Micro-Mechanical and 3D Fractal Analysis, Durability, and Thermal Behaviour of Nano-Modified Cementitious Lightweight Composites for Building Facades
Micro-Mechanical and 3D Fractal Analysis, Durability, and Thermal Behaviour of Nano-Modified...
Blankson, Marva Angela;Erdem, Savaş;Gürbüz, Ezgi
2021-02-26 00:00:00
buildings Article Micro-Mechanical and 3D Fractal Analysis, Durability, and Thermal Behaviour of Nano-Modified Cementitious Lightweight Composites for Building Facades 1 2 , 2 Marva Angela Blankson , Savas ¸ Erdem * and Ezgi Gürbüz Department of Civil Engineering, University Technology of Jamaica, 237 Kingston, Jamaica; mblankson@utech.edu.jm Department of Civil Engineering, Istanbul University-Cerrahpasa, 34200 Istanbul, Turkey; ezgi.gurbuz@istanbul.edu.tr * Correspondence: savas.erdem@istanbul.edu.tr Abstract: There are increasing research endeavours on the application of nanotechnology in the construction industry and lightweight composites. In this study, the influence of different percentage (1%, 2%, and 3% by weight of cement) colloidal nano-silica particles on the mechanical, thermal, and durability properties of lightweight cementitious composites was studied through measurement of compressive strength, flexural response, micro-hardness measurement, pore structure analysis, thermal conductivity, water permeability, and chloride penetration. Moreover, 3D X-ray Compute Tomography together with digital image analysis and 3D fractal analysis was used to characterize the nano-silica, micro-structures, and the fracture surfaces. The experimental results show that incorporating nano-silica particles resulted in a mechanical strength increase up to 45.4 % and a water permeability and chloride migration decrease up to 51.2% and 48.2%, respectively. The Citation: Blankson, M.A.; Erdem, S.; micro-structural and 3D fractal analysis also indicated that dense, flaw-free, and thus more resistant, Gürbüz, E. Micro-Mechanical and 3D interfaces to micro-cracks were formed and greater fractal dimensions were obtained with the increase Fractal Analysis, Durability, and of the nano-silica content. Finally, the 3D views confirmed that the nano-silica clusters were well Thermal Behaviour of Nano-Modified interconnected which further increase the carrying capacity and reducing the heat flow. Cementitious Lightweight Composites for Building Facades. Keywords: lightweight aggregate; nano-silica; mechanical property; micro-mechanical analysis Buildings 2021, 11, 85. https:// doi.org/10.3390/buildings11030085 Academic Editor: Francisco Almansa 1. Introduction Received: 1 February 2021 Lightweight concrete can be classified in three ways according to its density and Accepted: 22 February 2021 the purpose of its use: (I) having a density less than 800 kg/m , (II) a target density of Published: 26 February 2021 3 3 800–1400kg/m with moderate-strength, and (III) density ranges from 1400 kg/m to 2000 kg/m with structural concrete [1]. In recent years, there has been an increasing Publisher’s Note: MDPI stays neutral demand for use of structural concrete with lightweight aggregates in modern construction with regard to jurisdictional claims in methods, thanks to an appreciation of the advantage that a decreased density could result published maps and institutional affil- in a decrease in the cross-sections of load-carrying elements and a corresponding reduction iations. in the foundation size [2–4]. In addition, lightweight concrete would be characterized by its lower thermal conductivity compared to normal weight concretes because of the intrinsic porous nature of lightweight aggregates [5]. As a result, lightweight concrete has a huge potential for the use of many modern constructions, for example, lightweight bridge Copyright: © 2021 by the authors. decks [6], lightweight foamed concrete [7], lightweight concrete blocks [8], or thermal Licensee MDPI, Basel, Switzerland. isolation constructions [9]. However, lightweight concrete for the same strength level This article is an open access article exhibits a quite brittle behaviour compared to conventional concrete as a result of the distributed under the terms and low fracture toughness, the low tensile-compressive strength ratio, and residual tensile conditions of the Creative Commons strength [10]. Therefore, a deep understanding of its mechanical behaviour, the interfacial Attribution (CC BY) license (https:// bonding behaviour, energy absorption characteristics, and improvement of its ductility are creativecommons.org/licenses/by/ the key factors to promote its wide acceptance in a variety of structural applications. 4.0/). Buildings 2021, 11, 85. https://doi.org/10.3390/buildings11030085 https://www.mdpi.com/journal/buildings Buildings 2021, 11, 85 2 of 18 Structure-property relationships in cementitious-based materials are the heart of mod- ern concrete technology. Concrete is highly heterogeneous on multiple scales and has a complex structure made of three distinct constituents at a microscopic scale: aggregates, ce- ment paste matrix, and an interfacial transition zone (ITZ) between these two components. Establishing true structure-property relationships is now one of the greatest challenges for concrete technologists. In parallel, the distinct pore structure having many pores with different sizes has been considered one of the most important characteristics of cementi- tious materials. The pore structure of the composite controls other important properties including permeability, mechanical properties, dimensional stability, and durability-related properties, and can provide very valuable information regarding both internal structure and structural performance [11,12]. The enhancement of the pore structure is therefore of vital importance for assessing the material behaviour and improving the properties of lightweight concrete. The approach of nanoparticles as modifiers to improve the engineering properties and performance of concrete is considered promising. One of these nanomaterials is nano- silica which can be used in the form of compacted dry grains and colloidal suspension in mixtures [13]. Previous studies [14–17] have demonstrated that the addition of nano-silica has significantly improved the performance of concrete in terms of mechanical properties, and fracture toughness. Another study [18] has showed that the micro-structure of ITZ and the water permeability resistance capacity of concrete can be significantly enhanced with the addition of nano-SiO . In general, the improvement of the performance of cementitious composites with the addition of nano-silica can be attributed to the fact that the nano-silica particles fill the voids between the cement grains resulting in much denser the micro- structure and act as nucleating centers for the chemical generation of cement hydrates [19]. While knowledge regarding the effects of nano-silica modification on the cementitious composites with normal weight aggregates is widely established, only limited studies (especially with low dosage modification) are available in the literature. A considerable improvement in both early-age and late-age compressive and flexural tensile strengths has been reported in a recent study by Zhang et al. [20]. They studied the effect of a low dosage of nano-silica addition (0.1 to 1% by weight) on the lightweight aggregate concrete. However, the authors reported that the nano-silica modified specimens exhibited similar strengths compared with the corresponding control specimen when exceeding the optimal dosage which is called the neutralization effects of nano-particles. In another experimental study, Wang et al. [21] investigated the