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Control of the setting reaction and strength development of slag-blended volcanic ash-based phosphate geopolymer with the addition of boric acid

Control of the setting reaction and strength development of slag-blended volcanic ash-based... This work aimed to evaluate the role of the addition of blast furnace slag for the formation of reaction products and the strength development of volcanic ash-based phosphate geopolymer. Volcanic ash was replaced by 4 and 6 wt% of ground granulated blast furnace slag to accelerate the reaction kinetics. Then, the influence of boric acid for controlling the setting and kinetics reactions was also evaluated. The results demonstrated that the competition between the dissolution of boric acid and volcanic ash-slag particles is the main process controlling the setting and kinetics reaction. The addition of slag has significantly accelerated the initial and final setting times, whereas the addition of boric acid was beneficial for delaying the setting times. Consequently, it also enhanced the flowability of the paste. The compressive strength increased significantly with the addition of slag, and the optimum replaced rate was 4 wt% which resulted in 28 d strength of 27 MPa. Beyond that percentage, the strength was reduced because of the flash setting of the binder which does not allow a subsequent dissolution of the particles and their precipitation. The binders formed with the addition of slag and/or boric acid are beneficial for the improvement of the water stability of the volcanic ash-based phosphate geopolymer. . . . . Keywords Volcanic ash Phosphoric acid Blast furnace slag Boric acid Microstructure Introduction along with their amorphous polymeric structure, APC should be called phosphate geopolymer [7]. In general, the reaction Aluminosilicate phosphate cement (APC) or binder is a type product of acid phosphate-activated aluminosilicate is amor- of acid-base cement belonging to the chemically bonded phos- phous. This helps distinguish them from other CBPC made of phate cement (CBPC) group obtained at room or slightly ele- metal oxide whose reaction products are crystallized. vated temperature by a chemical reaction between a solid pre- However, depending on the synthesis conditions (curing con- cursor (aluminosilicate powder) and an acid phosphate [1, 2]. dition or addition of divalent metal oxides), semi-crystalline The reaction products as demonstrated in the literature are reaction products including struvite, monetite, brushite, and generally amorphous with a polymeric-like structure berlinite can be detected [8–10]. consisting mainly of repeating units of silicophosphate, The interest of developing phosphate cement relies on its aluminophosphate, silico-aluminophosphate, ferro-silico- good early properties that make them suitable for use as a aluminophosphate, and etc. [3–6]. Wagh suggested that be- rapid repair material for the restoration of concrete struc- cause of the 3-dimensional structure of some of those phases tures, and the waste management by the encapsulation of heavy metals [1, 2, 11]. The synthesis of APC that sets and hardens in a shorter time at room temperature depends mainly on the chemical composition of the aluminosilicate, * Jean Noël Yankwa Djobo noeldjobo@gmail.com; noel.djobo@campus.tu-berlin.de the type, and the dosage of the phosphate solution [3, 12, 13]. The initial setting time of metakaolin phosphate cement ranges from 20 h to several days depending on the reactivity Building Materials and Construction Chemistry, Technische Universität Berlin, Gustav-Meyer-Allee 25, 13355 Berlin, Germany of metakaolin [13, 14], while volcanic ash-based phosphate geopolymer has an initial setting time of few minutes at Local Materials Promotion Authority (MIPROMALO), MINRESI, 2396, Nkolbikok, Yaoundé, Cameroon room temperature [3]. However, not all volcanic ashes can 1146 J Aust Ceram Soc (2021) 57:1145–1154 display such a fast setting at room temperature because of Volcanic ash contains magnetite, hematite, anorthite the difference in their chemical and mineralogical compo- sodian, forsterite ferroan, and augite titanian as the main sition. It was demonstrated that the rapid setting of volcanic minerals while the blast furnace slag is entirely X-ray ash-based phosphate geopolymer is due to the availability amorphous. The final composition of phosphoric acid 2+ of divalent metals such as calcium (Ca )and magnesium used for the synthesis of the phosphate geopolymer binder 2+ (Mg ) in the initial composition [3]. These divalent metal is 35 wt%. It was obtained after dilution of the analytical ions are known to be useful for accelerating the setting re- grade of orthophosphoric acid 85 wt% (VWR action because of their high affinity with acid phosphate International GmbH, Germany). Boric acid with 99% pu- [15, 16]. Moreover, the reactivity of these divalent metal rity from VWR International GmbH, Germany was used oxides depends upon their degree of crystallinity. That as a setting retarder. means if it is too reactive (high content of amorphous phase), the setting will be too fast and will not allow proper Preparation of the phosphate cement dissolution and precipitation, which is necessary for the de- velopment of products with high strength and good durabil- Two series of mixes were carried out. The first one consists of ity [16–18]. On the other hand, if the metal oxide is less VA replaced with GGBFS at different rates, while the second reactive (low solubility in acid), this will delay the setting mix is composed of the previous ones with the addition of and strength