automatically complete the pouring process and realizes the smooth contour surface and complex feature model pouring by controlling the nozzle. In 2007, Dini demonstrated its D-shape technology [8]. D-shape technology uses hundreds of nozzles at the bottom of the printing device, which can spray magnesia binders. Fine sand containing magnesia powder is sprayed on the binders to gradually solidify to form a stone solid. The sand layers are glued together to form a stone building in the end. In 2009, Buswell et al. [8,9] of Loughborough University in the United Kingdom developed concrete printing technology (concrete printing, Con- print 3D ). Compared with the contour process, the preparation of the equipment and computer control program used in the concrete printing technology is simpler. In 2012, ETH Zurich [10,11] launched the mesh mold project to explore the possibility of digitally manufacturing concrete structures with high geometric complexity. This technology uses 3D-printing to build a polymer network. The mold selects the appropriate particle size aggregate according to the size of the mesh to design the concrete mix ratio, so as to realize the retention of the concrete by blocking the mesh. In 2015, ETH Zurich Lloret et al. [12] developed smart dynamic casting (SDC ) based on the fast sliding mold process. Com- pared with other 3D-printing building technologies, its significant advantage lies in the ability to form a sliding mold around the steel bar, which realizes the enhancement of the printing structure. In 2019, China Construction Industrial Technology Research Institute Co., Ltd. and China Construction Second Bureau South China Company jointly established a project to print “In-situ 3D-printing double-layer demonstration buildings”, marking a breakthrough in In-situ 3D-printing technology in the field of construction [13]. Figure 1 shows the development history of 3D-printing construction technology based on cement-based materials [14]. Since the concept of 3D-printing concrete was put forward for more than 20 years, the research and application of 3D-printing concrete technology has developed exponentially, relying on the advantages of short construction period, topology optimization, high mechanization, non-modeling, meeting personalized customization, complex structure building construction and shape Unique architectural construction and other advantages have entered a period of rapid development. The development of 3D- printing concrete technology presents the following characteristics: (1) The scale of the industry continues to grow; (2) New materials and new technologies continue to emerge; (3) New engineering applications continue to emerge. Appl. Appl. Sci. Sci. 2021 2021 , 11 , 11 , x, FOR 9822 PEER REVIEW  3  of 3 of 2425  Figure 1. Development history of 3D-printing construction technology. Reprinted with permission Figure 1. Development history of 3D‐printing construction technology, [14].  from ref. [14]. Copyright 1969 Elsevier. 3.3. Pe Performance rformance Re Requirements quirements ofof 3D 3D-printing ‐printing Concre Concrete te Mater Materials ials  3.1. Printability of 3D-printing Concrete Materials 3.1. Printability of 3D‐printing Concrete Materials  3D-printing concrete technology is different from traditional molding concrete tech- 3D‐printing concrete technology is different from traditional molding concrete tech‐ nology, and 3D-printing concrete technology has stricter requirements for materials. The nology, and 3D‐printing concrete technology has stricter requirements for materials. The  printing material not only needs to have enough fluidity to ensure the smooth pumping of printing material not only needs to have enough fluidity to ensure the smooth pumping  the material and continuous extrusion from the nozzle. It also needs to have good water of the material and continuous extrusion from the nozzle. It also needs to have good water  retention to avoid the clogging of the pumping tube due to material segregation, and it also retention to avoid the clogging of the pumping tube due to material segregation, and it  needs to have enough hardening speed to maintain the stable accumulation of subsequent also needs to have enough hardening speed to maintain the stable accumulation of sub‐ layers to build [14]. Therefore, the printability of the material mainly includes fluidity, sequent layers to build [14]. Therefore, the printability of the material mainly includes  extrudability, buildability, and setting time. fluidity, extrudability, buildability, and setting time.  3.1.1. Fluidity 3.1.1. Fluidity  Fluidity refers to the ability of concrete materials to be easily pumped, transported, and Fluidity refers to the ability of concrete materials to be easily pumped, transported,  smoothly extruded from the discharge port of the print head. It is an important parameter and smoothly extruded from the discharge port of the print head. It is an important pa‐ for evaluating printability. If the fluidity is small, it is likely to cause a high mechanical rameter for evaluating printability. If the fluidity is small, it is likely to cause a high me‐ wear rate and equipment blockage. If the fluidity is large, the printed components are easy chanical wear rate and equipment blockage. If the fluidity is large, the printed components  to collapse. Therefore, it is necessary to make reasonable adjustments to fluidity to meet are easy to collapse. Therefore, it is necessary to make reasonable adjustments to fluidity  printing needs. to meet printing needs.  The most important factor affecting fluidity is water content. If the water content is The most important factor affecting fluidity is water content. If the water content is  too small, the mixture will become dry and hard and cannot pass the conveying pipeline too small, the mixture will become dry and hard and cannot pass the conveying pipeline  smoothly. If the water content is too large, it will cause a large number of harmful pores in smoothly. If the water content is too large, it will cause a large number of harmful pores  the printed sample, which will affect the later strength. The water content of cement slurry in the printed sample, which will affect the later strength. The water content of cement  can be effectively controlled by adding high-performance water-reducing agent to improve slurry can be effectively controlled by adding high‐performance water‐reducing agent to  the fluidity of cement slurry. Perrot et al. [15] obtained a mixture that satisfies 3D-printing improve the fluidity of cement slurry. Perrot et al. [15] obtained a mixture that satisfies  which contains water-cement ratio of 0.41 and polycarboxylate polymer powder of 0.3% 3D‐printing which contains water‐cement ratio of 0.41 and polycarboxylate polymer pow‐ cement mass. Results showed the best construction speed was 1.1m/h. Le et al. [16] der of 0.3% cement mass. Results showed the best construction speed was 1.1m/h. Le et  obtained 3D printed mixture could be smoothly extruded through the nozzle and achieved al. [16] obtained 3D printed mixture could be smoothly extruded through the nozzle and  the construction of a specimen with the highest 61 layers. The gelling material is a water-to- achieved the construction of a specimen with the highest 61 layers. The gelling material is  binder ratio of 0.26 and a water reducing agent of 1%. The fluidity can also be improved by a water‐to‐binder ratio of 0.26 and a water reducing agent of 1%. The fluidity can also be  adding mineral admixtures to optimize the particle size gradation. The more continuous improved by adding mineral admixtures to optimize the particle size gradation. The more  the particle gradation, the more conducive to the formation of a densely packed state of the continuous  the  particle  gradation,  the  more  conducive  to  the  formation  of  a  densely  mixture, resulting in better fluidity. Güneyisi et al. [17] replaced the mixture produced by packed state of the mixture, resulting in better fluidity. Güneyisi et al. [17] replaced the  Portland cement with 50% fly ash, and the flow time reduced by 43.2%. Zhang et al. [18] mixture produced by Portland cement with 50% fly ash, and the flow time reduced by  studied the relationship between fluidity and aggregate content and aggregate fineness, 43 and .2%.the  Zha results ng [18] showed  et al. stu thatdied ther the e is relat a linear ionship relationship  between between  fluidityfluidity  and aggreg and aggr ate content egate, as  Appl. Sci. 2021, 11Appl. , x FOR  Sci. P EER 2021 ,RE  11V , xIE FOR W   PEER REVIEW  4  of  24  4  of  24  Appl. Sci. 2021, 11, 9822 4 of 25 and aggregateand  finen aggr ess,egat  ande  the finen results ess, an showed d the results  that there  showed  is a  tha lineta there r relationsh  is a line ipa rbetween  relationsh   ip between  fluidity and aggregate, as shown in Figure 2. Ting [19] and others used recycled glass as  fluidity and aggregate, as shown in Figure 2. Ting [19] and others used recycled glass as  fine aggregate and added it to concrete, which significantly improved the fluidity of con‐ fine aggregate and added it to concrete, which significantly improved the fluidity of con‐ shown in Figure 2. Ting [19] and others used recycled glass as fine aggregate and added it crete.  crete.  to concrete, which significantly improved the fluidity of concrete.     Figure 2. Relationship between the flowability of cement paste and the optimum amount of natural Figure 2. Relationship Figure  2. betwee  Relatinonship  the flowabi  betwee lityn  of the cem  flowabi ent pa litste y of and  cem the ent opt  paimum ste and amo  theu opt nt of imum  natu amo ‐ unt of natu‐ ral sand, [18].  ral sand.  sand Reprinted , [18].  with permission from ref. [18]. Copyright 2019 Elsevier Ltd. The fluidity test is generally determined by the jumping table test, and its expanded The fluidity test is generally determined by the jumping table test, and its expanded  The fluidity test is generally determined by the jumping table test, and its expanded  diameter is used as the fluidity index of the material, as shown in Figure 3. Zhang [20] and diameter is used as the fluidity index of the material, as shown in Figure 3. Zhang [20] and  diameter is used as the fluidity index of the material, as shown in Figure 3. Zhang [20] and  Ma [21] etc. used the jumping table test to measure the fluidity range of 170~226 mm and Ma [21] etc. used the jumping table test to measure the fluidity range of 170 ~ 226 mm and  Ma [21] etc. used the jumping table test to measure the fluidity range of 170 ~ 226 mm and  174~210 mm, respectively. Tay et al. [22] used slump test and slump flow test to study the 174 ~ 210 mm, respectively. Tay et al. [22] used slump test and slump flow test to study  174 ~ 210 mm, respectively. Tay et al. [22] used slump test and slump flow test to study  3D printable range of concrete. The results show that a mixture with a slump of 4–8 mm the 3D printable range of concrete. The results show that a mixture with a slump of 4‐8  the 3D printable range of concrete. The results show that a mixture with a slump of 4‐8  and a fluidity of 150–190 mm can have a better printing characteristic. mm and a fluidity of 150–190 mm can have a better printing characteristic.  mm and a fluidity of 150–190 mm can have a better printing characteristic.      Figure 3. Jump Figure table test  3.  Jump employed  table for  test fl empl owaboyed ility  meas for flurement, owability [21].  meas  urement, [21].  Figure 3. Jump table test employed for flowability measurement. Reprinted with permission from ref. [21]. Copyright 2017 Elsevier Ltd. 3.1.2. Extrudability  3.1.2. Extrudability  3.1.2. Extrudability Extrudability refers to the difficulty of 3D‐printing concrete in the extrusion process  Extrudability refers to the difficulty of 3D‐printing concrete in the extrusion process  Extrudability refers to the difficulty of 3D-printing concrete in the extrusion process and the continuity and surface quality after extrusion. The study of extrudability can en‐ and the continuity and surface quality after extrusion. The study of extrudability can en‐ and the continuity and surface quality after extrusion. The study of extrudability can sure that the slurry sure tha  cant  the be  cont slurry inuou  cans be ly  tra cont nsported inuously th tra roungsported h the feed  thr opipe ugh  an thed  feed depos pipe ited an  d deposited  ensure that the slurry can be continuously transported through the feed pipe and deposited smoothly through the nozzle of the print head. It is a guarantee for continuous printing  smoothly through the nozzle of the print head. It is a guarantee for continuous printing  smoothly through the nozzle of the print head. It is a guarantee for continuous printing construction and can ensure the integrity of the printing building.  construction and can ensure the integrity of the printing building.  