influence of nano-silica (1%, 2%, and 3%) addition on the static strength, shrinkage behaviour, and resistance to early-age cracking of the concrete. The improvement of the compressive strength of the lightweight aggregate concrete noticeably occurred at early ages (within 28 days). In terms of micro- structure modifications, the interfacial transition zone between the lightweight aggregate and surrounded cement matrix was much denser and more continuous compared to the lightweight mix without nano-silica. Moreover, with the incorporation of the nano- silica, the long-term shrinkage of the lightweight concrete did not seem to be affected significantly, while the total cracking area (as a reflection of the early cracking sensitivity) of the lightweight mixes showed a continuous decrease with the addition of nano-silica particles. A wide range (1, 2, 5, and 10 wt-%.) dosage of nano-silica was evaluated in the work of Sikora et al. [22]. They characterized the effects of nanosilica on the properties of lightweight aggregate concrete referring to an oven-dry density of 850 kg/m and ultra- lightweight aggregate concrete referring to an oven-dry density of 450 kg/m3, respectively. They observed considerable improvements for the mechanical and transport properties and a better efficiency than that of silica fume. Moreover, the 3D micro-structural analysis conducted by a micro-computed tomography also confirmed the production of a very dense void structure and the pore refinement contributing to a less permeable cement matrix. In contrast to the studies mentioned above, Vargas et al. [16] were evaluated the influence of the replacement of a high dosage of nano-silica (10 wt. %) on the performance of the lightweight aggregate concrete. They found that the characteristics of lightweight Buildings 2021, 11, 85 3 of 18 aggregates used in the manufacturing of lightweight concrete are the main parameters to control its compressive strength and durability performance. However, the nano-silica had a considerable effect on refinement of the pore structure resulting in a lower volume of voids and a decreased water absorption rate and thus, improved durability performance. Furthermore, a study [20] about the effect of colloidal nano-silica on lightweight concrete shows that relatively high dosage of nano-silica will create the internal stress resulting from the production of hydration product at the boundary of the lightweight aggregates and the surrounded cement matrix which, in turn, decreases the modification of the nano-particles. The literature review above clearly indicates that research in this field has mainly focused on the evaluation of mechanical properties, durability behaviour, and thermal performance at a macro level. However, research conducted so far is still less to evaluate comprehensively the microstructure associated mechanical and durability characterization of lightweight cementitious mixes modified with nano-silica, as well as 3D fractal cracking characterization. In addition, the potential structural application of this type of cementi- tious composites has been paid less attention. This leads to the aim of this study, which is to analyze the micro-structural damage characteristics of nano-modified lightweight cementitious composites that have experienced mechanical, thermal, and chemical effects, so as to assess the influence of the low dosage addition of the nano-particles. A comple- mentary objective is to make a comparison between the mechanical, thermal, durability, and bond behaviour of composite and its micro-structure as exemplified in mixes produced with sintered fly ash lightweight aggregates with and without nano-silica addition. This will be addressed via testing the mechanical properties (compressive strength, and flexural tensile strength), as well as the microstructural and crack analysis via; scanning electron microscopy, micro-hardness analysis, X-ray diffraction, mercury porosimetry analysis, energy dispersive X-ray analysis (EDX), and fractal analysis. 2. Experimental Program 2.1. Materials Used and Concrete Mixtures Portland Cement CEM I 52.5N, coarse lightweight aggregates, local fine sand having a specific gravity of 2.66, and a polycarboxylate ether-based superplasticizer, provided by BASF, Germany, were used to make the mixtures. The lightweight coarse aggregate used in this study was sintered fly ash produced by pelletizing the fly ash at a temperature of around 1100 C. As a result of this process, the aggregates had a rounded shape. The X-ray Diffraction (XRD) technique was used to analyze the mineralogy patterns of the aggregates and the analysis results are illustrated in Figure 1. The lightweight aggregates with a nominal maximum size of 10 mm had a specific gravity 1.35 g/cm and water absorption of 27.2 as % of dry mass. The XRD analysis demonstrated that the fly ash aggregate used would have significant amorphous content. The hump in the pattern would indicate that the amorphous phase is most likely silica having pozzolanic characteristics [23]. Nano-silica (Figure 2) has been selected in colloidal form (Figure 2) in order to provide a homogenous distribution of the nano-silica in the matrix and decrease the risk of nano-particles agglomeration [24]. Nano-silica used in this experimental work (50 wt. % of solid mass) has been provided by EKA Chemicals. The nano-particles with a particle size in the range of 5–20 nm have a density of 1.3 g/cm and a pH value of 9.4. Buildings 2021, 11, 85 4 of 18 Buildings 2021, 11, x FOR PEER REVIEW 4 of 19 Buildings 2021, 11, x FOR PEER REVIEW 4 of 19 Figure 1. XRD pattern of the lightweight fly ash aggregate. Figure 1. XRD pattern of the lightweight fly ash aggregate. Figure 1. XRD pattern of the lightweight fly ash aggregate. Figure 2. SEM photo and analysis for the nano silica particles. Figure 2. SEM photo and analysis for the nano silica particles. Figure 2. SEM photo and analysis for the nano silica particles. 2.2. Production of Concrete Mixtures 2.2. Production of Concrete Mixtures 2.2. Production of Concrete Mixtures The volume method is preferred for the design of the concrete mixtures, and the mix The volume method is preferred for the design of the concrete mixtures, and the mix proportions are illustrated in Table 1. The w/c ratio was the same as 0.35 for the mixtures, The volume method is preferred for the design of the concrete mixtures, and the mix proportions are illustrated in Table 1. The w/c ratio was the same as 0.35 for the mixtures, and the different dosages of the nano-particles (1%, 2%, and 3% by weight of cement) were proportions are illustrated in Table 1. The w/c ratio was the same as 0.35 for the mixtures, and the different dosages of the nano-particles (1%, 2%, and 3% by weight of cement) were and used the fordif man ferufactur ent dosages ing coof ncret the e mixe nano-particles s. Consider (1%, ing the re 2%, and lative 3% ly by high co weight st of of ncement) ano-par- used for manufacturing concrete mixes. Considering the relatively high cost of nano-par- ticles and the literature review (very limited studies to investigate the nano-modification were used for manufacturing concrete mixes. Considering the relatively high cost of nano- ticles and the literature review (very limited studies to investigate the nano-modification particles effects on and the the ligliteratur htweight e rc eview ementitious (very limited mixes), l studies ow repla to investigate cement ratios the nano-modification were chosen. Four effects on the lightweight cementitious mixes), low replacement ratios were chosen. Four ef different fects on m the ixes lightweight with/withcementitious out