development, so the control of the hardening boric acid (H BO ) as a setting retarder. The paste was obtain- 3 3 reaction should be considered when producing phosphate ed by first mixing VA and GGBFS for 2 min, and then the cement. The addition of boric acid or borax at different phosphoric acid (PA) solution is added with PA to the solid percentages (borax/MgO mass ratio up to 40%) was report- powder mass ratio of 0.45 and further mixed for 3 min using a ed effective for controlling hardening reaction [19–22]. KitchenAid. The details of different mix designs are summa- That acid performed well on delaying the initial setting time rized in Table 2. The formulation including VA + H PO 3 4+ of magnesium phosphate cement by forming a protective H BO was dismissed as the hardening time of that mix is 3 3 layer on particles of MgO, increasing the pH, and decreas- already long (3 days), so it is not relevant to retard its setting ing the temperature of the paste [21]. It is worth pointing out reaction with boric acid. Once the different pastes were ob- that the development and the understanding of the phos- tained, they were poured into 20-mm cubic molds and covered phate geopolymer are still at the early stage since very few with a plastic foil to prevent the rapid evaporation of water. papers have been published on the topic. Elsewhere the Then, the samples were stored at 20 °C and 65% relative potential of using boric acid for controlling the setting and humidity for at least 24 h before demolding and stored again kinetic reaction of phosphate geopolymers has not yet been in a plastic bag in the same conditions until the different tests investigated. were performed. This work aims at determining the reaction formation of low reactive volcanic ash-based phosphate cement modified Characterization of reaction products by blast furnace slag as an accelerator, and to unveil the mech- anism of the reaction of boric acid for controlling the setting The initial and final setting times were determined using an reaction phosphate geopolymers. automatic Vicat apparatus (Toni Technik GmbH, Berlin, Germany) according to EN 196–3. The formation of reaction products from blended volcanic Experimental methods ash-based phosphate geopolymer with and without boric acid was monitored by measuring the heat flow evolution, cumu- Materials lative heat of reaction, and rheological properties. The Isothermal Conduction Calorimetry (TAM Air 3 calorimeter, Volcanic ash (VA) was collected in the Cameroon volca- TA Instruments, New Castle, DE, USA) was performed at nic line and ground to the final particles size ≤200 μm. 20°C for 24 h. For this test, 7 g of powder was mixed exter- OPTERRA Zement GmbH, Germany supplied ground nally with phosphoric acid (according to the mix proportion in granulated blast furnace slag (GGBFS). The mean diam- Table 2) for 3 min using a vortex mixer, then immediately put eter determined by laser granulometry (Mastersizer 2000 in the calorimeter. from Malvern Instruments, Worcestershire, UK) is The rheological properties of the binder were measured 24.136 μm and 16.046 μm for VA and GGBFS respec- with a dynamic rheometer (Haake Rheometers, Thermo tively. Their chemical composition determined by X-ray Fisher Scientific, Karlsruhe, Germany). The flow curves were fluorescence (PW 2400 PHILIPS instrument, Eindhoven, recorded in rotational motion using Cylinder Sensor Systems the Netherlands) is reported in Table 1. The mineralogical (Z20DIN, composed of a mobile rotor and a static beaker). composition of both aluminosilicates is shown in Fig. 1. The test setup includes a pre-shearing phase to homogenize J Aust Ceram Soc (2021) 57:1145–1154 1147 Table 1 Chemical composition of volcanic ash and GGBF slag (wt%) Oxides SiO Al O Fe O CaO MgO NaOTiO KOMnO P O LOI Total 2 2 3 2 3 2 2 2 2 5 VA 41.7 16.0 13.5 9.3 8.18 2.42 3.01 1.34 0.21 0.89 2.61 99.1 GGBFS 38.0 10.8 0.3 40.1 7.24 0.31 0.83 0.64 0.17 0.02 – 98.4 the paste in the sample holder, followed by the measurement Results and discussion of the shear stress vs. shear rate. The pre-shearing phase con- −1 sists of rising the shear rate from 0 to 100 s within 30 s and Setting times −1 decreasing it from 100 to 0 s in the same period, followed by a resting time of 30 s. The subsequent measuring phase The influence of the slag replacement rate and addition of consisted of an ascending ramp, increasing the shear rate from boric on the setting times are reported in Table 2. The initial −1 0to100 s within 60 s followed by a sloping ramp from 100 setting time (IST) and final setting times (FST) of the phos- −1 to 0 s within 90 s. The measurements were performed at phate geopolymer with 100% volcanic ash are 6.5 h and 24 h 20°C, and for each test, 5 g of paste was used. respectively. When volcanic ash was replaced by 4 and 6 wt% The compressive strength evolution was measured at 3, 7, slags, the IST and FST are shortened significantly. The mix and 28 d according to EN 196–1 by using a compression with 6 wt% of slag replacement was too fast to be measured, testing machine (Toni Technik, Berlin, Germany). and only the final setting time was measured and is 0.16 h The changes in the mineralogy are determined from the (10 min). This may be due to the preferential fast dissolution Empyrean PANalytical diffractometer (Malvern Panalytical of calcium from slag and its precipitation with the available Ltd., Malvern, UK) with Ni filter CuKα radiation (k = phosphate species that accelerate the setting reaction [3, 23]. 