construction and can ensure the integrity of the printing building. An important factor affecting extrudability is the ratio of aggregate particle size to  An important factor affecting extrudability is the ratio of aggregate particle size to  An important factor affecting extrudability is the ratio of aggregate particle size to the the diameter of the extrusion nozzle. If the aggregate particle size is too large, the extru‐ the diameter of the extrusion nozzle. If the aggregate particle size is too large, the extru‐ diameter of the extrusion nozzle. If the aggregate particle size is too large, the extrusion sion nozzle will be blocked. Conversely, if the aggregate particle size is too small, its sur‐ sion nozzle will be blocked. Conversely, if the aggregate particle size is too small, its sur‐ nozzle will be blocked. Conversely, if the aggregate particle size is too small, its surface face area will increase, and the amount of slurry that required to wrap the surface of the  face area will increase, and the amount of slurry that required to wrap the surface of the  area will increase, and the amount of slurry that required to wrap the surface of the aggregate will increase, causing the concrete to easily crack. Liu [23] found that the use of Appl. Sci. 2021, 11, x FOR PEER REVIEW  5  of  24  Appl. Sci. 2021, 11, 9822 5 of 25 aggregate will increase, causing the concrete to easily crack. Liu [23] found that the use of  fine aggregate materials can ensure good extrudability and effectively prevent concrete  fine aggregate materials can ensure good extrudability and effectively prevent concrete materials from blocking the printing pipes and nozzles. Noura et al. [24] found that when  materials from blocking the printing pipes and nozzles. Noura et al. [24] found that when the maximum particle size of the sand in the mixture is 2 mm, and the weight ratio of the  the maximum particle size of the sand in the mixture is 2 mm, and the weight ratio of cement is 2, and when the ratio of the nozzle diameter to the maximum aggregate particle  the cement is 2, and when the ratio of the nozzle diameter to the maximum aggregate size particle   is  grsize eater is  tha greater n  5, than the  printing 5, the printing   matermaterial ial  will  will not  be not  extrude be extruded. d.  Bloc Blockage kage  occurred occurred. .  Malaeb Malaeb etet alal. . [2[5] 25 fou ] found nd tha that t while while red reducing ucing sa sand, nd, in incr creas easing ing the the am amount ount of of ce cement ment will will  ma make ke it itha have ve better better ext extr rudabil udability ity. When . When the the ratio ratio ofof fine fine agg aggr reg egate ate toto cement cement isis 1.1.28, 28, the the  ratio ratio ofof fin fine e aggreg aggreate gate toto sand sand isis 2.2. The The cecement ment ratio ratio rerquired equired for for proper proper ext extr ruda udability bility wi with th  minimum water is 0.48, the best nozzle diameter is 2 cm, and the extrudability is the best. minimum water is 0.48, the best nozzle diameter is 2 cm, and the extrudability is the best.  Hambach et al. [26] found that when the content of fiber mixed in cement-based materials Hambach et al. [26] found that when the content of fiber mixed in cement‐based materials  exceeds about 1.5%, it will cause the printing nozzle to block. exceeds about 1.5%, it will cause the printing nozzle to block.  The extrudability test is mainly evaluated by the apparent quality of the strip or the The extrudability test is mainly evaluated by the apparent quality of the strip or the  extrusion pressure. Le et al. [16] extruded a band with a total length of 4500 mm from a extrusion pressure. Le et al. [16] extruded a band with a total length of 4500 mm from a 9  9 mm wide nozzle without clogging or breaking, as a criterion for meeting extrudability, mm wide nozzle without clogging or breaking, as a criterion for meeting extrudability, as  as shown in Figure 4a. Lafhaj et al. [27] used the process parameters with a print nozzle shown in Figure 4a. Lafhaj et al. [27] used the process parameters with a print nozzle di‐ diameter of 15 mm and a printing speed of 100 mm/s, as shown in Figure 4b, and evaluated ameter of 15 mm and a printing speed of 100 mm/s, as shown in Figure 4b, and evaluated  the extrudability by observing whether the mixture was blocking the pipe during the the extrudability by observing whether the mixture was blocking the pipe during the pro‐ process of stacking 20 layers of strips with a length of 500 mm and a width of 350 mm. cess of stacking 20 layers of strips with a length of 500 mm and a width of 350 mm. Chen  Chen et al. [28] used a stamping extruder to quantify the extrusion pressure of different et al. [28] used a stamping extruder to quantify the extrusion pressure of different blends  blends to characterize the extrudability. to characterize the extrudability.  (a)  (b)  Figure 4. Test sample to evaluate extrudability. (a) Long strip evaluation chart. (b) Stacking evaluation chart, [16,27].  Figure 4. Test sample to evaluate extrudability. (a) Long strip evaluation chart. (b) Stacking evaluation chart. Reprinted with permission from ref. [16]. Copyright 2012, RILEM. Reprinted with 3.1.3. Buildability  permission from ref. [27]. Copyright 2019 MDPI. Buildability refers to the degree of deformation and overall stability of the 3D printed  3.1.3. Buildability cement‐based material after extrusion under its own weight and subsequent extrusion  Buildability refers to the degree of deformation and overall stability of the 3D printed and gravity of the printed layer. Because the 3D‐printing concrete technology does not  cement-based material after extrusion under its own weight and subsequent extrusion have templates, it is easy to have interlayer defects or settlement of the printing layer. The  and gravity of the printed layer. Because the 3D-printing concrete technology does not previous printing layer must support the subsequent printing layer with better bonding  have templates, it is easy to have interlayer defects or settlement of the printing layer. The ability with the subsequent printing layer, and the components do not collapse or deform.  previous printing layer must support the subsequent printing layer with better bonding Thus, the printing material needs to have interlayer support and shape retention. One of  ability with the subsequent printing layer, and the components do not collapse or deform. the basic requirements of 3D printed concrete is good buildability. The buildability is an  Thus, the printing material needs to have interlayer support and shape retention. One of index to measure the early stiffness of the structure, the foundation of the integrity of the  the basic requirements of 3D printed concrete is good buildability. The buildability is an printed components, and the key to the stability of the layers. Research on the buildability  index to measure the early stiffness of the structure, the foundation of the integrity of the can ensure the printed building stability.  printed components, and the key to the stability of the layers. Research on the buildability Buildability can be improved by increasing the amount of aggregate and adding min‐ can ensure the printed building stability. eral admixtures and additives. Zhang et al. [20] prepared the slurry by mixing 2% cement  Buildability can be improved by increasing the amount of aggregate and adding to  replace  silica  fume  and  nanoclay,  which  greatly  improved  the  construction  perfor‐ mineral admixtures and additives. Zhang et al. [20] prepared the slurry by mixing 2% mance of the slurry, and the construction height was increased from the original 72 mm  cement to replace silica fume and nanoclay, which greatly improved the construction Appl. Sci. 2021, 11, 9822 6 of 25 Appl. Sci. 2021, 11, x FOR PEER REVIEW  6  of  24  performance of the slurry, and the construction height was increased from the original to 260 mm. Shakor et al. [29] added 4.5% lithium carbonate to the mixture to promote rapid  72 mm to 260 mm. Shakor et al. [29] added 4.5% lithium carbonate to the mixture to promote rapid coagulation of the printing mixture, thereby achieving the purpose of coagulation of the printing mixture, thereby achieving the purpose of improving buildabil‐ improving buildability. Zhang et al. [18] found that the buildability of 3D printed concrete ity. Zhang et al. [18] found that the buildability of 3D printed concrete is determined by its  is determined by its yield stress, while the yield stress of concrete depends on the ratio of yield stress, while the yield stress of concrete depends on the ratio of cement paste and ag‐ cement paste and aggregate in the mixture. With the same aggregate content, the finer the gregate in the mixture. With the same aggregate content, the finer the concrete, the higher  concrete, the higher the concrete yield stress, which is easier to buildability. Long et al. [30] the concrete yield stress, which is easier to buildability. Long et al. [30] controlled the pro‐ controlled the proportion of cement-based materials by controlling a single variable of microcrystalline cellulose (MCC). The detailed parameters are shown in Table 1. The study portion of cement‐based materials by controlling a single variable of microcrystalline cellu‐ found that, as the addition of microcrystalline cellulose increases, the slump rate decreases. lose (MCC). The detailed parameters are shown in Table 1. The study found that, as the  The buildable height is increased, and the buildability is significantly improved, as shown addition  of  microcrystalline  cellulose  increases,  the  slump  rate  decreases.  The  buildable  in Figure 5. height is increased, and the buildability is significantly improved, as shown in Figure 5.  Table 1. Mix proportions of the cement mortars used in this study. Adapted with permission from ref. [30]. Copyright 2019 Elsevier Ltd. Table 1. Mix proportions of the cement mortars used in this study, [30].  Mix Cement Sand MCC HRWRA Li CO 2 3 Mix No.  Cement (g)  SF (g)  FA (g)  SFSand (g)  (g FA) (g) w/b  MCC w/b  (g)  HRWRA (g)  Li2CO3 (g)  No. (g) (g) (g) (g) (g) M‐1  780  130  390  1300  0.35  0  4.55  13  M-1 780 130 390 1300 0.35 0 4.55 13 M‐2  780  130  390  1300  0.35  6.5  4.55  13  M-2 780 130 390 1300 0.35 6.5 4.55 13 M‐3  780  130  390  1300  0.35  13  4.55  13  M-3 780 130 390 1300 0.35 13 4.55 13 M‐4  780  130  390  1300  0.35  19.5  4.55  13  M-4 780 130 390 1300 0.35 19.5 4.55 13 R‐2  780  130  390  1300  0.3  0  4.55  13  R-2 780 130 390 1300 0.3 0 4.55 13 Figure 5. Buildability of different mix proportions: heights of the built layers. Reprinted with Figure 5. Buildability of different mix proportions: heights of the built layers, [30].  permission from ref. [30]. Copyright 2019 Elsevier Ltd. The buildability test can directly explore the non-deformation of the stacking and The buildability test can directly explore the non‐deformation of the stacking and  accumulation of concrete through traditional construction test methods. Long et al. [30] accumulation of concrete through traditional construction test methods. Long et al. [30]  used a print nozzle with a diameter of 20 mm and a printing speed of 80 mm/s to prepare used a print nozzle with a diameter of 20 mm and a printing speed of 80 mm/s to prepare  a 5-layer hollow structure with a size of 400 mm  150 mm  100 mm, as shown in a 5‐layer hollow structure with a size of 400 mm × 150 mm × 100 mm, as shown in Figure 6.  Figure 6. The structure slump was used to characterize the buildability of the mixture. Yuan et al. [31] used a loading device to detect the deformation of the 20-layer printing The structure slump was used to characterize the buildability of the mixture. Yuan et al.  structure. The loading rate was set according to the printing construction plan. When the [31] used a loading device to detect the deformation of the 20‐layer printing structure. The  deformation was less than 0.2%, it was deemed to meet the construction requirements. loading rate was set according to the printing construction plan. When the deformation  Bhattacherjee et al. [32] printed a 300 mm  300 mm rectangular hollow thin-walled was less than 0.2%, it was deemed to meet the construction requirements. Bhattacherjee  et al. [32] printed a 300 mm × 300 mm rectangular hollow thin‐walled structure, stacked  20 layers vertically. Then the difference between the actual build height and the design  height was calculated. The above difference value and the thickness of first printed layer  were used to evaluate the buildability. Le et al. [33] proposed quantifying the buildability  as the number of filament layers that can be established without significant deformation  of the lower layer. The shear strength is measured by the shear blade strain gauge, and  the shear strength is in the range of 0.3 ~ 0.9 kPa, which can meet the construction require‐ ments, and the best shear strength is 0.55 kPa. Ma et al. [21] printed 20 layers of extruded  filaments with a vertical stacking length of 250 mm and a width of 30 mm, with a layer  Appl. Sci. 2021, 11, 9822 7 of 25 structure, stacked 20 layers vertically. Then the difference between the actual build height and the design height was calculated. The above difference value and the thickness of first printed layer were used to evaluate the buildability. Le et al. [33] proposed quantifying the buildability as the number of filament layers that can be established without significant Appl. Sci. 2021, 11, x FOR PEER REVIEW  7  of  24  deformation of the lower layer. The shear strength is measured by the shear blade strain gauge, and the shear strength is in the range of 0.3~0.9 kPa, which can meet the construction requirements, and the best shear strength is 0.55 kPa. Ma et al. [21] printed 20 layers of extruded filaments with a vertical stacking length of 250 mm and a width of 30 mm, with height of 8 mm, and meeting the construction requirements without collapsing within 10  a layer height of 8 mm, and meeting the construction requirements without collapsing min of standing time. Ogura et al. [34] proposed to build a wall of 1000 mm long, 30 mm  within 10 min of standing time. Ogura et al. [34] proposed to build a wall of 1000 mm wide, and 120 mm high long, 30 comm mpo wide, sed of and 7  120 layers mm high of fila composed ments.  