the nano-mixes), particles low were replacement cast and test ratios ed, nwer amel e y chosen. control Four con- different mixes with/without the nano-particles were cast and tested, namely control con- dif cret fer e ent conmi tainin xesg with/without lightweight coar the se nano-particles without nano-wer silica e cast (RC)and , ligh tested, tweight namely aggrega contr te con ol- cret concr e con ete ta containing ining lightlightweight weight coarse coarse without without nanonano-silica -silica (RC), (RC), lightw lightweight eight aggrega aggr teegate con- crete with 1% nano-silica (LAC-1), lightweight aggregate concrete with 2% nano-silica crete with 1% nano-silica (LAC-1), lightweight aggregate concrete with 2% nano-silica (LAC-2), and lightweight aggregate concrete with 3% nano-silica (LAC-3). (LAC-2), and lightweight aggregate concrete with 3% nano-silica (LAC-3). Buildings 2021, 11, 85 5 of 18 concrete with 1% nano-silica (LAC-1), lightweight aggregate concrete with 2% nano-silica (LAC-2), and lightweight aggregate concrete with 3% nano-silica (LAC-3). Table 1. Mix proportions for 1 m . Fine Lightweight Cement Water Nano-Silica Density Mixture Aggregate Coarse (kg) (kg) (kg) (kg/m ) (kg) Aggregate (kg) RC 500 175 566 554 - 1645 LAC-1 495 175 566 554 10 1633 LAC-2 490 175 566 554 20 1622 LAC-3 485 175 566 554 30 1664 The fly ash lightweight aggregates have been used in the saturated surface dry condi- tion. For that purpose, the lightweight aggregates were immersed in water for a period of 24 to provide fully saturated particles. Then, the lightweight aggregates were spread on a pan and exposed to air-dry conditions for 24 hours in which the lightweight aggregates absorb no water during the mixing process [25]. The fresh concrete was placed into the moulds in two layers and then vibration was applied to remove entrapped air bubbles as well as levels the fresh concrete. Finally, the de-molded specimens were cured in the environment of 20 2 C and 65% relative humidity. 2.3. Tests Conducted Three cubes (100 mm) specimens for the compressive strength of each mixture were tested at the age of 7, 28, and 56 days based on TS-EN 12390-3 [26]. The ultrasonic pulse velocity of cube specimens was performed using a PUNDIT equipment (portable ultrasonic non-destructive digital indicating tester). 100 100 400 mm of prism specimens were tested to determine the flexural tensile strengths of the mixes at the age of 7, 28, and 56 days. An open access ImageJ software was then employed to characterize the flexural load-induced fracture surfaces and compute the fractal dimensions of the mixtures were calculated. Then, the fracture energies of the concretes were estimated as a function of the surface macro-cracks by the following Formula 1 [27]: D1 d Ws/Gf = a (/a) (1) where a is the height of the cross-section of the tested sample, is the maximum size 1 d of the fine particle, and D is the fractal dimension of the fracture surface or a crack, respectively. A Vickers micro-hardness test was performed to measure the micro-hardness of the ITZs. A load of 0.01 kg with 10s contact time has been applied to the region of 100 m away from the surface of aggregates. The chloride migration test was performed according to the NT BUILD 492 [28] procedure. A silver nitrite indicator solution (Figure 3) was used to measure the chloride migration depths of 100 50 mm cylinder specimens and the non-steady-state migration coefficients (Dnssm) were computed based on the formulas reported by Luo and Schutter [29]. The water penetration tests were conducted according to TS-EN 12390-8 [30] at the age of 28 days. For that purpose, one side of 150 mm cube specimens was exposed to water for a pressure of 500 50 kPa for 72 hours. The specimens were then split into two halves by using the compressive test set-up. At the final stage, the water penetration sections on the lightweight cement composite surface were highlighted and the maximum depth of the penetration of the water in specimens was recorded. Buildings 2021, 11, 85 6 of 18 Buildings 2021, 11, x FOR PEER REVIEW 6 of 19 Figure 3. Chloride migration test set up. Figure 3. Chloride migration test set up. A C–Therm/ Tci thermal conductivity equipment was used to measure thermal con- ductivity coefficients (k) of the sample parts which were used for compressive strength A C–Therm/ Tci thermal conductivity equipment was used to measure thermal test. The measurement has been done according to ASTM Standard E1952 [31] with five conductivity coefficients (k) of the sample parts which were used for compressive strength replicated specimens. In this study, a mercury intrusion porosimetry device was also used to characterize test. The measurement has been done according to ASTM Standard E1952 [31] with five the pore structure of the lightweight mixes. The relationship between the applied pressure replicated specimens. and the volume of intruded mercury was quantitatively observed for each lightweight In this study, a mercury intrusion porosimetry device was also used to characterize mix. Pull-out tests were also carried out to assess, to some extent, the performance of this the pore structure of the lightweight mixes. The relationship between the applied pressure nano-modified lightweight cementitious composites in structural applications. For that and the volume of intruded mercury was quantitatively observed for each lightweight mix. purpose, the reinforcing ribbed steel-bar of 14 mm diameter was extracted from the light- weight cementitious composites cube specimens with a size of 150 × 150 × 150 mm at the Pull-out tests were also carried out to assess, to some extent, the performance of age of 28 days (Figure 4). A controlled displacement test according to ASTM E1512-01[32] this nano-modified lightweight cementitious composites in structural applications. For was carried out at a fixed displacement rate of 0.5 mm/min, and the reactive load was that recorded purpose, and then, the bon the reinfor d stress cing was cal ribbed culated us steel-bar ing by Formul ofa 14 2. mm diameter was extracted from the Calculation of bond stress, lightweight cementitious composites cube specimens with a size of 150 150 150 mm at τ = Applied Force / π ϕ l (2) the age of 28 days (Figure 4). A controlled displacement test according to ASTM E1512- where; 01 [32] was carried out at a fixed displacement rate of 0.5 mm/min, and the reactive load τ = Bond stress (N/mm ) was recorded and then, the bond stress was calculated using by Formula 2. φ= Reinforcement bar diameter (mm) l = Anchor Calculation age length of— bond mm (len str gth of ess, reinforcement embedded in concrete) = Applied Force/ l (2) where; = Bond stress (N/mm ) = Reinforcement bar diameter (mm) Buildings 2021, 11, x FOR PEER REVIEW 7 of 19 l = Anchorage length—mm (length of reinforcement embedded in concrete) Figure 4. Pull-out test set up. Figure 4. Pull-out test set up. 3. Results and Discussions 3.1. Micro-Structure Associated Compressive Strength Compressive strength test results of the lightweight and nano-modified lightweight mixtures at 7, 28, and 56 days are illustrated in Figure 5. The results, in general, indicate an increasing trend of the compressive strength when the nano-silica was incorporated into the mixes. Compared to the reference mix, the 28 days compressive strength improvement ra- tios of the samples including nano-silica particles with the proportions of 1%, 2%, and 3% by weight of cement were 12.70%, 26.8%, and 35.1%, respectively. This confirms that an increase in the rate of nano-silica particles in the lightweight mix resulted in the enhance- ment in the compressive strength. By