1.540598 Å). The binder was characterized by measuring When boric acid was added, the IST and FST were delayed to the mass loss and phase transformation while the temperature 4.5 h and 18 h respectively for the mix with 4 wt%. For the rises by using thermal gravimetry analysis and differential mix with 6 wt% slags, the initial and final setting times were scanning calorimetry (3+ SARe System, Mettler Toledo, also delayed to 0.08 h (5 min) and 0.90 h (55 min). In litera- Columbus, OH, USA). The test was performed in the temper- ture, it was reported that the retardation effect of boric acid in ature range 25–1000°C at a heating rate 5 K/min with synthet- the hydration of magnesium phosphate cement depends upon ic air flowing at 70 mL/min. the dosage in the mix. With a lower dosage, it mainly acts on The microstructure and the chemistry of the binder were hindering the precipitation reaction (delay of the initial setting 2+ determined by using a scanning electron microscope (SEM) time) by stabilizing Mg ions with the formation of the com- + 2+ coupled with energy-dispersive X-ray spectroscopy (Zeiss plex MgB(OH) which makes the Mg ions less available to Gemini SEM 500 NanoVP microscope, Oberkochen, react with phosphate species [20], while for a higher dosage, it Germany). The device used is equipped with a backscattered rather delays the dissolution reaction (delay of the final setting electron detector (BSD) that operates at low-vacuum mode time) [22]. In this work, the initial and final setting times were with 15 kV acceleration voltage. delayed with the presence of boric acid. Therefore, one can Fig. 1 X-ray patterns of a volcanic ash and b ground blast furnace slag 1148 J Aust Ceram Soc (2021) 57:1145–1154 Table 2 Details of the different VA (wt%) GGBFS (wt%) Mass ratio Boric acid/ Initial setting Final setting mix designs and setting times (VA+GGBFS) (wt%) time (h) time (h) Phosphoric acid/ (VA+GGBFS) 100 0 0.45 0 6.50 24 96 4 0.45 0 0.17 0.70 94 6 0.45 0 flash 0.16 96 4 0.45 5 4.50 18 94 6 0.45 5 0.08 0.92 say that the inhibition of both dissolution and precipitation of temperature of up to 95°C depending on the content of the volcanic ash slag during the reaction is responsible for the reagents [27]. Knowing that the dissolution of silica and alu- delaying of the setting time. mina is lower at room temperature, it is evident that the addi- tional heat supplied by slag dissolution has improved the dis- solution rate of volcanic ash [5, 12, 18, 28]. Reaction kinetics The addition of boric acid significantly decreased the height of the exothermic peak (Fig. 2b). A similar decrease The effects of slag and boric acid on the reaction kinetics of is also observed in the total heat released after 24 h. The total the formation of volcanic ash phosphate geopolymer binder heat achieved after 24 h (Fig. 3a) by the slag-blended volcanic are depicted in Figs. 2 and 3. The heat flow curves (Fig. 2) ash phosphate geopolymer binder is 83 J/g, 107 J/g, and 114 J/ display a single exothermic peak whose maximum is reached g for samples with 0, 4, and 6 wt% of slag, respectively. With within 6 min after mixing. Similar behavior was also observed the addition of boric acid, the total heat released after 24 h in an acid phosphate-activated high-calcium fly ash where the (Fig. 3b) is now 74 J/g, 91 J/g, and 101 J/g for samples with 0, maximum peak of the heat evolution was reached within 2– 4, and 6 wt% respectively. This trend translates that the addi- 4min [24]. In the case of volcanic ash phosphate geopolymer tion of boric acid has decreased the rate and extent of the binder, the exothermic peak corresponds to an increase in the dissolution-precipitation reaction. This corroborates the re- reaction temperature [25]. This corresponds to the dissolution sults of the initial and final setting times. Such behavior can of the initial raw materials and the precipitation of the dis- be explained by the fact that when all components are mixed, solved species to give the reaction products [3]. When slag there is a competition between the dissolution of boric acid is added to the mix, the heat of the reaction rises significantly (endothermic process) and the dissolution of volcanic ash slag with the increase of the slag content. That change is related to (exothermic process). It is worth noting that the reaction be- the additional heat provided by the dissolution of slag, which tween boric acid (H BO ) and phosphoric acid (H PO )gives 3 3 3 4 is rich in calcium. The latter was reported as other divalent rise to boron phosphate (BPO )and water [29]. This means in metal oxides to generate excessive heat when mixed with an our case that part of the phosphoric acid was consumed for the acid phosphate [26]. This sometimes leads to a reaction Fig. 2 Effect of blast furnace slag (a)and boric acid (b) on the heat flow evolution of slag-volcanic ash phosphate geopolymer binder J Aust Ceram Soc (2021) 57:1145–1154 1149 Fig. 3 Effect of blast furnace slag (a)and boric acid (b) on the total heat evolution of slag-volcanic ash-based phosphate geopolymer binder dissolution of boric acid, which limits the availability of pro- shear rate in the non-Newtonian fluid. It is worth noting tons (H ) that requires for the subsequent dissolution of vol- that for a non-Newtonian fluid, when n < 1, it means that canic ash and slag. Therefore, it contributes to the decrease in the paste behaves like a shear-thinning fluid and when reaction temperature or heat [19, 21, 22]. However, no delay n > 1, it is a shear-thickening fluid [30, 31]. In Table 3, was observed in the time for reaching the peak maximum of the fluidity index is lower than 1, indicating that the the heat released during the reaction. This indicates that the blended slag-volcanic ash-based phosphate geopolymer