of If 7the layers  wall of ca filaments. n maint Ifathe in it wall s  can maintain its geometric shape and no obvious deformation is observed, it is considered to geometric shape and no obvious deformation is observed, it is considered to be buildable.  be buildable. (a)  (b)  Figure 6. Print shape adopted for evaluating the buildability. (a) Shape map. (b) Evaluation Chart. Reprinted with Figure 6. Print shape adopted for evaluating the buildability. (a) Shape map. (b) Evaluation Chart, [30].  permission from ref. [30]. Copyright 2019 Elsevier Ltd. 3.1.4. Setting Time  3.1.4. Setting Time Setting time refers to the time that the 3D printed concrete mixture is mixed with Setting time refers to the time that the 3D printed concrete mixture is mixed with  water to maintain the printing performance. Setting time is an important parameter of the water to maintain the printing performance. Setting time is an important parameter of the  performance indicators of 3D-printing materials. A longer setting time can obtain good performance indicators of 3D‐printing materials. A longer setting time can obtain good  fluidity and extrudability, and a shorter setting time can obtain sufficient early strength. fluidity and extrudability, and a shorter setting time can obtain sufficient early strength.  Due to the different scales of structures, the setting time of materials should be adjusted Due to the different accor scales ding of to structur the scalees, of the the printed  setting str  uctur time e to ofmeet  materia differlent s shou needs. ld  Ther be adju efore,st the edsetting   time of 3D printed concrete should be adjustable within a certain range. according to the scale of the printed structure to meet different needs. Therefore, the set‐ Setting time can be adjusted by coagulant, retarder, or by changing the gelling material. ting time of 3D printed concrete should be adjustable within a certain range.  Le et al. [16] found that stirring the cementitious material can increase the setting time; by Setting time can be adjusted by coagulant, retarder, or by changing the gelling material.  adding a retarder and a superplasticizer, respectively, the influence of the two on the setting Le et al. [16] found that stirring the cementitious material can increase the setting time; by  time was obtained, as shown in Figure 7. The setting time of the superplasticizer and retarder with the dosage of 1% and 0.5% is 68 min and 100 min, respectively. Zareiyan et al. [35] mixed adding a retarder and a superplasticizer, respectively, the influence of the two on the setting  60% calcium sulfoaluminate and 40% type I ordinary Portland cement to shorten the setting time was obtained, as shown in Figure 7. The setting time of the superplasticizer and re‐ time of the cementitious material to 45 min. Khalil et al. [24] mixed 93% ordinary Portland tarder with the dosage of 1% and 0.5% is 68 min and 100 min, respectively. Zareiyan et al.  cement and 7% sulphoaluminate cement, and the obtained mixtures had initial setting and [35] mixed 60% calcium sulfoaluminate and 40% type I ordinary Portland cement to shorten  final setting times of 110 min and 150 min, respectively. Kazemian et al. [36] obtained an the setting time of the in ce itiamenti l settint gious time ma of 1t6e 3ria miln to by 45 ad min. ding 3 Khalil % calciu et m al. chl o[24] ride .mixed 93% ordinary  Portland cement and 7% sulphoaluminate cement, and the obtained mixtures had initial  setting and final setting times of 110 min and 150 min, respectively. Kazemian et al. [36]  obtained an initial setting time of 163 min by adding 3% calcium chloride.      (a)  (b)  Figure 7. (a) Retarder dosage versus open time. (b) Superplasticiser dosage versus open time, [16].  The setting time test is based on different standards, and the instruments and meth‐ ods used by the researchers are not the same. Chen et al. [37] started printing a strip of  Appl. Sci. 2021, 11, x FOR PEER REVIEW  7  of  24  height of 8 mm, and meeting the construction requirements without collapsing within 10  min of standing time. Ogura et al. [34] proposed to build a wall of 1000 mm long, 30 mm  wide, and 120 mm high composed of 7 layers of filaments. If the wall can maintain its  geometric shape and no obvious deformation is observed, it is considered to be buildable.  (a)  (b)  Figure 6. Print shape adopted for evaluating the buildability. (a) Shape map. (b) Evaluation Chart, [30].  3.1.4. Setting Time  Setting time refers to the time that the 3D printed concrete mixture is mixed with  water to maintain the printing performance. Setting time is an important parameter of the  performance indicators of 3D‐printing materials. A longer setting time can obtain good  fluidity and extrudability, and a shorter setting time can obtain sufficient early strength.  Due to the different scales of structures, the setting time of materials should be adjusted  according to the scale of the printed structure to meet different needs. Therefore, the set‐ ting time of 3D printed concrete should be adjustable within a certain range.  Setting time can be adjusted by coagulant, retarder, or by changing the gelling material.  Le et al. [16] found that stirring the cementitious material can increase the setting time; by  adding a retarder and a superplasticizer, respectively, the influence of the two on the setting  time was obtained, as shown in Figure 7. The setting time of the superplasticizer and re‐ tarder with the dosage of 1% and 0.5% is 68 min and 100 min, respectively. Zareiyan et al.  [35] mixed 60% calcium sulfoaluminate and 40% type I ordinary Portland cement to shorten  the setting time of the cementitious material to 45 min. Khalil et al. [24] mixed 93% ordinary  Portland cement and 7% sulphoaluminate cement, and the obtained mixtures had initial  Appl. Sci. 2021, 11, 9822 8 of 25 setting and final setting times of 110 min and 150 min, respectively. Kazemian et al. [36]  obtained an initial setting time of 163 min by adding 3% calcium chloride.  (a)  (b)  Figure 7. (a) Retarder dosage versus open time. (b) Superplasticiser dosage versus open time. Reprinted with permission Figure 7. (a) Retarder dosage versus open time. (b) Superplasticiser dosage versus open time, [16].  from ref. [16]. Copyright 2012, RILEM. The setting time test is based on different standards, and the instruments and meth‐ The setting time test is based on different standards, and the instruments and methods ods used by the researchers are not the same. Chen et al. [37] started printing a strip of  used by the researchers are not the same. Chen et al. [37] started printing a strip of 800 mm 40 mm every 10 min after a static time of 30 min and recorded the time of rupture as the setting time. Kazemian et al. [36] used a penetration resistance meter to test the initial setting time of the mixture based on the ASTM C403 standard, and found that the initial setting time of the mixture with 3% calcium chloride was 163 min. Aqel et al. [38] used a Vicat tester to measure the initial setting time of cement slurry containing limestone fillers according to ASTMC191-08, and found that with the increase of limestone content and its fineness, the initial setting time of the mixture was shortened. Khalil et al. [35] obtained the initial setting and final setting time of 99 min for the mixture to meet the requirements of 3D-printing through the Vicat tester. Ma et al. [39] used the penetration resistance method to determine the stiffness development of the cementitious material during the coagulation process in accordance with the Chinese national testing standard GB/T50080-2002 and determined the setting time of the mixture based on the penetration resistance measured at different times. However, now the setting time of printing materials is still relatively long, which makes the advantages of 3D-printing rapid manufacturing unable to be fully utilized. 3.2. Mechanical Properties of 3D-Printing Concrete During the construction of 3D-printing concrete, slurry is piled up layer by layer. Slurry should not only support its own weight, but also be able to withstand the weight of the slurry accumulated on the upper layer. Thus, the slurry needs certain mechanical properties, strong early strength, and higher late strength, otherwise it will collapse and deform due to insufficient strength. Compressive strength and flexural strength are used to characterize the mechanical properties of 3D printed concrete, which can be improved by adding mineral admixtures and optimizing the gradation. Paul et al. [40] used ordinary cement, fly ash, silica fume, fine sand, glass fiber and other materials to develop new 3D-printing materials. The 28 d compressive strength is in the range of 36 to 57 MPa, and the flexural strength is about 10 MPa. Le et al. [33] developed a high-performance fiber concrete suitable for 3D- printing by reducing the water-binder ratio and optimizing the material gradation method, which has high compressive strength (about 100 MPa) and flexural strength (11 MPa). Hambach et al. [26] found that glass, basalt, and carbon fiber can effectively improve the bending strength of concrete. Shakor et al. [29] studied the 3D-printing calcium aluminate cement water-based adhesive and found that it can effectively enhance the strength of the design structure, and studied the impact of saturation and other compressive strength, at the saturation level of 170% When, the maximum compressive strength of 3DP cementitious material is 8.26MPa, as shown in Figure 8. Appl. Sci. 2021, 11, x FOR PEER REVIEW  8  of  24  800 mm × 40 mm every 10 min after a static time of 30 min and recorded the time of rup‐ ture as the setting time. Kazemian et al. [36] used a penetration resistance meter to test the  initial setting time of the mixture based on the ASTM C403 standard, and found that the  initial setting time of the mixture with 3% calcium chloride was 163 min. Aqel et al. [38]  used a Vicat tester to measure the initial setting time of cement slurry containing limestone  fillers according to ASTMC191‐08, and found that with the increase of limestone content  and its fineness, the initial setting time of the mixture was shortened. Khalil et al. [35]  obtained the initial setting and final setting time of 99 min for the mixture to meet the  requirements of 3D‐printing through the Vicat tester. Ma et al. [39] used the penetration  resistance method to determine the stiffness development of the cementitious material  during the coagulation process in accordance with the Chinese national testing standard  GB/T50080‐2002 and determined the setting time of the mixture based on the penetration  resistance measured at different times. However, now the setting time of printing materi‐ als is still relatively long, which makes the advantages of 3D‐printing rapid manufactur‐ ing unable to be fully utilized.  3.2. Mechanical Properties of 3D‐Printing Concrete  During the construction of 3D‐printing concrete, slurry is piled up layer by layer.  Slurry should not only support its own weight, but also be able to withstand the weight  of the slurry accumulated on the upper layer. Thus, the slurry needs certain mechanical  properties, strong early strength, and higher late strength, otherwise it will collapse and  deform due to insufficient strength.  Compressive strength and flexural strength are used to characterize the mechanical  properties of 3D printed concrete, which can be improved by adding mineral admixtures  and optimizing the gradation. Paul et al. [40] used ordinary cement, fly ash, silica fume,  fine sand, glass fiber and other materials to develop new 3D‐printing materials. The 28 d  compressive strength is in the range of 36 to 57 MPa, and the flexural strength is about 10  MPa. Le et al. [33] developed a high‐performance fiber concrete suitable for 3D‐printing  by reducing the water‐binder ratio and optimizing the material gradation method, which  has high compressive strength (about 100 MPa) and flexural strength (11 MPa). Hambach  et al. [26] found that glass, basalt, and carbon fiber can effectively improve the bending  strength of concrete. Shakor et al. [29] studied the 3D‐printing calcium aluminate cement  water‐based adhesive and found that it can effectively enhance the strength of the design  structure, and studied the impact of saturation and other compressive strength, at the sat‐ uration level of 170% When, the maximum compressive strength of 3DP cementitious ma‐ terial is 8.26MPa, as shown in Figure 8.  