adding the nanoparticles, the bond between the lightweight aggregate particles and the matrix was improved as a result of the advanced hydration degree. This is mainly credited to the high degree of pozzolanic reaction in the nano-silica modified lightweight system. The distinct hump shown by a red dash line in the XRD analysis in Figure 2 indicates that the presence of considerable amounts of amor- phous silica material and the pozzolanic characteristic. The significant amorphous silica content of the particles reacted with calcium hydroxide and fill the pores existing in the matrix with C-S-H gels (Figure 6). The mercury porosimetry test result presented in Sec- tion 3.3 also provides good support for this theory and indicates that the percentage of pore volume in the nano-modified lightweight mixes was significantly lower than the ref- erence mix. Following that, the increased integrity of the concrete matrix may improve the connected area and carrying capacity for the matrix materials that flow and deform around the lightweight aggregate against the compressive loading. Buildings 2021, 11, 85 7 of 18 3. Results and Discussions 3.1. Micro-Structure Associated Compressive Strength Compressive strength test results of the lightweight and nano-modified lightweight Buildings 2021, 11, x FOR PEER REVIEW 8 of 19 mixtures at 7, 28, and 56 days are illustrated in Figure 5. The results, in general, indicate an increasing trend of the compressive strength when the nano-silica was incorporated into the mixes. 7 days 28 days 56 days RC LAC-1 LAC-2 LAC-3 Figure 5. The compressive strength test results of the mixes. Figure 5. The compressive strength test results of the mixes. Compared to the reference mix, the 28 days compressive strength improvement ratios of the samples including nano-silica particles with the proportions of 1%, 2%, and 3% by weight of cement were 12.70%, 26.8%, and 35.1%, respectively. This confirms that an increase in the rate of nano-silica particles in the lightweight mix resulted in the enhancement in the compressive strength. By adding the nanoparticles, the bond between the lightweight aggregate particles and the matrix was improved as a result of the advanced hydration degree. This is mainly credited to the high degree of pozzolanic reaction in the nano-silica modified lightweight system. The distinct hump shown by a red dash line in the XRD analysis in Figure 2 indicates that the presence of considerable amounts of amorphous silica material and the pozzolanic characteristic. The significant amorphous silica content of the particles reacted with calcium hydroxide and fill the pores existing in the matrix with C-S-H gels (Figure 6). The mercury porosimetry test result presented in Section 3.3 also provides good support for this theory and indicates that the percentage of pore volume in the nano-modified lightweight mixes was significantly lower than the reference mix. Following that, the increased integrity of the concrete matrix may improve the connected area and carrying capacity for the matrix materials that flow and deform around the lightweight aggregate against the compressive loading. Figure 6. Scanning electron microscope views of the micro-structure. Figure 7 illustrates the relationship between the average micro-hardness (strength) distribution measured in the interfacial transition zone of each lightweight mix and the distance from the aggregate surface. The results clearly show that there are noticeable dif- ferences between the specimen behaviors. In the mix encompassing 3% nano-silica parti- cles, the micro-hardness at the distance of 50 µ m away from the sintered fly ash aggregate surface reached a level of 38 HV 0.01, while it was only 29 HV 0.01 for the reference mix which does not contain any nano-particles. As previously stated by Erdem [33] and Perkins [34], the greater the crystallinity the stiffer the composites, or the higher the strength. From a microstructural and chemical science point of view, adding nano-silica particles into the lightweight mixes would in- crease the crystallinity of the mixes which, in turn, resulted in a higher micro-hardness Compressive Strength (MPa) Buildings 2021, 11, x FOR PEER REVIEW 8 of 19 7 days 28 days 56 days RC LAC-1 LAC-2 LAC-3 Buildings 2021, 11, 85 8 of 18 Figure 5. The compressive strength test results of the mixes. Figure 6. Figure Scanning 6. Scelectr anning on el micr ectron oscope microsc views ope of vi the ews micr of the mic o-structur ro-e. structure. Figure 7 illustrates the relationship between the average micro-hardness (strength) Figure 7 illustrates the relationship between the average micro-hardness (strength) Buildings 2021, 11, x FOR PEER REVIEW 9 of 19 distribution measured in the interfacial transition zone of each lightweight mix and the distribution measured in the interfacial transition zone of each lightweight mix and the distance from the aggregate surface. The results clearly show that there are noticeable distance from the aggregate surface. The results clearly show that there are noticeable dif- differences between the specimen behaviors. In the mix encompassing 3% nano-silica ferences between the specimen behaviors. In the mix encompassing 3% nano-silica parti- value compared to the reference mix. This is mainly credited to a higher crystallinite pro- particles, the micro-hardness at the distance of 50 m away from the sintered fly ash cles, the micro-hardness at the distance of 50 µ m away from the sintered fly ash aggregate portion of the nano-silica (as confirmed in Figure 2) as a result of a significant number of aggregate surface reached a level of 38 HV 0.01, while it was only 29 HV 0.01 for the surface reached a level of 38 HV 0.01, while it was only 29 HV 0.01 for the reference mix unsaturated bonds. reference mix which does not contain any nano-particles. which does not contain any nano-particles. As previously stated by Erdem [33] and Perkins [34], the greater the crystallinity the stiffer the composites, or the higher the strength. From a microstructural and chemical science point of view, adding nano-silica particles into the lightweight mixes would in- crease the crystallinity of the mixes which, in turn, resulted in a higher micro-hardness 25 RC LAC-1 LAC-2 LAC-3 0 20 40 60 80 100 120 Distance from aggregate surface Figure 7. Micro-hardness values of the ITZs of the mixes. Figure 7. Micro-hardness values of the ITZs of the mixes. As previously stated by Erdem [33] and Perkins [34], the greater the crystallinity the In parallel, the 3D micro-computed tomography views of the mix with nano-silica stiffer the composites, or the higher the strength. From a microstructural and chemical particle science s h point ave al of so view support , adding ed this nano-silica theory to some particles extent into . It the can lightweight be seen from mixes Figure would 8 that in- thcr e ease inclus the ion crystallinity of the nano-of sili the ca p mixes articles which, into th in e turn, lightweight resulted mix in le a d higher to den micr sifico-har ation dness and hom value ogeneity compar ofed the to interf the racial eference matrix mix. and This prois vide mainly d superio credited r bonding. to a higher In addition, crystallinite the colloidal nano-silica particles were distributed in a very homogeneous way in the matrix, which indicates that dense, flaw-free, and thus more resistant interfaces to micro-cracks were formed with the increase of the nano-silica content. Vickers Microhardness (HV0.01) Compressive Strength (MPa) Buildings 2021, 11, x FOR PEER REVIEW 9 of 19 value compared to the reference mix. This is mainly credited to a higher crystallinite pro- portion of the nano-silica (as confirmed in Figure 2) as a result of a significant number of unsaturated bonds. 