boric acid in the current system does not retard the reaction pastes behave like a shear-thinning fluid. This implies that process but rather inhibits the dissolution-precipitation. This the viscosity of the pastes increases with the decrease of occurs because, in addition to the limited availability of pro- the shear rate. This observation also holds for the mixes tons (H ) for the dissolution of the aluminosilicate, there is a with boric acid. However, the fluidity index of blended lack of sufficient phosphate species for undergoing the precip- slag-volcanic ash-based phosphate geopolymer pates re- itation, so there is a reduction in the yield of the reaction mains lower than those with boric acid and decreases with products formed at the end of the process. That can be as- the slag content, so the addition of boric acid is beneficial sumed as the main mechanism for controlling the setting re- for lowering the viscosity of the paste. This agrees with action and the reaction kinetics of volcanic ash-slag phosphate the literature where an improvement of the flowability of geopolymer binder with boric acid. the paste of magnesium potassium phosphate cement with increasing dosage of boric acid was observed [21]. Rheological properties Compressive strength development Figure 4 shows one hysteresis cycle (shear stress vs. shear rate) of different mixes. The shear stress increases non- The results of the compressive strength evolution in dry con- linearly with the shear rate, so the rheological properties were ditions and after immersion in water for 24 h are depicted in computerized by applying the non-linear fitting on the down Fig. 5. The compressive strength increases over time and is ramp curve using the Herschel-Bulkley function. The latter optimal when volcanic ash is replaced by 4 wt% of slag. consists of τ = τ +Kɣ (where τ is the shear stress (Pa), ɣ is Further increase in the replacement rate leads to a decrease ̇ ̇ the shear rate (1/s), τ is the yield stress (Pa), n is the fluidity in the compressive strength. After immersion into water for index, and K is the consistency coefficient (Pa s)). The yield 24 h, the compressive strength drops sharply by up to 40.3%. stress (τ ) is the minimum force that requires to apply on the However, the mixes with slag show the maximum 28 d paste to flow [30]. In Table 3, reporting the rheological prop- strength loss of 36.4% and 24.7% respectively with 4 wt% erties, the yield stress increases with slag content even after the and 6 wt% of volcanic ash replacement. This demonstrates addition of boric acid. This illustrates the high interaction that the addition of slag is beneficial for improving the water among dissolved species in the mixes and the acceleration of stability of the volcanic ash phosphate geopolymer binder the hardening. when compared with the reference sample. That is because The flow index (n) decreases with increasing the slag slag also takes part in the reaction by consuming some proton content. It describes the dependence of the viscosity to (H ) for its dissolution and the dissolved calcium condenses 1150 J Aust Ceram Soc (2021) 57:1145–1154 Table 3 Rheological properties VA GGBFS Boric acid/ Yield stress, Fluidity of slag-blended volcanic ash (wt%) (wt%) (VA+GGBFS) τ (Pa) index, n phosphate cement 0 (wt%) 100 0 0 0.393 0.856 96 4 0 0.664 0.843 94 6 0 1.426 0.834 96 4 5 1.032 0.896 94 6 5 1.358 0.847 with the phosphate ions to form water-stable reaction phases replacement. This result correlates with the inhibitor role [12, 32–34], whereas the flash reaction of calcium with acid played by boric acid in the setting time and reaction phosphate does not allow a good subsequent dissolution of kinetics of the slag-blended volcanic ash-based phos- volcanic ash when replaced by a higher content of slag. It is phate geopolymer as shown in this work (see Figs. 2 worth noting that the calcium-containing phases are preferen- and 3). Indeed, the competition between the dissolution tially dissolved in phosphoric acid at the early reaction stage of boric acid and slag-volcanic ash hinders the forma- and precipitate with the phosphate species to form amorphous tion of high-volume reaction products. Consequently, calcium phosphate phases [9, 24], so the addition of slag will the compressive strength of the slag blended is also induce the formation of a high amount of calcium phosphate diminished. Moreover, the water stability seems to be phase. The latter was described as responsible of the flash better than the mixes without boric acid, since the max- initial setting time reported in Table 2 for phosphate binder imum strength loss here is around 22%. Thus, it can be with 6 wt% of slag. Furthermore, it was reported that alumi- suggested that boric acid has taken part in the reaction num and iron are the main elements contributing to the formation of the binder and that a new phase including strength development of volcanic ash phosphate binder [3], boron would have been formed. This will be ascertained so the limitations in the availability of these elements during and discussed further in the next sections of this paper. the reaction process will hinder the compressive strength development. Mineralogy The effect of the addition of boric acid on the strength development of slag-blended volcanic ash- Figure 7 showsthe X-raypatternsinthe rangeof5–65° based phosphate geopolymer is depicted in Fig. 6.The 2θ of binders obtained with 4 wt% of slag and/or with the 28 d compressive strength is the most affected by the addition of boric acid. It should be noted that beyond that addition of boric acid, with a compressive strength loss