Appl. Sci. 2021, 11, 9822 9 of 25 Appl. Sci. 2021, 11, x FOR PEER REVIEW  9  of  24  Figure 8. Compressive strength of 3DP cubic samples with lithium carbonate. Reprinted with Figure 8. Compressive strength of 3DP cubic samples with lithium carbonate, [29].  permission from ref. [29]. Copyright 2017 Elsevier Ltd. Fiber reinforcement is also an effective means to improve mechanical properties such  Fiber reinforcement is also an effective means to improve mechanical properties such as compressive strength and flexural strength, but too high fiber content will reduce flu‐ as compressive strength and flexural strength, but too high fiber content will reduce fluidity, idity, increase the risk of nozzle clogging, and reduce bonding strength. Christ et al. [41]  increase the risk of nozzle clogging, and reduce bonding strength. Christ et al. [41] studied studied the fiber reinforcement effect of different fibers, as shown in Figure 9. The me‐ the fiber reinforcement effect of different fibers, as shown in Figure 9. The mechanical test chanical test of the printed sample showed that the bending strength was increased by  of the printed sample showed that the bending strength was increased by 180%. Le [33] 180%. Le [33] and Paul [40] etc. respectively selected 12 mm/8 mm (length/diameter) pol‐ and Paul [40] etc. respectively selected 12 mm/8 mm (length/diameter) polypropylene ypropylene fiber and 0.5% alkali‐resistant glass fiber (6 mm) to improve the surface prop‐ fiber and 0.5% alkali-resistant glass fiber (6 mm) to improve the surface properties of the erties of the sample. Hambach et al. [26] found that the compressive strength of glass fiber  sample. Hambach et al. [26] found that the compressive strength of glass fiber and basalt and basalt fiber sample with 1% volume content is only 13 MPa, while the compressive  fiber sample with 1% volume content is only 13 MPa, while the compressive strength of strength of carbon fiber sample with the same content can reach 30 MPa.  carbon fiber sample with the same content can reach 30 MPa. Figure 9. Flexural strength of fiber-reinforced samples printed in x and y direction. Reprinted with Figure 9. Flexural strength of fiber‐reinforced samples printed in x and y direction, [41].  permission from ref. [41]. Copyright 2014 Elsevier B.V. 3D‐printing concrete has high requirements for compressive strength as conventional  3D-printing concrete has high requirements for compressive strength as conventional concrete, the interlayer bond strength is also a necessary condition for maintaining struc‐ concrete, the interlayer bond strength is also a necessary condition for maintaining struc- tural stability. Studies showed the 3D print concrete component is easily destroyed at the  tural stability. Studies showed the 3D print concrete component is easily destroyed at interlayer bonding when the load is received [42]. The weakness of the 3D‐printing con‐ the interlayer bonding when the load is received [42]. The weakness of the 3D-printing crete structure is an inter‐interlayer adhesion, and the interlayer bonding performance  significantly affects structural mechanical properties.  Interlayer print interval is a key factor affecting bond strength [33]. Panda et al. [43]  found that the methods that increasing the interval, print speed and printhead height can  reduce the interlayer bond strength of 3D‐printing concrete structure. Nerella et al. [44]  observed that the interlayer bond strength decreases over time intervals. When the inter‐ val time interval is 1min, the bond strength is reduced by 50%, and when the interval time  interval is 1d, the bond strength is reduced by 90%.  Adding a mortar between the interlayer can effectively improve the interlayer bond  strength. Hosseini et al. [45] added resin mortar to the interlayer, which was composed of  black charcoal particles, sulfur, and sand. Results showed that the use of epoxy resins and  Kefla fibers can increase the interlayer bond strength 20%. Ma et al. [46] proposed to add  the cellulose fiber mortar between the print layers, which can effectively improve the in‐ terlayer bonding performance. When the interval is 60 min, the interlayer bond strength  is still higher than 1.91 MPa.  Later maintenance can also improve the mechanical properties of the components.  Because there is no template that constraints the components, a long‐range evaporation of  the surface of the member generate surface cracking. The cracking not only reduces the  Appl. Sci. 2021, 11, 9822 10 of 25 concrete structure is an inter-interlayer adhesion, and the interlayer bonding performance significantly affects structural mechanical properties. Interlayer print interval is a key factor affecting bond strength [33]. Panda et al. [43] found that the methods that increasing the interval, print speed and printhead height can reduce the interlayer bond strength of 3D-printing concrete structure. Nerella et al. [44] observed that the interlayer bond strength decreases over time intervals. When the interval time interval is 1min, the bond strength is reduced by 50%, and when the interval time interval is 1d, the bond strength is reduced by 90%. Adding a mortar between the interlayer can effectively improve the interlayer bond strength. Hosseini et al. [45] added resin mortar to the interlayer, which was composed of black charcoal particles, sulfur, and sand. Results showed that the use of epoxy resins and Kefla fibers can increase the interlayer bond strength 20%. Ma et al. [46] proposed to add the cellulose fiber mortar between the print layers, which can effectively improve the interlayer bonding performance. When the interval is 60 min, the interlayer bond strength is still higher than 1.91 MPa. Later maintenance can also improve the mechanical properties of the components. Because there is no template that constraints the components, a long-range evaporation of the surface of the member generate surface cracking. The cracking not only reduces the strength of the member, but also affects durability. Li et al. [47] obtained that under standard curing and water care conditions, the anti-bending strength of the sample is substantially equivalent (3.5 MPa), and under steam conservation conditions it is 12.93 Mpa. Xia et al. [48] studied the effect of maintenance temperature (25 C, 40 C, 60 C and 80 C) on the strength of the fly ash geological polymer, and found that temperature can increase the strength of 3D-printing geological polymer samples. However, due to large structural dimensions, in a full-size 3D-printing structure, it is more difficult to improve their intensity by high temperature conservation. 3.3. The Durable Performance of 3D-Printing Concrete There are fewer reports on the durability of 3D-printing concrete materials, but this is a very important performance indicator, which directly affects the safety of 3D-printing concrete buildings and building structures. Once a problem occurs, it will not only waste a lot of manpower and material resources, but also threaten life safety. Thus, it is necessary to study the durability of 3D-printing concrete materials. In 3D-printing buildings, there are many factors that affect the durability of materials, such as temperature, external forces, and chemical effects. Developing materials with good anti-seepage and anti-freezing properties can ensure the durability of the structure. Weng et al. [49] used peeling test, four-point bending test, and compressive strength test to study the fire resistance and high temperature resistance of 3D-printing fiber-reinforced cement-based composites, and found that PVA fiber can effectively prevent cement-based materials from flaking and bursting when exposed to high temperatures. The bending and compressive strength of 3DPFRCC at different temperatures are higher than those of 3D printed plain concrete, as shown in Figure 10. Weger et al. [50] tested the freeze-thaw resistance and carbonization resistance of 3D printed cement-based materials formed by the powder bonding process. Results showed that the freeze-thaw resistance properties of 3D printed cement-based materials meet the standard requirements. With normal or increasing CO concentration, the specimen did not undergo carbonization. 2 Appl. Sci. 2021, 11, x FOR PEER REVIEW  10  of  24  strength of the member, but also affects durability. Li et al. [47] obtained that under stand‐ ard curing and water care conditions, the anti‐bending strength of the sample is substan‐ tially equivalent (3.5 MPa), and under steam conservation conditions it is 12.93 Mpa. Xia  et al. [48] studied the effect of maintenance temperature (25 °C, 40 °C, 60 °C and 80 °C) on  the strength of the fly ash geological polymer, and found that temperature can increase  the strength of 3D‐printing geological polymer samples. However, due to large structural  dimensions, in a full‐size 3D‐printing structure, it is more difficult to improve their inten‐ sity by high temperature conservation.  3.3. The Durable Performance of 3D‐Printing Concrete  There are fewer reports on the durability of 3D‐printing concrete materials, but this  is a very important performance indicator, which directly affects the safety of 3D‐printing  concrete buildings and building structures. Once a problem occurs, it will not only waste  a lot of manpower and material resources, but also threaten life safety. Thus, it is necessary  to study the durability of 3D‐printing concrete materials.  In 3D‐printing buildings, there are many factors that affect the durability of materi‐ als, such as temperature, external forces, and chemical effects. Developing materials with  good anti‐seepage and anti‐freezing properties can ensure the durability of the structure.  Weng et al. [49] used peeling test, four‐point bending test, and compressive strength test  to study the fire resistance and high temperature resistance of 3D‐printing fiber‐reinforced  cement‐based composites, and found that PVA fiber can effectively prevent cement‐based  materials from flaking and bursting when exposed to high temperatures. The bending and  compressive strength of 3DPFRCC at different temperatures are higher than those of 3D  printed plain concrete, as shown in Figure 10. Weger et al. [50] tested the freeze‐thaw re‐ sistance and carbonization resistance of 3D printed cement‐based materials formed by the  powder bonding process. Results showed that the freeze‐thaw resistance properties of 3D  printed cement‐based materials meet the standard requirements. With normal or increas‐ Appl. Sci. 2021, 11, 9822 11 of 25 ing CO2 concentration, the specimen did not undergo carbonization.  Figure 10. Mechanical properties at elevated temperature. Reprinted with permission from ref. [49]. Copyright 2018 Elsevier. Figure 10. Mechanical properties at elevated temperature, [49].  4. Key Preparation Technology The preparation of printing materials is the key of 3D-printing concrete technology. 4. Key Preparation Technology  Now concrete is used as the base material of the printing materials, and cementing materials, The preparation of printing materials is the key of 3D‐printing concrete technology.  aggregates, admixtures, admixtures, special fibers, etc., are added to it to meet the required of actual project 3D printed concrete. Now concrete is used as the base material of the printing materials, and cementing mate‐ rials, aggregates, admixtures, 4.1. Cementitious  admiMaterials xtures, special fibers, etc., are added to it to meet the  The types of 3D-printing concrete cementing materials are wide, mainly including required of actual project 3D printed concrete.  Portland cement, sulphoaluminate cement, resin, geopolymer, and so on. Cementitious ma- terials have a certain adjustment effect on the setting time, strength, bonding performance, 4.1. Cementitious Materials and stability of 3D printed concrete structures. Portland cement is the most common coagulation material in the construction industry, The types of 3D‐printing concrete cementing materials are wide, mainly including  but studies have found that ordinary Portland cement cannot bend moments, and it has disadvantageous shear forces, bonding ability, and setting time between layers, which Portland cement, sulphoaluminate cement, resin, geopolymer, and so on. Cementitious  makes it difficult to meet the needs of 3D-printing [33,51]. It is necessary to improve the materials have a certain adjustment effect on the setting time, strength, bonding perfor‐ fineness and mineral composition of cement. Sun Jianzhi et al. [52] modified the properties of Portland cement by adding polymer and other emulsions to reduce shrinkage and mance, and stability of 3D printed concrete structures.  