25 RC LAC-1 LAC-2 LAC-3 0 20 40 60 80 100 120 Buildings 2021, 11, 85 9 of 18 Distance from aggregate surface Figure 7. Micro-hardness values of the ITZs of the mixes. proportion of the nano-silica (as confirmed in Figure 2) as a result of a significant number of unsaturated bonds. In parallel, the 3D micro-computed tomography views of the mix with nano-silica In parallel, the 3D micro-computed tomography views of the mix with nano-silica particles have also supported this theory to some extent. It can be seen from Figure 8 that particles have also supported this theory to some extent. It can be seen from Figure 8 that the inclusion of the nano-silica particles into the lightweight mix led to densification and the inclusion of the nano-silica particles into the lightweight mix led to densification and homogeneity of the interfacial matrix and provided superior bonding. In addition, the homogeneity of the interfacial matrix and provided superior bonding. In addition, the colloidal nano-silica particles were distributed in a very homogeneous way in the matrix, colloidal nano-silica particles were distributed in a very homogeneous way in the matrix, which indicates that dense, flaw-free, and thus more resistant interfaces to micro-cracks which indicates that dense, flaw-free, and thus more resistant interfaces to micro-cracks were formed with the increase of the nano-silica content. were formed with the increase of the nano-silica content. Buildings 2021, 11, x FOR PEER REVIEW 10 of 19 Figure 8. 3D X-ray micro-tomography views of the nano-silica modification. Figure 8. 3D X-ray micro-tomography views of the nano-silica modification. 3.2. Flexural Strength and 3D Crack Analysis 3.2. Flexural Strength and 3D Crack Analysis In general, the flexural strength of the lightweight mix increases in parallel with the In general, the flexural strength of the lightweight mix increases in parallel with the increase in the percentage of nano-silica particles (Figure 9). The results, in general, indicate increase in the percentage of nano-silica particles (Figure 9). The results, in general, indi- an increasing trend of the flexural tensile strength when the nano-silica particles were cate an increasing trend of the flexural tensile strength when the nano-silica particles were incorporated into the mixes. incorporated into the mixes. Compared to the reference mix, the 56 days flexural tensile strength improvement ratios of the samples including nano-silica particles with the proportions of 1%, 2%, and 3 % by weight of cement were 9.90%, 27.7%, and 45.4%, respectively. With the aggregate in the four lightweight mixes being the same, it would be deduced that the higher level of tensile stresses occurred in the interfacial transition zone between the aggregate and the surrounded matrix. The stronger ITZ (as confirmed by the micro-hardness results) in the mix reinforced with nano-particles could efficiently contribute to the stress transfer mech- anism which, in turn, results in greater flexural strength. Vickers Microhardness (HV0.01) Buildings 2021, 11, x FOR PEER REVIEW 11 of 19 Buildings 2021, 11, 85 10 of 18 4.5 3.5 2.5 1.5 0.5 7 days 28 days 56 days RC LAC-1 LAC-2 LAC-3 Figure 9. The flexural strength test results of the mixes. Figure 9. The flexural strength test results of the mixes. Compared to the reference mix, the 56 days flexural tensile strength improvement After the flexural tensile strength tests of 28-day samples, the fractal analysis has been ratios of the samples including nano-silica particles with the proportions of 1%, 2%, and conducted on fractured specimens of the lightweight mixes. Then, the fractal dimensions 3 % by weight of cement were 9.90%, 27.7%, and 45.4%, respectively. With the aggregate of the surface cracks were computed for each mix. Although various methods (cube count- in the four lightweight mixes being the same, it would be deduced that the higher level ing, variance methods, etc.) are available in the literature for calculating the fractal size, of tensile stresses occurred in the interfacial transition zone between the aggregate and the most practical and used method is box counting, which is based on measuring the the surrounded matrix. The stronger ITZ (as confirmed by the micro-hardness results) in boundaries of a shape using by the lengths between points on it considering square boxes. the mix reinforced with nano-particles could efficiently contribute to the stress transfer The details of the methods were discussed in a previous study by Erdem and Blankson mechanism which, in turn, results in greater flexural strength. [35]. Following that, the fracture energy (Ws/Gf) of the samples in mm was calculated at After the flexural tensile strength tests of 28-day samples, the fractal analysis has been the macro level using the formulation proposed by Guo et al. [27]. The value of Ws/Gf conducted on fractured specimens of the lightweight mixes. Then, the fractal dimensions of indicates the ratio of the total energy (Ws), which is released during crack propagation, to the surface cracks were computed for each mix. Although various methods (cube counting, the fracture energy (Gf). variance methods, etc.) are available in the literature for calculating the fractal size, the most The fractal size values of the fracture surfaces obtained by Image J program are illus- practical and used method is box counting, which is based on measuring the boundaries of trated in Figure 10. The results clearly show that the lightweight mix with a percentage of a shape using by the lengths between points on it considering square boxes. The details of 3 % nano-silica particles (LAC-3) has the highest fractal dimension value among the mixes. the methods were discussed in a previous study by Erdem and Blankson [35]. Following In addition, the other nano-silica reinforced lightweight samples had also a fractal dimen- that, the fracture energy (Ws/Gf) of the samples in mm was calculated at the macro level sion greater than the reference lightweight mix (RC). The higher fractal dimension of the using the formulation proposed by Guo et al. [27]. The value of Ws/Gf indicates the LAC-3 mix led to the dissipation of a higher fracture energy at the macro scale level as ratio of the total energy (Ws), which is released during crack propagation, to the fracture confirmed by the results presented in Table 2. The higher fractal dimension values asso- energy (Gf). ciated with the addition of nano-silica particles revealed that the unsaturated bonds sig- The fractal size values of the fracture surfaces obtained by Image J program are nificantly reduced the hydrophobicity and static contact angle for the nano-silica improv- illustrated in Figure 10. The results clearly show that the lightweight mix with a percentage ing the bond ability between them and hydration products and producing the toughening of 3 % nano-silica particles (LAC-3) has the highest fractal dimension value among the mechanism along the crack front and voids. mixes. In addition, the other nano-silica reinforced lightweight samples had also a fractal Three-dimensional images of the fractured surface roughness of the samples are il- dimension greater than the reference lightweight mix (RC). The higher fractal dimension lustrated in Figure 11. According to the above-mentioned results, the sample with 3% of the LAC-3 mix led to the dissipation of a higher fracture energy at the macro scale nano-silica particles gave the highest energy value during crack occurring and propaga- level as confirmed by the