range, there is no more additional information from the (as regards to the strength of the mixes without boric spectra. All the initial minerals present in volcanic ash are acid, Fig. 5) of 46.4% and 36% respectively for the still observed in the different binders. The main changes mixes with 4 wt% and 6 wt% of volcanic ash are the diminution of the peak intensity of some minerals Fig. 4 Flow curves of slag blended (a) and mixed with boric acid (b) J Aust Ceram Soc (2021) 57:1145–1154 1151 Fig. 5 Compressive strength evolution of slag-blended volcanic ash- based phosphate geopolymer (a), after immersion in water for 24 h prior Fig. 7 XRD patterns of volcanic ash (a), volcanic ash-based phosphate measurement (b) geopolymer (b), volcanic ash-based phosphate geopolymer blended with 4 wt% slag (c) volcanic ash-based phosphate geopolymer blended with or their disappearance and the formation of the broad 4 wt% slag, and boric acid (d) hump in synthesized cement paste in the range of 20– 40°. That hump is characteristic of amorphous reaction as observed in Fig. 8. Slag shows a very low mass loss products. This indicates that minerals have taken part in starting at room temperature up to 650°C corresponding the reaction, as demonstrated in previous works [3]. The to the removal of residual water both physically and reaction products were then characterized by thermal anal- chemically bonded. This is followed by mass gain, which ysis as the XRD cannot detect X-ray amorphous phases. It can be identified in the DSC curve (700–850°C) as an is worth recalling that the amorphous phase is not stable exothermic peak and is due to the oxidation process of at elevated temperatures, so the rise of temperature could sulphites. Volcanic ash and phosphate geopolymer show a trigger their crystallization which can indicate the type of single mass loss which is accompanied by an endothermic phase or its composition. peak with maximum appearing before 100°C. That corre- The thermal analysis shows the differences in the mass sponds to the removal of different types of water [4]. In change and heat flow with the increase of the temperature the phosphate cement, a very weak exothermic peak at around 650°C is identified which might be attributed to the transformation of amorphous iron phosphate phases from the binder into ferrous/ferric phosphate minerals [4]. Microstructure and phase composition The SEM images of phosphate geopolymer with 4 wt% slag and/or boric acid are depicted in Fig. 9. The micro- structure of all samples is heterogeneous with the pres- ence of a few cracks. The EDX analysis of different sam- ples reported in Table 4 helped to observe the diversity of the composition of the phases in different mixes. The asterisk put on the boron in Table 4 is to recall that the quantification of boron with EDX measurement is not accurate because it is a light element, so the molar ratio B/P presented in Table 4 is only for the qualitative pur- posetoshowhow boronreactsinthesystem.Inthesam- ples without any additives, 3 distinct phases named A, B, Fig. 6 Compressive strength evolution of blended slag-volcanic ash- and C were identified. The phase marked as A in the based phosphate geopolymer with the addition of boric acid (a), after immersion in water for 24 h prior measurement (b) micrograph is silicon-rich corresponding to unreacted 1152 J Aust Ceram Soc (2021) 57:1145–1154 Fig. 8 Thermogravimetry and differential scanning calorimetry of volcanic ash, slag, and slag-blended volcanic ash-based phosphate geopolymer particles of volcanic ash [3]. The phases B and C corre- be either physically or chemically bonded. Other new phases spond to the newly formed binder. The composition of named D and E are present. Their chemical composition re- phase B shows that it is silicon and aluminum-free phase, vealed that in phase D, apart from all elements observed in while phase C is a mix-up of all elements present in vol- phase C, there is also boron with high content of phosphorus. canic ash in which atomic ratios with phosphorus allow This phase could result from the partial reaction of boric acid its identification as the main binder phase. The latter was with the dissolved species during the formation of phase C. identified in all samples. The fact that the binders B and C The consequence of this is the lowering of the yield of reac- are shapeless confirms that the reaction product is amor- tion as discussed in the setting time and reaction kinetic in the phous, as shown in the XRD. previous section.This agrees withtheprevious statement and The addition of slag leads to the formation of a single confirms the mechanism of the reaction of boric acid as de- binding phase besides the unreacted particles. When boric scribed in this paper. The micrographs of the binder with acid is added to the previous mix, the resulting sample dis- 4 wt% slag + boric acid clearly show segregation between plays a new unreacted phase marked as AA whose composi- phase C and other phases, with a well-defined boundary. All tion includes boron with very low content of phosphorus. these observations demonstrate that boric acid partly reacted This suggests a possible formation of an intermediate with volcanic ash and slag while another part contributes to unreacted phase of boron-volcanic ash particles which could the formation of a new binder. Table 4 EDX analysis of different phases identified in volcanic ash phosphate cement blended with 4 wt% slag and/or boric acid Samples Atomic ratios Area Phases Si/P Al/P Fe/P Ca/P Mg/P Ti/P Na/P K/P B /P 0 wt% slag 11.46 5.85 1.86 1.51 0.48 0.43 2.85 3.66 0 A Unreacted particles 0 0 1.08 0.20 2.63 0.20 