improve adhesion. It is also possible to consider mixing ordinary Portland cement and sulfoaluminate cement into 3D printed concrete to adjust the early hydration speed and early strength of the concrete. Sulphoaluminate cement has the characteristics of early strength, and the initial setting time is about 6 min. Its early strength is relatively high, applying it to the proportion of printed concrete can get a significant quick setting effect [53]. Li et al. [54] found that magnesium oxysulfide cement does not harden in water, and has the advantages of light weight, fast setting speed, early strength, and strong adhesion. The compressive strength and flexural strength of magnesium phosphate cement are higher than other types of cement. The setting and hardening time is 1~10 min, which can meet the requirements of the bearing capacity of 3D printed concrete structures. Resin-based cementitious materials can greatly improve the bearing capacity of com- ponents. Issa et al. [55] used epoxy resin to repair concrete cracks. Due to the existence of cracks, the compressive strength of the cube decreased by 40.9%, and only 8% after repair. Ahmad et al. [56] injected epoxy resin into the flexural-shear cracks to strengthen the cracked reinforced concrete members, which increased the bearing capacity by 49%. Geopolymer cementitious materials can repair cracks, improving the mechanical properties of components. The strength of components can also be improved by increasing the ratio of geopolymers. Porto et al. [57] used polymer-modified cement mortar to repair the reinforced concrete column surface layer, and the axial stiffness of the repaired column Appl. Sci. 2021, 11, 9822 12 of 25 was only reduced by 13%. Pellegrino et al. [58] used geopolymer modified cement mortar to repair the tension zone of the beam, and the load value of the first crack increased by 30%. Sakka et al. [59] added 7.5% styrene-butadiene rubber polymer to cement-based materials, forming polymer membranes to better combine the cement hydration products which effectively reduced the adverse effects caused by the interlayer tensile strength of the printed specimens. Hosseini et al. [45] mixed sand aggregate and carbon-sulfur polymer in a ratio of 3:2 to prepare carbon-sulfur polymer modified cement mortar which can enhance the inter-layer tensile properties of the cement mortar. The tensile strength is 100% higher than that of non-adhesive bonded specimens. Yang et al. [60] mixed cement and styrene-butadiene emulsion in a ratio of 3:2 to prepare a styrene-butadiene emulsion modified cement paste to enhance the interlayer tensile properties of the concrete. The stretched strength of styrene-butadiene emulsion bonded specimens 7 d and 28 d interlayer increased by 144% and 96% respectively. Bong et al. [61] optimized the ratio of printable geopolymer materials, and the direct tensile strength of the 28d layers of the printed specimen was 2.7 MPa, which was 3 times the direct tensile strength of the 7d layers, as Appl. Sci. 2021, 11, x FOR PEER REVIEW  12  of  24  shown in Figure 11. Komljenovic ´ et al. [51] found that the types of catalysts HS and SS are the factors that determine the rate of increase in compressive strength, because the increase in the mass ratio of SS/HS increases the soluble silicate content in the geopolymerization system, which increases the compressive strength. Figure 11. Interlayer bond strength of Na-N-2.5 (the optimum 3D printable mixture). Reprinted with Figure 11. Interlayer bond strength of Na‐N‐2.5 (the optimum 3D printable mixture), [61].  permission from ref. [61]. Copyright 2019 MDPI. In order to facilitate the comparison of the effects of different cementing materials on Table 2. The effect of different cementitious materials on components.  the components, they are summarized, as shown in Table 2. Cementitious Material Name  Effect  Reference  Table 2. The effect of different cementitious materials on components. Portland cement  The most commonly used condensing material, with high early strength  [33,51]  Cementitious Material Name Effect Reference High early strength, good quick‐setting effect, high compressive and  Sulphoaluminate cement  [53,54]  The most commonly used condensing flexural strength  Portland cement [33,51] material, with high early strength Resin  Significantly improve component bearing capacity  [55,56]  High early strength, good quick-setting Sulphoaluminate cement [53,54] It can repair cracks with good repair effect and improve the strength of  effect, high compressive and flexural strength Geopolymer  [45,53,57–61]  components  Significantly improve component bearing Resin [55,56] capacity It can repair cracks with good repair effect 4.2. Aggregate  Geopolymer [45,53,57–61] and improve the strength of components Aggregate affects the rheology, viscosity, stress characteristics before and after hard‐ ening and durability of concrete directly or indirectly from the internal structure [62]. The  aggregate particle size has a great influence on the 3D printed concrete. When the aggre‐ gate particle size is too large, it will block the extrusion nozzle. On the contrary, if the  aggregate particle size is too small, its specific surface area will increase, and the amount  of paste increases that used to wrap the surface of the aggregate which will make the con‐ crete easy to crack. When the ratio of the nozzle diameter to the maximum aggregate par‐ ticle size is greater than 5, the printing material will not be blocked during the extrusion  process. When the maximum particle size of the aggregate is less than 1/10 of the nozzle  diameter, the cementitious material can be smoothly extruded through the nozzle [24,25].  Lin et al. [63] measured the fluidity, compressive strength, and flexural strength of mortar  influenced by sand of different fineness. Within a certain range, as the fineness modulus  of sand increases, the fluidity of mortar, 7 d compressive strength and flexural strength  will be improved to varying degrees, as shown in Figure 12.  Appl. Sci. 2021, 11, 9822 13 of 25 4.2. Aggregate Aggregate affects the rheology, viscosity, stress characteristics before and after harden- ing and durability of concrete directly or indirectly from the internal structure [62]. The aggregate particle size has a great influence on the 3D printed concrete. When the aggregate particle size is too large, it will block the extrusion nozzle. On the contrary, if the aggregate particle size is too small, its specific surface area will increase, and the amount of paste increases that used to wrap the surface of the aggregate which will make the concrete easy to crack. When the ratio of the nozzle diameter to the maximum aggregate particle size is greater than 5, the printing material will not be blocked during the extrusion process. When the maximum particle size of the aggregate is less than 1/10 of the nozzle diameter, the cementitious material can be smoothly extruded through the nozzle [24,25]. Lin et al. [63] measured the fluidity, compressive strength, and flexural strength of mortar influenced by Appl. Sci. 2021, 11, x FOR PEER REVIEW  13  of  24  sand of different fineness. Within a certain range, as the fineness modulus of sand increases, the fluidity of mortar, 7 d compressive strength and flexural strength will be improved to varying degrees, as shown in Figure 12. (a)  (b)  Figure 12. (a) Effect of fineness modulus on Fluidity. (b) The Mechanical Properties of Mortar. Reprinted with permission Figure 12. (a) Effect of fineness modulus on Fluidity. (b) The Mechanical Properties of Mortar, [63].  from ref. [63]. Copyright 2017 IPO. 4.3. Mineral Admixture 4.3. Mineral Admixture  The active ingredients of mineral admixtures can greatly increase the strength of 3D The active ingredients of mineral admixtures can greatly increase the strength of 3D  printed concrete components, the density of the structure, and improve the durability of the material and the service life of the structure. printed concrete components, the density of the structure, and improve the durability of  The incorporation of fly ash can effectively improve the working performance, me- the material and the service life of the structure.  chanical properties, and durability of printed concrete. Fly ash is the main admixture for The incorporation of fly ash can effectively improve the working performance, me‐ preparing high-performance 3D printed concrete. However, the incorporation of a large amount of fly ash causes the slow development of the early strength of the printed concrete, chanical properties, and durability of printed concrete. Fly ash is the main admixture for  and the obvious reduction of carbonization resistance and frost resistance. Therefore, fly preparing high‐performance 3D printed concrete. However, the incorporation of a large  ash is generally mixed with other admixtures. Kang Jingfu et al. [64] mixed fly ash and GH ore powder to prepare high-strength and high-performance concrete. Voigt et al. [65] amount of fly ash causes the slow development of the early strength of the printed con‐ observed that the addition of fly ash improved the fluidity of cement-based materials, but crete, and the obvious reduction of carbonization resistance and frost resistance. There‐ reduced the shape stability of the material. The cement-based material obtained by mixing fore, fly ash is generally mixed with other admixtures. Kang Jingfu et al. [64] mixed fly  sintered clay and fly ash has good fluidity and shape stability. Silica fume has high activity. As an admixture for concrete, it not only increases the ash and GH ore powder to prepare high‐strength and high‐performance concrete. Voigt  initial strength of the printed concrete, but also improves the later flexural strength of the et al. [65] observed that the addition of fly ash improved the fluidity of cement‐based ma‐ printed concrete. The silica fume particles are very small, which can be used to fill the pores terials,  but  reduced  the ar  ound shape the st aggr abi egate, lity  impr of  othe ving  ma the t bonding erial.  perf The ormance   cemeof ntthe‐ba concr sedete,   ma and teria enhancing l  ob‐ the transition zone between the cement slurry and the aggregate interface [66]. March- tained by mixing sintered clay and fly ash has good fluidity and shape stability.  ment et al. [67] added silica fume, which accounts for 10% of the cementitious material, to Silica fume has high activity. As an admixture for concrete, it not only increases the  initial strength of the printed concrete, but also improves the later flexural strength of the  printed concrete. The silica fume particles are very small, which can be used to fill the  pores around the aggregate, improving the bonding performance of the concrete, and en‐ hancing the transition zone between the cement slurry and the aggregate interface [66].  Marchment et al. [67] added silica fume, which accounts for 10% of the cementitious ma‐ terial, to improve the adhesion and extrusion properties of the printing material. Rahul et  al. [68] found that the addition of silica fume can optimize the particle size distribution of  the printing material. After adding silicon powder, the actual and ideal particle size dis‐ tribution can be more closely matched, as shown in Figure 13, increasing its yield stress,  and improving the buildability of the printing material. However, it will reduce the fluid‐ ity of the material. Zhang et al. [69] added 2% of the cement mass of nano‐clay and silica  fume into concrete to prepare a concrete material with high thixotropy.  Appl. Sci. 2021, 11, 9822 14 of 25 improve the adhesion and extrusion properties of the printing material. Rahul et al. [68] found that the addition of silica fume can optimize the particle size distribution of the printing material. After adding silicon powder, the actual and ideal particle size distribu- tion can be more closely matched, as shown in Figure 13, increasing its yield stress, and Appl. Sci. 2021, 11, x FOR PEER REVIEW  14  of  24  improving the buildability of the printing material. However, it will reduce the fluidity of the material. Zhang et al. [69] added 2% of the cement mass of nano-clay and silica fume into concrete to prepare a concrete material with high thixotropy. Figure 13. Comparison of the combined particle size distribution of all dry ingredients after adding Figure 13. Comparison of the combined particle size distribution of all dry ingredients after add‐ silica fume and the optimum distribution suggested by the Andreassen model. Reprinted with ing silica fume and the optimum distribution suggested by the Andreassen model, [68].  