results presented in Table 2. The higher fractal dimension tion. Figure 11 shows 3D dimensional surface curves of the mixes associated with the values associated with the addition of nano-silica particles revealed that the unsaturated depths of the points of the samples. The results revealed the crack width and depth at the bonds significantly reduced the hydrophobicity and static contact angle for the nano-silica LAC-3 was high, and at the same time, the cracks spread reached to the upper fibers with improving the bond ability between them and hydration products and producing the a much larger surface area leading to the greater fracture energy. toughening mechanism along the crack front and voids. Flexural Strength (MPa) Buildings 2021, 11, x FOR PEER REVIEW 12 of 19 Buildings 2021, 11, 85 11 of 18 Figure 10. Fractal dimensions of the mixes. Figure 10. Fractal dimensions of the mixes. Table 2. Fractal dimensions vs. Ws/Gf values. Table 2. Fractal dimensions vs. Ws/Gf values. Mix ID Fractal Dimension -D Ws/Gf (mm) Mix ID Fractal Dimension -D Ws/Gf (mm) RC RC 1.25191.2519 71.44 71.44 LAC-1 1.2930 78.53 LAC-1 1.2930 78.53 LAC-2 1.3846 96.98 LAC-2 1.3846 96.98 LAC-3 1.5586 144.76 LAC-3 1.5586 144.76 Three-dimensional images of the fractured surface roughness of the samples are illustrated in Figure 11. According to the above-mentioned results, the sample with 3% nano-silica particles gave the highest energy value during crack occurring and propagation. Figure 11 shows 3D dimensional surface curves of the mixes associated with the depths of the points of the samples. The results revealed the crack width and depth at the LAC-3 was high, and at the same time, the cracks spread reached to the upper fibers with a much larger surface area leading to the greater fracture energy. Buildings 2021, 11, x FOR PEER REVIEW 13 of 19 Buildings 2021, 11, 85 12 of 18 Figure 11. 3D surface plots of the fracture surfaces of the mixes. Figure 11. 3D surface plots of the fracture surfaces of the mixes. 3.3. Water Permeability and Chloride Migration 3.3. Water Permeability and Chloride Migration In the four types of lightweight cementitious composites, the transport properties In the four types of lightweight cementitious composites, the transport properties were also studied in order to understand the durability behaviour. It is very well-known were also studied in order to understand the durability behaviour. It is very well-known that one of the degradations mechanisms of the cementitious composites is sourced from that one of the degradations mechanisms of the cementitious composites is sourced from the aggressive media such as sulphate and chloride riches environments in which the the aggressive media such as sulphate and chloride riches environments in which the chemicals penetrate the concrete matrix through the voids. The water permeability and the chemicals penetrate the concrete matrix through the voids. The water permeability and non-steady state chloride migration tests would therefore provide good indicators of the the non-steady state chloride migration tests would therefore provide good indicators of durability of the composite [36]. the durability of the composite [36]. Table 3 illustrates the average results after these two durability tests were conducted Table 3 illustrates the average results after these two durability tests were conducted on the representative lightweight and nano-silica modified lightweight samples. At 28 days, on the representative lightweight and nano-silica modified lightweight samples. At 28 the depth of water penetration was 51.2% higher in the lightweight mix (RC) than that with days, the depth of water penetration was 51.2% higher in the lightweight mix (RC) than 3% nano-silica particles (LAC-3). It is also shown that, in comparison to the reference mix that with 3% nano-silica particles (LAC-3). It is also shown that, in comparison to the ref- (RC), the computed chloride migration coefficient from non-steady state migration (D ) nssm erence mix (RC), the computed chloride migration coefficient from non-steady state mi- at 28 days was 48.22% higher than in the lightweight cementitious composites modified gration (Dnssm) at 28 days was 48.22% higher than in the lightweight cementitious compo- with 3% nano-silica particles. sites modified with 3% nano-silica particles. Buildings 2021, 11, 85 13 of 18 Table 3. Water permeability and chloride penetrability test. Water Permeability Chloride Mix ID (mm) Penetration Charge Passed Depth (mm) D 10 nssm (Coulombs) Buildings 2021, 11, x FOR PEER REVIEW RC 37 12 12.4 2678 14 of 19 LAC-1 32 9 9.56 1989 LAC-2 26 7 7.12 1892 Table 3. Water permeability and chloride penetrability test. LAC-3 18 6 6.42 1746 Water Chloride Pene- Mix ID Permeability According to the evaluation criteria by NT BUILD 492 [28], all the nano-modified tration (mm) lightweight cementitious mixes can be described as having high resistance to chloride penetration, whereas the reference lightweight mix can be classified as having medium Charge Passed −12 Depth (mm) Dnssm × 10 resistance to chloride ion penetration. In addition, in this test, the higher D measure- nssm (Coulombs) ment shows that any steel reinforcement that is placed in the lightweight cementitious RC 37 12 12.4 2678 mixes without nanoparticles would be more susceptible to corrosion. While other features LAC-1 32 9 9.56 1989 may have helped to reduce the transport property of the lightweight cementitious mixes LAC-2 26 7 7.12 1892 with nano-silica modification, it is plausible that a refinement of pore structure (the volume LAC-3 18 6 6.42 1746 of micro pores is 65% bigger [26]) with the addition of the colloidal nano-silica particles has occurred as shown in Table 4 and Figure 12. The pore refinement would provide the According to the evaluation criteria by NT BUILD 492 [28], all the nano-modified stronger interfacial transition zone which makes the difficult the movement of the chemical lightweight cementitious mixes can be described as having high resistance to chloride substances or water across its length. penetration, whereas the reference lightweight mix can be classified as having medium resistance to chloride ion penetration. In addition, in this test, the higher Dnssm measure- Table 4. The calculated percentage of the micro and macro pores. ment shows that any steel reinforcement that is placed in the lightweight cementitious Mix ID Micro Pores (%) Macro Pores (%) mixes without nanoparticles would be more susceptible to corrosion. While other features may have helped to reduce the transport property of the lightweight cementitious mixes RC 68.40 31.6 with nano-silica modification, it is plausible that a refinement of pore structure (the vol- LAC-1 72.30 27.7 ume of micro pores is 65% bigger [26]) with the addition of the colloidal nano-silica parti- LAC-2 77.40 22.6 cles has occurred as shown in Table 4 and Figure 12. The pore refinement would provide LAC-3 81.10 18.9 the stronger interfacial transition zone which makes the difficult the movement of the chemical substances or water across its length. Figure 12. The pore structure analysis of the mixes. Figure 12. The pore structure analysis of the mixes. Table 4. The calculated percentage of the micro and macro pores. Mix ID Micro Pores (%) Macro Pores (%) RC 68.40 31.6 LAC-1 72.30 27.7 LAC-2 77.40 22.6 LAC-3 81.10 18.9 Buildings 2021, 11, x FOR PEER REVIEW 15 of 19 Buildings 2021, 11, x FOR PEER REVIEW 15 of 19 Buildings 2021, 11, 85 14 of 18 3.4. Thermal Conductivity 3.4. Thermal Conductivity 3.4. Thermal Conductivity Figure 13 represents the results of thermal conductivity of the lightweight cementi- Figure 13 