0.17 0.05 0 B Binder 1 0.73 1.37 2.81 0.66 0.43 0.58 0.30 0.10 0 C Binder 2 4 wt% slag 10.28 1.16 11.57 1.35 16.58 0.15 0.162 0.16 0 A Unreacted particles 1.88 1.53 3.94 1.73 1.08 0.62 0.37 0.16 0 C Binder 2 4 wt% slag + boric acid 6.35 1.63 2.91 4.50 6.22 1.84 1.28 0.81 1.73 AA Unreacted phase 3.90 3.40 15.65 0.82 0.93 3.87 0.37 0.32 0 A Unreacted particles 1.09 1.37 2.88 1.70 0.87 0.63 0.42 0.18 0 C Binder 2 1.55 1.30 2.38 1.32 0.58 0.26 0.35 0.17 0.68 D Binder 3 0 0.87 2.59 1.44 0.21 0.33 0.19 0.06 0.04 E Binder 4 J Aust Ceram Soc (2021) 57:1145–1154 1153 Fig. 9 Backscattered SEM images at different magnifications (×450, ×1000, and ×2000) of volcanic ash-based phosphate geopolymer binder Conclusion Acknowledgements The technical assistance of Mr. Tobias Dorn and David Dahncke for collecting experimental test results is appreciated. The work has reported the role of blast furnace slag and boric acid addition on the reaction and strength development of Author contribution Jean Noel Yankwa Djobo: research design, acqui- volcanic ash phosphate cement. The main findings are sum- sition, analysis and interpretation of data, and drafting of the paper. marized as follows: Dietmar Stephan: advisor, critical revision and editing, and approval of the submitted and final versions. – The partial substitution of volcanic ash with blast furnace Funding Open Access funding enabled and organized by Projekt DEAL. slag speeds up the reaction kinetics of volcanic ash phos- The authors received financial support from the Alexander von Humboldt phate geopolymer binders. Foundation through the Georg Foster Postdoctoral Fellowship (CM- – The addition of boric acid mainly contributes to the inhi- 1201499-GF-P) awarded to Dr. Djobo. bition of the dissolution-precipitation reaction by con- suming phosphate species. Declarations – Although boric acid succeeded to delay the setting time Conflict of interest The authors declare no competing interests. and improves the fluidity of the paste, it was found detri- mental for strength development. Open Access This article is licensed under a Creative Commons – The compressive strength was significantly increased Attribution 4.0 International License, which permits use, sharing, adap- with the replacement of volcanic ash with blast furnace tation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, pro- slag, while the incorporation of boric acid slows down the vide a link to the Creative Commons licence, and indicate if changes were compressive strength development. made. The images or other third party material in this article are included – The reaction products formed with the addition of slag in the article's Creative Commons licence, unless indicated otherwise in a and/or boric acid contribute to the improvement of the credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by water stability of volcanic ash phosphate cement. statutory regulation or exceeds the permitted use, you will need to obtain – The microstructure is composed of various types of permission directly from the copyright holder. To view a copy of this binders including boron as proof of the partial reaction licence, visit http://creativecommons.org/licenses/by/4.0/. of boric acid with reacted phases from volcanic ash and blast furnace slag. 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Wang, Y.-S., Dai, J.-G., Ding, Z., Xu, W.-T.: Phosphate-based metakaolin on properties and microstructure of magnesium phos- geopolymer: formation mechanism and thermal stability. Mater Lett. phate cement. Constr Build Mater. 234, 117353 (2020). https://doi. 190, 209–212 (2017). https://doi.org/10.1016/j.matlet.2017.01.022 org/10.1016/j.conbuildmat.2019.117353 16. Wagh, A.S., Grover, S., Jeong, S.Y.: Chemically bonded phosphate 34. Xu, B., Lothenbach, B., Ma, H.: Properties of fly ash blended mag- ceramics: II, warm-temperature process for alumina ceramics. J Am nesium potassium phosphate mortars: effect of the ratio between fly Ceram Soc. 86,1845–1849 (2003). https://doi.org/10.1111/j.1151- ash and magnesia. Cem Concr Compos. 90, 169–177 (2018). 2916.2003.tb03570.x https://doi.org/10.1016/j.cemconcomp.2018.04.002 17. Wagh, A.S., Jeong, S.Y.: Chemically bonded phosphate ceramics: III, reduction mechanism and its application to iron phosphate ce- ramics. J Am Ceram Soc. 86,1850–1855 (2003). https://doi.org/10. Publisher’snote Springer Nature remains neutral with regard to jurisdic- 1111/j.1151-2916.2003.tb03571.x tional claims in published maps and institutional affiliations. http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Journal of the Australian Ceramic Society Springer Journals

Control of the setting reaction and strength development of slag-blended volcanic ash-based phosphate geopolymer with the addition of boric acid

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10.1007/s41779-021-00610-4
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Abstract