permission from ref. [68]. Copyright 2018 Elsevier Ltd. 4.4. Fiber Material 4.4. Fiber Material  Fibrous materials can significantly improve the crack resistance of the printed con- Fibrous materials can significantly improve the crack resistance of the printed con‐ crete and the elastic modulus of the maximum load. It can also significantly improve crete and the elastic modulus of the maximum load. It can also significantly improve the  the toughness, ductility of the printed concrete, delay the surface deterioration of the toughness, ductility of the printed concrete, delay the surface deterioration of the concrete  concrete product, and improve its durability. Tohamy et al. [70] showed that the addi- tion of polypropylene fiber can prevent the printed concrete samples from peeling off, product, and improve its durability. Tohamy et al. [70] showed that the addition of poly‐ optimize the extrusion process of the concrete in the printer output port to a certain ex- propylene fiber can prevent the printed concrete samples from peeling off, optimize the  tent, and obtain a uniform and continuous printed sample structure. However, when the extrusion process of the concrete in the printer output port to a certain extent, and obtain  content of polypropylene fiber is too much, it will reduce the impermeability of concrete. a uniform and continuous printed sample structure. However, when the content of poly‐ Feng et al. [71] used gypsum as the main printing material and tested it with GFRP (Glass propylene fiber is too much, it will reduce the impermeability of concrete. Feng et al. [71]  Reinforced Plastics) materials, and found that the printing column has a significant in- crease in axial load-bearing capacity and ductility. Mechtcherine et al. [72] found that the used gypsum as the main printing material and tested it with GFRP (Glass Reinforced  continuous embedded carbon fiber during the printing process can effectively improve Plastics) materials, and found that the printing column has a significant increase in axial  the bending strength and deformation ability of the specimen. Ding et al. [73] used PE load‐bearing capacity and ductility. Mechtcherine et al. [72] found that the continuous  (polyethylene) fiber to reinforce the printing matrix material and found that the addition of embedded carbon fiber during the printing process can effectively improve the bending  PE fiber significantly improved the bending strength of the specimen in different directions, as shown in Figure 14. Arunothayan et al. [74] added steel fibers to concrete materials, strength and deformation ability of the specimen. Ding et al. [73] used PE (polyethylene)  which greatly improved the bending resistance of the printed specimens. Ma et al. [75] fiber to reinforce the printing matrix material and found that the addition of PE fiber sig‐ added 0.5% basalt fiber to concrete materials, and the basic mechanical properties and nificantly improved the bending strength of the specimen in different directions, as shown  anisotropy of printable cement-based materials were significantly improved. in Figure 14. Arunothayan et al. [74] added steel fibers to concrete materials, which greatly  improved the bending resistance of the printed specimens. Ma et al. [75] added 0.5% basalt  fiber to concrete materials, and the basic mechanical properties and anisotropy of printa‐ ble cement‐based materials were significantly improved.  In order to facilitate the comparison of the effects of different fiber materials on the  components, they are summarized, as shown in Table 3.  Figure 14. Tensile strength of specimens with various fiber ratios, [73].  Appl. Sci. 2021, 11, x FOR PEER REVIEW  14  of  24  Figure 13. Comparison of the combined particle size distribution of all dry ingredients after add‐ ing silica fume and the optimum distribution suggested by the Andreassen model, [68].  4.4. Fiber Material  Fibrous materials can significantly improve the crack resistance of the printed con‐ crete and the elastic modulus of the maximum load. It can also significantly improve the  toughness, ductility of the printed concrete, delay the surface deterioration of the concrete  product, and improve its durability. Tohamy et al. [70] showed that the addition of poly‐ propylene fiber can prevent the printed concrete samples from peeling off, optimize the  extrusion process of the concrete in the printer output port to a certain extent, and obtain  a uniform and continuous printed sample structure. However, when the content of poly‐ propylene fiber is too much, it will reduce the impermeability of concrete. Feng et al. [71]  used gypsum as the main printing material and tested it with GFRP (Glass Reinforced  Plastics) materials, and found that the printing column has a significant increase in axial  load‐bearing capacity and ductility. Mechtcherine et al. [72] found that the continuous  embedded carbon fiber during the printing process can effectively improve the bending  strength and deformation ability of the specimen. Ding et al. [73] used PE (polyethylene)  fiber to reinforce the printing matrix material and found that the addition of PE fiber sig‐ nificantly improved the bending strength of the specimen in different directions, as shown  in Figure 14. Arunothayan et al. [74] added steel fibers to concrete materials, which greatly  improved the bending resistance of the printed specimens. Ma et al. [75] added 0.5% basalt  fiber to concrete materials, and the basic mechanical properties and anisotropy of printa‐ ble cement‐based materials were significantly improved.  Appl. Sci. 2021, 11, 9822 15 of 25 In order to facilitate the comparison of the effects of different fiber materials on the  components, they are summarized, as shown in Table 3.  Figure 14. Tensile strength of specimens with various fiber ratios. Reprinted with permission from Figure 14. Tensile strength of specimens with various fiber ratios, [73].  ref. [73]. Copyright 2020 Elsevier Ltd. In order to facilitate the comparison of the effects of different fiber materials on the components, they are summarized, as shown in Table 3. Table 3. The effect of different fiber materials on components. Fiber Material Name Effect Reference Anti-flaking, make the extrusion more uniform Polypropylene fibers [70] and continuous GFRP (Glass fiber reinforced Improve axial load carrying capacity and [71] plastic) fiber ductility Improve bending strength and deformation Carbon fiber [72] ability PE (Polyethylene) fiber Significantly improve the bending strength in [73,74] Steel fiber different directions Improve anisotropy and basic mechanical Basalt fiber [75] properties 4.5. Admixture Admixtures are an important part of 3D printed concrete materials. They can signifi- cantly adjust and improve the fluidity, setting time, and mechanical properties of concrete. Adding admixtures to 3D printed concrete appropriately will get a small amount, but have a big effect. Slavcheva et al. [76] found that by increasing the amount of water re- ducing agent, the total pore volume of the printed sample was reduced, and the content of nanopores was increased. It also caused an increase in the compactness of the sample, and the drying and shrinkage reduced. Perrot et al. [15] found that xanthan gum and Brunei gum thickeners can significantly improve the moldability of the accumulated layer of printed concrete test strips. Le et al. [16] used 1% water-reducing agent gelling materials to improve fluidity and realized the construction of specimens with the highest 61 layers. 5. Printing Parameters In addition to the material composition, the final performance of 3D printed concrete is also affected by the printing parameters to a large extent. A reasonable choice of printing parameters will greatly improve the printing quality of the components. The printing parameters of 3D-printing include the nozzle size selected by the printer, the height of the nozzle from the printing platform, the movement speed of the print head, and the interval time between layers. Printing process control and printing parameter selection are the key links in the formation of concrete 3D-printing structures, and they are also important factors that affect the final printing quality of concrete. Appl. Sci. 2021, 11, 9822 16 of 25 5.1. Design of the Print Head The key technology of 3D-printing concrete lies in whether the prepared concrete can pass through the nozzle smoothly, and realize layer-by-layer bonding and solidification to form a whole, so the design of the print head is very important. Khoshnevis et al. [77,78] used a large three-dimensional extrusion device and a nozzle with a spatula to achieve layered printing of concrete, which can solve the problem of uneven 3D-printing surface and can be used for printing large buildings. The print head is shown in Figure 15. Buswell and Lim et al. [8,9,79] used the main components of concrete container, conveying pipe, printing pump, output pipe, nozzle to form the printing head. The printing of solid components is realized through the cross operation of spraying and extruding material layered printing and implanting transverse steel mesh. It has higher precision and freedom, Appl. Sci. 2021, 11, x FOR PEER REVIEW  16  of  24  and has great advantages in printing small and heterogeneous building components, but it has a slight disadvantage compared with printing in large buildings and whole houses.    (a)  (b)  Figure 15. Installation of the print head: (a) extruder sits on an x‐y‐zgantry robot. (b) details of nozzle assembly, [77].  Figure 15. Installation of the print head: (a) extruder sits on an x-y-zgantry robot. (b) details of nozzle assembly. Reprinted with permission from ref. [77]. Copyright 2003 Elsevier B.V. 5.2. Nozzle size.  5.2. Nozzle size The nozzle size and shape in the printing parameters will have different effects on  The nozzle size and shape in the printing parameters will have different effects on the printing effect, and different nozzle shapes and sizes should be adopted for different  the printing effect, and different nozzle shapes and sizes should be adopted for different printing conditions. Paul et al. [40] found that the strength of specimens printed with rec‐ printing conditions. Paul et al. [40] found that the strength of specimens printed with tangular nozzle holes is higher than that of specimens printed with circular nozzle holes,  rectangular nozzle holes is higher than that of specimens printed with circular nozzle and circular nozzles are more suitable for printing complex structures. Because the rec‐ holes, and circular nozzles are more suitable for printing complex structures. Because the tangular nozzle has fewer interlayer gaps than the circular nozzle,   the strength is higher.  rectangular nozzle has fewer interlayer gaps than the circular nozzle, the strength is higher. Sun Xiaoyan [80] and others found that under the same extrusion flow conditions, the  Sun Xiaoyan [80] and others found that under the same extrusion flow conditions, the printed specimens with a triangular extruded shape have the fewest number of interlayer  printed specimens with a triangular extruded shape have the fewest number of interlayer defects and the best mechanical properties. The defects are reduced, and the mechanical  defects and the best mechanical properties. The defects are reduced, and the mechanical properties are improved with the extrusion size larger for the printed specimens of the  properties are improved with the extrusion size larger for the printed specimens of the same shape. When the extrusion shape and size are same, the staggered arrangement of  same shape. When the extrusion shape and size are same, the staggered arrangement of the the printing ports can improve the mechanical properties of the printed specimen by 13%  printing ports can improve the mechanical properties of the printed specimen by 13% to to 47%. Tay et al. [81] found that when the print nozzle area is equal to the cross‐sectional  47%. Tay et al. [81] found that when the print nozzle area is equal to the cross-sectional area area of the actual mortar strip, the printed mortar strip is better. The setting of the print  of the actual mortar strip, the printed mortar strip is better. The setting of the print nozzle nozzle aperture must be consistent with the content and size of the aggregate to avoid  aperture must be consistent with the content and size of the aggregate to avoid clogging. clogging.  