represents the results of thermal conductivity of the lightweight cementi- Figure 13 represents the results of thermal conductivity of the lightweight cementitious tious composite specimens. Figure 14 shows an inverse relationship between the micro- tious composite specimens. Figure 14 shows an inverse relationship between the micro- composite specimens. Figure 14 shows an inverse relationship between the microstruc- structural homogeneity (as measured by the ultrasonic pulse velocity measurements) and structural homogeneity (as measured by the ultrasonic pulse velocity measurements) and tural homogeneity (as measured by the ultrasonic pulse velocity measurements) and the the thermal conductivity for the lightweight mixes. There were no significant differences the thermal conductivity for the lightweight mixes. There were no significant differences thermal conductivity for the lightweight mixes. There were no significant differences between the specimens in terms of the measured thermal conductivity values, and all the between the specimens in terms of the measured thermal conductivity values, and all the between the specimens in terms of the measured thermal conductivity values, and all the lightweight specimens tested exhibited relatively low thermal conductivity values (lower lightweight specimens tested exhibited relatively low thermal conductivity values (lower lightweight specimens tested exhibited relatively low thermal conductivity values (lower than 0.55 Wm/K). than 0.55 Wm/K). than 0.55 Wm/K). 0.6 0.6 0.5 [] [] 0.5 [] [] 0.4 [] 0.4 [] [] [] 0.3 0.3 0.2 0.2 0.1 0.1 RC LAC-1 LAC-2 LAC-3 RC LAC-1 LAC-2 LAC-3 Figure Figure 13. 13. Therm Thermal al conducti conductivity vity of of the mixes the mixes. . Figure 13. Thermal conductivity of the mixes. 0.6 0.6 0.5 0.5 LAC-3 LAC-2 LAC-3 LAC-2 0.4 0.4 LAC-1 RC LAC-1 RC 0.3 0.3 0.2 0.2 0.1 0.1 3 3.2 3.4 3.6 3.8 4 3 3.2 3.4 3.6 3.8 4 Ultrasonic Wave Velocities (km/s) Ultrasonic Wave Velocities (km/s) Figure 14. The thermal conductivity vs. the ultrasonic velocities of the mixes. Figure 14. The thermal conductivity vs. the ultrasonic velocities of the mixes. Figure 14. The thermal conductivity vs. the ultrasonic velocities of the mixes. The results also confirm that the addition of nano-silica particles has a beneficial effect The results also confirm that the addition of nano-silica particles has a beneficial ef- on the densification of the micro-structure accompanied by the lower porosity and a mini- The results also confirm that the addition of nano-silica particles has a beneficial ef- fect mal onincr the ease densific in the ation thermal of thconductivity e micro-structure . The enhanced accompanied inter by -particle the lower contact poros configuration ity and a fect on the densification of the micro-structure accompanied by the lower porosity and a minimal increase in the thermal conductivity. The enhanced inter-particle contact config- with the pore refinement by the addition of smaller size nano-silica can provide such minimal increase in the thermal conductivity. The enhanced inter-particle contact config- uration with the pore refinement by the addition of smaller size nano-silica can provide modification on the thermal performance of the lightweight mixes. uration with the pore refinement by the addition of smaller size nano-silica can provide such mod Figur ific eation on 15 is an the t X-ray herm CT image al perfo thr rmance o ough a lightweight f the lightw cementitious eight mixes. concr ete core taken such modification on the thermal performance of the lightweight mixes. from Figthe ureconcr 15 is ete an slab X-ray containing CT image 3%th nano-silica rough a lig particles. htweight It cem indicates entitiothat us con thecnano-silica rete core Figure 15 is an X-ray CT image through a lightweight cementitious concrete core clusters were well interconnected which further reducing their opportunity to convey heat taken from the concrete slab containing 3% nano-silica particles. It indicates that the nano- taken from the concrete slab containing 3% nano-silica particles. It indicates that the nano- silener ica cgy lust ef ers fectively were we thr ll ough interco the nnec composite ted which andfu may rther deliver reducia nconductivity g their opportun of 0.52 ity to W/m con-K. silica clusters were well interconnected which further reducing their opportunity to con- vey heat energy effectively through the composite and may deliver a conductivity of 0.52 vey heat energy effectively through the composite and may deliver a conductivity of 0.52 W/m K. W/m K. Th Th erma erma l l Condu Condu ctivit ctivit y y (W/mK) (W/mK) Th Th erma erma l l Condu Condu ctivit ctivit y y (W/mK) (W/mK) Buildings 2021, 11, 85 15 of 18 Buildings 2021, 11, x FOR PEER REVIEW 16 of 19 Figure 15. 3D X-ray micro-tomography views of the LAC-3 mix. Figure 15. 3D X-ray micro-tomography views of the LAC-3 mix. 3.5. 3.5. Bond Bond Str Streength ngth The The influence influence of of th the e n nano-silica ano-silica mo modification dification on on ththe e bobond nd strengt strength h of th ofe the light lightweight weight cementitious cementitious m mixes ixes is is sho shown wn in in Fi Figur guree 16. 16.Th The e addit addition ion ofof nano nano-silica -silica to th toe the cemen cementitious titious matrix resulted in an increase in the bond strength of the lightweight concrete. Compared matrix resulted in an increase in the bond strength of the lightweight concrete. Compared to to the reference mix, the 28 days bond strength improvement ratios of the samples includ- the reference mix, the 28 days bond strength improvement ratios of the samples including Buildings 2021, 11, x FOR PEER REVIEW 17 of 19 ing nano-silica particles with the proportions of 1%, 2%, and 3% by weight of cement were nano-silica particles with the proportions of 1%, 2%, and 3% by weight of cement were 4.91%, 51.7%, and 132.1%, respectively. 4.91%, 51.7%, and 132.1%, respectively. Two observations will be used to explain the significant improvement in the bond strength of the steel rebar and the nano-modified lightweight mixes. Firstly, it has been 8.00 observed that the depth of the projection of the particle on the ribbed steel bars was sig- nificant which, in turn, provides a significant resistance to slippage. It was also observed 7.00 that the spaces between the sand particles were not narrow due to the nano-silica densifi- 6.00 cation of the interfacial transition zone of the lightweight aggregate and the surrounded cement matrix. From these observations, it can be concluded that the space that is availa- 5.00 ble between the particles could accommodate a considerable amount of the nano-silica reinforced matrix to resist slippage. Moreover, it can also be plausibly presumed that in 4.00 the nano-modified lightweight composites, especially with the higher dosages, at the in- 3.00 terface between the lightweight concrete and the sand particles, the nano-silica would have reduced the contact between the concrete and the steel rebar. The mercury porosim- 2.00 etry test result presented in Section 3.3 also provides good support for this theory and indicates that the percentage of pore volume in the nano-modified lightweight mixes was 1.00 significantly lower than the reference mix. This reduction in contact would contribute to 0.00 a reduction in friction at the lightweight concrete and steel rebar interface resulting in an RC LAC-1 LAC-2 LAC-3 arrestor for the cracks formed in the cementitious matrix and contributed to improvement in resistance to bond failure. The highlighted factors collectively mitigated the effective- Figure 16. Bond strength test