This work aimed to evaluate the role of the addition of blast furnace slag for the formation of reaction products and the strength development of volcanic ash-based phosphate geopolymer. Volcanic ash was replaced by 4 and 6 wt% of ground granulated blast furnace slag to accelerate the reaction kinetics. Then, the influence of boric acid for controlling the setting and kinetics reactions was also evaluated. The results demonstrated that the competition between the dissolution of boric acid and volcanic ash-slag particles is the main process controlling the setting and kinetics reaction. The addition of slag has significantly accelerated the initial and final setting times, whereas the addition of boric acid was beneficial for delaying the setting times. Consequently, it also enhanced the flowability of the paste. The compressive strength increased significantly with the addition of slag, and the optimum replaced rate was 4 wt% which resulted in 28 d strength of 27 MPa. Beyond that percentage, the strength was reduced because of the flash setting of the binder which does not allow a subsequent dissolution of the particles and their precipitation. The binders formed with the addition of slag and/or boric acid are beneficial for the improvement of the water stability of the volcanic ash-based phosphate geopolymer. . . . . Keywords Volcanic ash Phosphoric acid Blast furnace slag Boric acid Microstructure Introduction along with their amorphous polymeric structure, APC should be called phosphate geopolymer [7]. In general, the reaction Aluminosilicate phosphate cement (APC) or binder is a type product of acid phosphate-activated aluminosilicate is amor- of acid-base cement belonging to the chemically bonded phos- phous. This helps distinguish them from other CBPC made of phate cement (CBPC) group obtained at room or slightly ele- metal oxide whose reaction products are crystallized. vated temperature by a chemical reaction between a solid pre- However, depending on the synthesis conditions (curing con- cursor (aluminosilicate powder) and an acid phosphate [1, 2]. dition or addition of divalent metal oxides), semi-crystalline The reaction products as demonstrated in the literature are reaction products including struvite, monetite, brushite, and generally amorphous with a polymeric-like structure berlinite can be detected [8–10]. consisting mainly of repeating units of silicophosphate, The interest of developing phosphate cement relies on its aluminophosphate, silico-aluminophosphate, ferro-silico- good early properties that make them suitable for use as a aluminophosphate, and etc. [3–6]. Wagh suggested that be- rapid repair material for the restoration of concrete struc- cause of the 3-dimensional structure of some of those phases tures, and the waste management by the encapsulation of heavy metals [1, 2, 11]. The synthesis of APC that sets and hardens in a shorter time at room temperature depends mainly on the chemical composition of the aluminosilicate, * Jean Noël Yankwa Djobo noeldjobo@gmail.com; noel.djobo@campus.tu-berlin.de the type, and the dosage of the phosphate solution [3, 12, 13]. The initial setting time of metakaolin phosphate cement ranges from 20 h to several days depending on the reactivity Building Materials and Construction Chemistry, Technische Universität Berlin, Gustav-Meyer-Allee 25, 13355 Berlin, Germany of metakaolin [13, 14], while volcanic ash-based phosphate geopolymer has an initial setting time of few minutes at Local Materials Promotion Authority (MIPROMALO), MINRESI, 2396, Nkolbikok, Yaoundé, Cameroon room temperature [3]. However, not all volcanic ashes can 1146 J Aust Ceram Soc (2021) 57:1145–1154 display such a fast setting at room temperature because of Volcanic ash contains magnetite, hematite, anorthite the difference in their chemical and mineralogical compo- sodian, forsterite ferroan, and augite titanian as the main sition. It was demonstrated that the rapid setting of volcanic minerals while the blast furnace slag is entirely X-ray ash-based phosphate geopolymer is due to the availability amorphous. The final composition of phosphoric acid 2+ of divalent metals such as calcium (Ca )and magnesium used for the synthesis of the phosphate geopolymer binder 2+ (Mg ) in the initial composition [3]. These divalent metal is 35 wt%. It was obtained after dilution of the analytical ions are known to be useful for accelerating the setting re- grade of orthophosphoric acid 85 wt% (VWR action because of their high affinity with acid phosphate International GmbH, Germany). Boric acid with 99% pu- [15, 16]. Moreover, the reactivity of these divalent metal rity from VWR International GmbH, Germany was used oxides depends upon their degree of crystallinity. That as a setting retarder. means if it is too reactive (high content of amorphous phase), the setting will be too fast and will not allow proper Preparation of the phosphate cement dissolution and precipitation, which is necessary for the de- velopment of products with high strength and good durabil- Two series of mixes were carried out. The first one consists of ity [16–18]. On the other hand, if the metal oxide is less VA replaced with GGBFS at different rates, while the second reactive (low solubility in acid), this will delay the setting mix is composed of the previous ones with the addition of and strength development, so the control of the hardening boric acid (H BO ) as a setting retarder. The paste was obtain- 3 3 reaction should be considered when producing phosphate ed by first mixing VA and GGBFS for 2 min, and then the cement. The addition of boric acid or borax at different phosphoric acid (PA) solution is added with PA to the solid percentages (borax/MgO mass ratio up to 40%) was report- powder mass ratio of 0.45 and further mixed for 3 min using a ed effective for controlling hardening reaction [19–22]. KitchenAid. The details of different mix designs are summa- That acid performed well on delaying the initial setting time rized in Table 2. The formulation including VA + H PO 3 4+ of magnesium phosphate cement by forming a protective H BO was dismissed as the hardening time of that mix is 3 3 layer on particles of MgO, increasing the pH, and decreas- already long (3 days), so it is not relevant to retard its setting ing the temperature of the paste [21]. It is worth pointing out reaction with boric acid. Once