5.3. Print Height 5.3. Print Height  Printing height is the height of the nozzle from the printing platform. Printing height Printing height is the height of the nozzle from the printing platform. Printing height  is a parameter that affects the bonding strength and surface quality between layers. Printing is a parameter that affects the bonding strength and surface quality between layers. Print‐ height will also affect the settlement of the printed mortar strips. When the printing height ing height will also affect the settlement of the printed mortar strips. When the printing  is too high, the interlayer bonding strength of the printed mortar strip will be greatly height  is  too  high,  the  interlayer  bonding  strength  of  the  printed  mortar  strip  will  be  greatly reduced, and the shape of the printed mortar strip will be unstable. When the  printing height is too low, the printing nozzle will generate additional pressure on the  printed mortar strip, resulting in more Great settlement. Wolfs et al. [82] believe that the  print height should be equal to the width of the nozzle section, because it can smoothly  deposit the extruded mortar strip and avoid the interaction between the print head and  the mortar strip. Panda et al. [43] based their tests on the direct tensile test method and  found that when the height of the printing nozzle is smaller than the height of the printing  mortar strip, the bond strength between layers increases as the height decreases, as shown  in Figure 16. Panda adjusted the print height of the nozzle to increase the tensile strength  between layers. Results show that the direct interlayer tensile strength of the sample pre‐ pared with a nozzle height of 0 mm is 53.3% higher than that of a sample prepared with a  nozzle height of 4 mm. Zareiyan et al. [83] reduced the height of the print bar to increase  the interlayer cleavage strength, and found that the 28 d interlayer cleavage strength of  the 25.4 mm layer thickness sample was 11.4% higher than when the layer thickness was  Appl. Sci. 2021, 11, 9822 17 of 25 reduced, and the shape of the printed mortar strip will be unstable. When the printing height is too low, the printing nozzle will generate additional pressure on the printed mortar strip, resulting in more Great settlement. Wolfs et al. [82] believe that the print height should be equal to the width of the nozzle section, because it can smoothly deposit the extruded mortar strip and avoid the interaction between the print head and the mortar strip. Panda et al. [43] based their tests on the direct tensile test method and found that when the height of the printing nozzle is smaller than the height of the printing mortar strip, the bond strength between layers increases as the height decreases, as shown in Figure 16. Panda adjusted the print height of the nozzle to increase the tensile strength between layers. Results show that the direct interlayer tensile strength of the sample prepared with a nozzle Appl. Sci. 2021, 11, x FOR PEER REVIEW  17  of  24  height of 0 mm is 53.3% higher than that of a sample prepared with a nozzle height of 4 mm. Zareiyan et al. [83] reduced the height of the print bar to increase the interlayer cleavage strength, and found that the 28 d interlayer cleavage strength of the 25.4 mm layer thickness sample was 11.4% higher than when the layer thickness was 50.8 mm, and the 50.8 mm, and the 25.4 mm layer thickness sample shows a higher splitting crack growth  25.4 mm layer thickness sample shows a higher splitting crack growth rate. rate.  Figure 16. Effect of nozzle standoff distance on tensile bond strength, [43].  Figure 16. Effect of nozzle standoff distance on tensile bond strength. Reprinted with permission from ref. [43]. Copyright 2017 Elsevier Ltd. 5.4. Speed of Print Head Movement  5.4. Speed of Print Head Movement The moving speed of the print head is an important printing parameter of 3D‐print‐ The moving speed of the print head is an important printing parameter of 3D-printing ing technology, and the selection of the moving speed of the print head has a great influ‐ technology, and the selection of the moving speed of the print head has a great influence ence on the bonding strength between layers and the surface quality. Tay et al. [84] chose  on the bonding strength between layers and the surface quality. Tay et al. [84] chose a a 30 mm × 15 mm rectangular nozzle for printing, and found that the nozzle’s moving  30 mm  15 mm rectangular nozzle for printing, and found that the nozzle’s moving speed speed has a great influence on the cross‐sectional shape of the printed mortar strip. Rahul  has a great influence on the cross-sectional shape of the printed mortar strip. Rahul et al. [68] et al. [68] believe that the extrusion speed of the printer is the same as the nozzle move‐ believe that the extrusion speed of the printer is the same as the nozzle movement speed, ment speed, which can maintain the shape stability of the printed mortar. Kruger et al.  which can maintain the shape stability of the printed mortar. Kruger et al. [85] developed [85] developed a design model for 3D concrete printing and predicted the printing speed  a design model for 3D concrete printing and predicted the printing speed to prevent to prevent structural failure under given conditions. Panda et al. [43] studied the effect of  structural failure under given conditions. Panda et al. [43] studied the effect of different different nozzle speeds on the performance of fresh concrete for 3D‐printing, and found  nozzle speeds on the performance of fresh concrete for 3D-printing, and found that the that the adhesive strength of the sample decreased slightly as the nozzle speed increased,  adhesive strength of the sample decreased slightly as the nozzle speed increased, as shown as shown in Figure 17. Panda adjusted the running speed of the nozzle to increase the  in Figure 17. Panda adjusted the running speed of the nozzle to increase the tensile tensile strength between layers. The results show that the direct interlayer tensile strength  strength between layers. The results show that the direct interlayer tensile strength of of the samples made with the 70 mm/s nozzle operating speed is 10.5% higher than that  the samples made with the 70 mm/s nozzle operating speed is 10.5% higher than that of of the samples made with the 110 mm/s nozzle operating speed. The printing speed used  the samples made with the 110 mm/s nozzle operating speed. The printing speed used by Nerella et al. [86] is about 75 mm/s. The initial setting time of concrete materials is  by Nerella et al. [86] is about 75 mm/s. The initial setting time of concrete materials is controlled within 3 min through the accelerator, and the interval between layers is 30 s,  controlled within 3 min through the accelerator, and the interval between layers is 30 s, which greatly optimizes the rheological properties of concrete.  which greatly optimizes the rheological properties of concrete. Figure 17. Effect of nozzle speed on tensile bond strength, [43].  5.5. Interlayer Interval Time  Appl. Sci. 2021, 11, x FOR PEER REVIEW  17  of  24  50.8 mm, and the 25.4 mm layer thickness sample shows a higher splitting crack growth  rate.  Figure 16. Effect of nozzle standoff distance on tensile bond strength, [43].  5.4. Speed of Print Head Movement  The moving speed of the print head is an important printing parameter of 3D‐print‐ ing technology, and the selection of the moving speed of the print head has a great influ‐ ence on the bonding strength between layers and the surface quality. Tay et al. [84] chose  a 30 mm × 15 mm rectangular nozzle for printing, and found that the nozzle’s moving  speed has a great influence on the cross‐sectional shape of the printed mortar strip. Rahul  et al. [68] believe that the extrusion speed of the printer is the same as the nozzle move‐ ment speed, which can maintain the shape stability of the printed mortar. Kruger et al.  [85] developed a design model for 3D concrete printing and predicted the printing speed  to prevent structural failure under given conditions. Panda et al. [43] studied the effect of  different nozzle speeds on the performance of fresh concrete for 3D‐printing, and found  that the adhesive strength of the sample decreased slightly as the nozzle speed increased,  as shown in Figure 17. Panda adjusted the running speed of the nozzle to increase the  tensile strength between layers. The results show that the direct interlayer tensile strength  of the samples made with the 70 mm/s nozzle operating speed is 10.5% higher than that  of the samples made with the 110 mm/s nozzle operating speed. The printing speed used  by Nerella et al. [86] is about 75 mm/s. The initial setting time of concrete materials is  Appl. Sci. 2021, 11, 9822 18 of 25 controlled within 3 min through the accelerator, and the interval between layers is 30 s,  which greatly optimizes the rheological properties of concrete.  Figure 17. Effect of nozzle speed on tensile bond strength. Reprinted with permission from ref. [43]. Figure 17. Effect of nozzle speed on tensile bond strength, [43].  Appl. Sci. 2021, 11, x FOR PEER REVIEW  18  of  24  Copyright 2017 Elsevier Ltd. 5.5. Interlayer Interval Time  5.5. Interlayer Interval Time The interlayer interval time is a key factor affecting the bond strength. Optimize the  The interlayer interval time is a key factor affecting the bond strength. Optimize the int interlayer erlayer int interval erval time time to to pr prevent event significant  significantdeformation  deformation or or collapse  collapse and  and ensur  ensure e acceptable  accepta‐ ble bonding  bonding str ength strengbetween th between printed  printed layers.  layers. Panda  Panda et etal. al.[ 43 [43 ]]showed  showed that that incr increasing easing the the  time interval between layers, printing speed and print head height reduces the interlayer time interval between layers, printing speed and print head height reduces the interlayer  bonding bonding str strength ength of of3D  3Dprinted  printed concr  concret ete e str structur uctures,es, as as shown  shown in Figur in Fig eu 18 re. 18 Ner . Ne ellarell eta al. et [ 44 al].  showed that the bonding strength between layers decreases with the extension of the time [44] showed that the bonding strength between layers decreases with the extension of the  interval. When the time interval between layers is 1 min, the bonding strength decreases time interval. When the time interval between layers is 1 min, the bonding strength de‐ by 50%, and when the time interval between layers is 1 d, the bonding strength decreases creases by 50%, and when the time interval between layers is 1 d, the bonding strength  by 90%. Wolfs et al. [87] optimized the interlayer interval and surface dehydration to decreases by 90%. Wolfs et al. [87] optimized the interlayer interval and surface dehydra‐ increase the interlayer tensile strength. Results show that reducing the interlayer interval tion to increase the interlayer tensile strength. Results show that reducing the interlayer  can increase the interlayer tensile strength. The interlayer tensile strength and splitting of interval can increase the interlayer tensile strength. The interlayer tensile strength and  the specimens with an interval of 15 s is 19% and 27% higher than that of the 24 h specimen, splitting of the specimens with an interval of 15 s is 19% and 27% higher than that of the  respectively. 24 h specimen, respectively.  Figure Figure 18. 18. Effe . Effect ct ofof time time gay gay bebetween tween layers layers on on tensile tensile bond bond streng strength. th, [43] Reprinted .  with permission from ref. [43]. Copyright 2017 Elsevier Ltd. 6. Print Quality Evaluation Index  6. Print Quality Evaluation Index The forming quality of printed components is also a key issue of 3D‐printing concrete  The forming quality of printed components is also a key issue of 3D-printing concrete technology. It mainly includes geometric quality problems such as the shape and size of  technology. It mainly includes geometric quality problems such as the shape and size of the printed components and surface quality problems such as surface roughness. Basic  the printed components and surface quality problems such as surface roughness. Basic requirements of the construction process are to print components with good forming qual‐ requirements of the construction process are to print components with good forming ity. Therefore, it is very important to systematically evaluate the forming quality of 3D  quality. Therefore, it is very important to systematically evaluate the forming quality of 3D printed concrete components.  printed concrete components. The quality of concrete 3D‐printing is affected by many factors such as printing ma‐ The quality of concrete 3D-printing is affected by many factors such as printing terials, printer systems, pumps, control systems, print heads, and the shape of the dis‐ materials, printer systems, pumps, control systems, print heads, and the shape of the charge nozzle [88]. Lim et al. [9] proposed that the surface quality of 3D‐printing cement‐ based  materials  can  be  judged  by  the  three  criteria  of  no  defects  on  the  surface,  clear  square  boundaries  of  the  printed  layer,  and  meeting  the  requirements  of  surface  con‐ sistency and homogeneity.  Printing parameters are the most critical factor affecting the surface quality of the  parts; thus, scholars focus mainly on adaptive layering and optimization of various print‐ ing parameters. Nadiyapara et al. [89] believe that, compared with equal layer thickness  layering, adaptive layering can set a smaller layer thickness in the feature‐intensive area  of the model, so that the dense features are not easy to lose, thereby improving the surface  quality of the part. The algorithm is complex; the quality of the algorithm determines the  accuracy of the layering. If the algorithm is unreasonable, it will reduce the surface quality  of the model. Kim et al. [90] believe that the fused deposition printing parameters have  strong coupling and influence each other during the printing process, which ultimately  determines the surface quality of the parts. Optimizing the molding parameters can sig‐ nificantly improve the poor surface finish of the molded parts. Li Yanru et al. [91] effec‐ tively optimized the surface quality of the part by selecting the optimal forming angle,  optimal layer thickness, and optimal extrusion magnification.  