results of the mixes. ness of the nano-modified lightweight concrete in structural applications and obliterated Figure 16. Bond strength test results of the mixes. a significant improvement on the adherence between the ribbed steel bar and the concrete. Two observations will be used to explain the significant improvement in the bond 4. Conclusions strength of the steel rebar and the nano-modified lightweight mixes. Firstly, it has been ob- served that the depth of the projection of the particle on the ribbed steel bars was significant Based on the results obtained in this experimental and micro-structural study, the which, in turn, provides a significant resistance to slippage. It was also observed that the following conclusions can be summarized: spaces between the sand particles were not narrow due to the nano-silica densification of The increased integrity of the lightweight mix with nano-particle addition matrix im- the interfacial transition zone of the lightweight aggregate and the surrounded cement ma- proved the connected area and carrying capacity for the matrix materials that flow and trix. From these observations, it can be concluded that the space that is available between deform around the lightweight aggregate against the mechanical loading. the particles could accommodate a considerable amount of the nano-silica reinforced matrix Adding nano-silica particles into the lightweight mixes would increase the crystal- to resist slippage. Moreover, it can also be plausibly presumed that in the nano-modified linity of the mixes which, in turn, resulted in a higher micro-hardness value compared to lightweight composites, especially with the higher dosages, at the interface between the the reference mix. lightweight concrete and the sand particles, the nano-silica would have reduced the contact The higher fractal dimension values associated with the addition of nano-silica par- ticles revealed that the unsaturated bonds significantly reduced the hydrophobicity and static contact angle for the nano-silica improving the bond ability between them and hy- dration products and producing the toughening mechanism along the crack front and voids. Comparison of the transport properties of the lightweight concrete with nano-silica particles indicates that there is a consistently lower level of penetrability in the lightweight concrete with nano-silica over time. Nano-silica particles could have created a more tortuous path for heat and water flow, and hence the movement of substances in the ITZ of the irregular nano-silica parti- cles would serve to reduce the movement and minimize the transport mechanism. The refining and densification of the cementitious matrix with the addition of nano- silica make it less favor the migration of water and chloride ions from the outside to im- prove the flexural and compressive strengths and the thermal performance inside, and noticeably. This, in turn, increases the potential use of this type of cementitious compo- sites in building facades, marine environment, and bridge decks. Based on the bond behaviour results and larger Poisson’s ratio than normal weight concrete, it is possible to extend the use of the nano-modified lightweight cementitious composites in composite structural elements in high-rise buildings. The nano-modified cementitious lightweight concrete can be used in concrete-filled steel tube column. This would be particularly beneficial to decrease the affected seismic load. Author Contributions: Conceptualization, M.A.B., and S.E.; Methodology, M.A.B., S.E., and E.G.; Formal Analysis, M.A.B., S.E., and E.G.; Investigation, M.A.B. and S.E.; Resources, E.G.; Writing— Original Draft Preparation, M.A.B.; Writing—Review and Editing, M.A.B., S.E., and E.G.; Visuali- zation, S.E. All authors have read and agreed to the published version of the manuscript. Bond Strength (MPa) Buildings 2021, 11, 85 16 of 18 between the concrete and the steel rebar. The mercury porosimetry test result presented in Section 3.3 also provides good support for this theory and indicates that the percentage of pore volume in the nano-modified lightweight mixes was significantly lower than the reference mix. This reduction in contact would contribute to a reduction in friction at the lightweight concrete and steel rebar interface resulting in an arrestor for the cracks formed in the cementitious matrix and contributed to improvement in resistance to bond fail- ure. The highlighted factors collectively mitigated the effectiveness of the nano-modified lightweight concrete in structural applications and obliterated a significant improvement on the adherence between the ribbed steel bar and the concrete. 4. Conclusions Based on the results obtained in this experimental and micro-structural study, the following conclusions can be summarized: The increased integrity of the lightweight mix with nano-particle addition matrix improved the connected area and carrying capacity for the matrix materials that flow and deform around the lightweight aggregate against the mechanical loading. Adding nano-silica particles into the lightweight mixes would increase the crystallinity of the mixes which, in turn, resulted in a higher micro-hardness value compared to the reference mix. The higher fractal dimension values associated with the addition of nano-silica parti- cles revealed that the unsaturated bonds significantly reduced the hydrophobicity and static contact angle for the nano-silica improving the bond ability between them and hydration products and producing the toughening mechanism along the crack front and voids. Comparison of the transport properties of the lightweight concrete with nano-silica particles indicates that there is a consistently lower level of penetrability in the lightweight concrete with nano-silica over time. Nano-silica particles could have created a more tortuous path for heat and water flow, and hence the movement of substances in the ITZ of the irregular nano-silica particles would serve to reduce the movement and minimize the transport mechanism. The refining and densification of the cementitious matrix with the addition of nano- silica make it less favor the migration of water and chloride ions from the outside to improve the flexural and compressive strengths and the thermal performance inside, and noticeably. This, in turn, increases the potential use of this type of cementitious composites in building facades, marine environment, and bridge decks. Based on the bond behaviour results and larger Poisson’s ratio than normal weight concrete, it is possible to extend the use of the nano-modified lightweight cementitious composites in composite structural elements in high-rise buildings. The nano-modified cementitious lightweight concrete can be used in concrete-filled steel tube column. This would be particularly beneficial to decrease the affected seismic load. Author Contributions: Conceptualization, M.A.B., and S.E.; Methodology, M.A.B., S.E., and E.G.; Formal Analysis, M.A.B., S.E., and E.G.; Investigation, M.A.B. and S.E.; Resources, E.G.; Writing— Original Draft Preparation, M.A.B.; Writing—Review and Editing, M.A.B., S.E., and E.G.; Visualiza- tion, S.E. All authors have read and agreed to the published version of the manuscript. Funding: The authors would like to thanks to the University Technology of Jamaica for the support. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: The data presented in this study are available on request from the corresponding author. Conflicts of Interest: The authors declare no conflict of interest. Buildings 2021, 11, 85 17 of 18 References 1. 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