Appl. Sci. 2021, 11, 9822 19 of 25 discharge nozzle [88]. Lim et al. [9] proposed that the surface quality of 3D-printing cement-based materials can be judged by the three criteria of no defects on the surface, clear square boundaries of the printed layer, and meeting the requirements of surface consistency and homogeneity. Printing parameters are the most critical factor affecting the surface quality of the parts; thus, scholars focus mainly on adaptive layering and optimization of various printing parameters. Nadiyapara et al. [89] believe that, compared with equal layer thickness layering, adaptive layering can set a smaller layer thickness in the feature-intensive area of the model, so that the dense features are not easy to lose, thereby improving the surface quality of the part. The algorithm is complex; the quality of the algorithm determines the accuracy of the layering. If the algorithm is unreasonable, it will reduce the surface quality of the model. Kim et al. [90] believe that the fused deposition printing parameters have strong coupling and influence each other during the printing process, which ultimately determines the surface quality of the parts. Optimizing the molding parameters can significantly improve the poor surface finish of the molded parts. Li Yanru et al. [91] effectively optimized the surface quality of the part by selecting the optimal forming angle, optimal layer thickness, and optimal extrusion magnification. 7. The Problems and Prospects of 3D-Printing Concrete 7.1. Existing Problems Many challenges and technical problems have always existed in the field of 3D- printing concrete, although much research has been undertaken. A brief list is as follows: (1) The problem of accurate conversion between software design and real architecture. In contrast to the traditional building structure, 3D-printing construction technology requires the design of the building model on the computer, and then converts it into a physical object through an automated program. The design software has become an important part of the preparation phase of the 3D-printing construction technology, but there are certain errors between the software 3D design and the entity in architectural printing, which lead to construction quality problems and limit the development of 3D-printing concrete technology. How to realize the conversion between software and reality is an indispensable step in the development of printing concrete. (2) The feasibility of complex architectural printing. The mechanical properties of 3D- printing construction technology are still at the stage of research and development. Although it can be applied to the construction of low-rise and large-area buildings, one-time 3D-printing cannot be done for common high-rise buildings. Thus, the feasible method is to print prefabricated ones first and re-assembly of parts, which is similar to prefabricated buildings. Thus, the advantage of rapid prototyping is lost. Difficulties lie in the high-rise buildings of tens of meters or even hundreds of meters. First of all, it requires a printer compatible with the height of the building. Secondly, to obtain the structural strength of the high-rise building and the problem of the steel bar structure in the building structure, a reasonable solution is needed. (3) The requirements of building materials is another problem. Most 3D printed build- ings are made of high-strength special concrete materials. Ordinary cement cannot meet the requirements of building performance and printing process. It can be seen that the requirements of 3D-printing for material performance must be considered. The 3D-printing building mainly uses printer nozzles to spray materials to build walls. This spraying method not only requires certain stress and mechanical molding characteristics, but also has certain requirements for the quality of coarse and fine aggregates. The requirements for the quality of aggregates are very high, and even new crushing processes are needed. The effect of admixtures in ordinary concrete may be due to the change of materials, which makes it play a role in the specific system of 3D-printing buildings, and even its mechanism of action also changes. Thus, to Appl. Sci. 2021, 11, 9822 20 of 25 realize the printing of three-dimensional buildings, materials are the first problem to be solved. (4) The problem of molding height. The current 3D-printing concrete technology is still in the stage of plane expansion. It can be applied to the construction of low-rise and large-area buildings. However, it is not possible to print the widely used high-rise buildings. It can only be printed by first printing prefabs and then assembling them. To print dozens of floors of buildings, it is necessary to design a giant 3D printer to solve the problem of structural strength of large buildings and the printing of steel bars in buildings. (5) The problem of anisotropy. This is one of the main challenges to limit the development of 3D printed concrete. The main reason for the anisotropy is that the internal structure of the layers or the internal structure of the material is different due to the layer-by- layer printing, which causes the difference of mechanical properties when the printed components loaded in different directions. It is also related to various factors such as material properties, nozzle types, and steel bar layout besides the impact of the layer-by-layer printing construction process. (6) Although the 3D printed concrete over traditional concrete pouring methods can adapt to complex designs and free-style buildings, the true connotations of opti- mization, sustainability and effective weight reduction have not been fully explored. Topology optimization is a mathematical method that has been widely used in opti- mizing the appearance and layout of materials. Now a topology optimization method has been introduced in the numerical simulation stage of 3D printed concrete, with the purpose of optimizing the printed structure. However, it is difficult to apply this method directly to the structural design of concrete because it fails to consider the nonlinear behavior of the material (such as the significant difference between the compressive and tensile strength of concrete). (7) The adhesion between different layers of 3D printed concrete is mainly characterized by tensile and shear strength. Most of the current research focuses on tensile strength, and the research on shear strength is relatively insufficient. In addition, 3D printed concrete will have major changes in the production process, safety measures, etc., which will cause more or less problems. These all require further research. 7.2. Development Direction and Key Technology Now 3D-printing concrete technology is in the initial stage of development. In order to realize the universalization of the application of 3D-printing concrete technology, it is necessary to solve its existing problems with key technologies. The future research and development of 3D-printing concrete technology can be carried out from the following aspects: (1) Research on concrete materials. Although a lot of research on 3D-printing concrete ma- terials has been carried out, and some properties that 3D-printing concrete needs have been proposed, it is mainly based on the preliminary understanding of 3D-printing concrete technology, and there is a lack of systematic, theoretical, and in-depth re- search. It is necessary to conduct systematic and in-depth research on concrete raw materials and compounding theory for 3D-printing concrete technology to develop rapidly. Thus, the new concrete mix theory and new raw material requirements need to be improved according to performance requirements. This will be the main research topic in the development of 3D-printing concrete technology. (2) Software and hardware collaboration. The 3D printer needs to recognize the given three-dimensional model to print the building based on the model. The modeling and design software of the architectural field and the CNC software of the 3D printer are effectively docked to realize the seamless recognition of information which is the future software development direction. In addition, using the CNC software to control the hardware device precisely also needs further development. Appl. Sci. 2021, 11, 9822 21 of 25 (3) Research on printing technology. How to reasonably arrange the printing direction and order, control the all-round climb of the print head, etc., are important factors that determine the printing efficiency, and it is also an aspect that needs attention in future research. The achievable printing height of the current 3D architectural printing process is greatly restricted, but the development of the society has gradually made the concrete buildings develop toward the towering direction. It requires a new printing process to achieve high-rise printing and development to the high-rise to solve the problem including the structural strength, reinforcement, and other issues. (4) Application of micro characterization technology. The current research still focuses on analysis of macroscopic mechanical properties, while the research on micro-scale can provide theoretical support at the specific material level for the macro-performance. The analysis at the micro level will further deepen and improve the understanding of the mechanical properties of 3D printed concrete. (5) Durability of printing materials. The main focus of study is rheology, buildability, anisotropy of mechanical properties, and interlayer adhesion for extrusion-type 3D- printing concrete. It is necessary to conduct a detailed study on the durability of its material structure considering the service time limit and environment of the printing structure and the building. Quantitatively evaluating the durability of 3D printed concrete is necessary in order to compare the durability of different 3D printed concretes, and finally realize the prediction and improvement of the durability of 3D printed concrete. (6) Hardening performance is one of the important indicators of 3D-printing concrete materials. Due to the existence of weak surfaces between layers, 3D-printing concrete materials have obvious anisotropy. Although researchers have conducted a lot of re- search on the factors that influence the interlayer bonding performance of 3D-printing concrete materials, and also have proposed some effective interface enhancement methods, the current methods lack flexibility and universality. It is necessary to further study flexible and effective interlayer performance enhancement methods, and optimize the design of the printing structure and path. (7) Strengthening and toughening methods are an important guarantee for the safety of 3D printed concrete structures. Adding chopped fibers can effectively improve the tensile strength and toughness of 3D printed concrete materials, but the increase in tensile strength is limited and it is difficult to achieve the enough reinforcement of steel bars. Although the continuous rib co-printing method is effective, its enhancement effect on the structure is limited. Other co-printing methods are in the conceptual design stage and need to be studied further. As a new type of construction technology in the construction industry, concrete 3D- printing has the characteristics of moldless construction, high efficiency, and high precision, showing a good application prospect. Although there are still some problems with the development of materials and equipment, and the development of construction technology, ideas such as rapid in-situ printing of large building structures on site, simultaneous and coordinated printing of multiple printers, and fine printing of factory components and assembly construction will gradually be realized. This technology will surely lead to great vitality in the field of construction and become an important supplement to traditional construction methods. Author Contributions: F.L., D.Z., X.H., L.S. and Q.Z., all contributed to the collection of data and preparation of the paper. All authors have read and agreed to the published version of the manuscript. Funding: This work is supported by National Science Foundation under Grant No. 5210041604, 52174144, 52174120, China Postdoctoral Science Foundation under Grant No. 2021M691967, and Natural Science Foundation of Shandong Province, China under Grant No. ZR202103010529. 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Journal

Applied SciencesMultidisciplinary Digital Publishing Institute

Published: Oct 20, 2021

Keywords: 3D-printing; concrete; material properties; preparation technology; printing parameters

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