the different pastes were ob- that the development and the understanding of the phos- tained, they were poured into 20-mm cubic molds and covered phate geopolymer are still at the early stage since very few with a plastic foil to prevent the rapid evaporation of water. papers have been published on the topic. Elsewhere the Then, the samples were stored at 20 °C and 65% relative potential of using boric acid for controlling the setting and humidity for at least 24 h before demolding and stored again kinetic reaction of phosphate geopolymers has not yet been in a plastic bag in the same conditions until the different tests investigated. were performed. This work aims at determining the reaction formation of low reactive volcanic ash-based phosphate cement modified Characterization of reaction products by blast furnace slag as an accelerator, and to unveil the mech- anism of the reaction of boric acid for controlling the setting The initial and final setting times were determined using an reaction phosphate geopolymers. automatic Vicat apparatus (Toni Technik GmbH, Berlin, Germany) according to EN 196–3. The formation of reaction products from blended volcanic Experimental methods ash-based phosphate geopolymer with and without boric acid was monitored by measuring the heat flow evolution, cumu- Materials lative heat of reaction, and rheological properties. The Isothermal Conduction Calorimetry (TAM Air 3 calorimeter, Volcanic ash (VA) was collected in the Cameroon volca- TA Instruments, New Castle, DE, USA) was performed at nic line and ground to the final particles size ≤200 μm. 20°C for 24 h. For this test, 7 g of powder was mixed exter- OPTERRA Zement GmbH, Germany supplied ground nally with phosphoric acid (according to the mix proportion in granulated blast furnace slag (GGBFS). The mean diam- Table 2) for 3 min using a vortex mixer, then immediately put eter determined by laser granulometry (Mastersizer 2000 in the calorimeter. from Malvern Instruments, Worcestershire, UK) is The rheological properties of the binder were measured 24.136 μm and 16.046 μm for VA and GGBFS respec- with a dynamic rheometer (Haake Rheometers, Thermo tively. Their chemical composition determined by X-ray Fisher Scientific, Karlsruhe, Germany). The flow curves were fluorescence (PW 2400 PHILIPS instrument, Eindhoven, recorded in rotational motion using Cylinder Sensor Systems the Netherlands) is reported in Table 1. The mineralogical (Z20DIN, composed of a mobile rotor and a static beaker). composition of both aluminosilicates is shown in Fig. 1. The test setup includes a pre-shearing phase to homogenize J Aust Ceram Soc (2021) 57:1145–1154 1147 Table 1 Chemical composition of volcanic ash and GGBF slag (wt%) Oxides SiO Al O Fe O CaO MgO NaOTiO KOMnO P O LOI Total 2 2 3 2 3 2 2 2 2 5 VA 41.7 16.0 13.5 9.3 8.18 2.42 3.01 1.34 0.21 0.89 2.61 99.1 GGBFS 38.0 10.8 0.3 40.1 7.24 0.31 0.83 0.64 0.17 0.02 – 98.4 the paste in the sample holder, followed by the measurement Results and discussion of the shear stress vs. shear rate. The pre-shearing phase con- −1 sists of rising the shear rate from 0 to 100 s within 30 s and Setting times −1 decreasing it from 100 to 0 s in the same period, followed by a resting time of 30 s. The subsequent measuring phase The influence of the slag replacement rate and addition of consisted of an ascending ramp, increasing the shear rate from boric on the setting times are reported in Table 2. The initial −1 0to100 s within 60 s followed by a sloping ramp from 100 setting time (IST) and final setting times (FST) of the phos- −1 to 0 s within 90 s. The measurements were performed at phate geopolymer with 100% volcanic ash are 6.5 h and 24 h 20°C, and for each test, 5 g of paste was used. respectively. When volcanic ash was replaced by 4 and 6 wt% The compressive strength evolution was measured at 3, 7, slags, the IST and FST are shortened significantly. The mix and 28 d according to EN 196–1 by using a compression with 6 wt% of slag replacement was too fast to be measured, testing machine (Toni Technik, Berlin, Germany). and only the final setting time was measured and is 0.16 h The changes in the mineralogy are determined from the (10 min). This may be due to the preferential fast dissolution Empyrean PANalytical diffractometer (Malvern Panalytical of calcium from slag and its precipitation with the available Ltd., Malvern, UK) with Ni filter CuKα radiation (k = phosphate species that accelerate the setting reaction [3, 23]. 1.540598 Å). The binder was characterized by measuring When boric acid was added, the IST and FST were delayed to the mass loss and phase transformation while the temperature 4.5 h and 18 h respectively for the mix with 4 wt%. For the rises by using thermal gravimetry analysis and differential mix with 6 wt% slags, the initial and final setting times were scanning calorimetry (3+ SARe System, Mettler Toledo, also delayed to 0.08 h (5 min) and 0.90 h (55 min). In litera- Columbus, OH, USA). The test was performed in the temper- ture, it was reported that the retardation effect of boric acid in ature range 25–1000°C at a heating rate 5 K/min with synthet- the hydration of magnesium phosphate cement depends upon ic air flowing at 70 mL/min. the dosage in the mix. With a lower dosage, it mainly acts on The microstructure and the chemistry of the binder were hindering the precipitation reaction (delay of the initial setting 2+ determined by using a scanning electron microscope (SEM) time) by stabilizing Mg ions with the formation of the com- + 2+ coupled with energy-dispersive X-ray spectroscopy (Zeiss plex MgB(OH) which makes the Mg ions less available to Gemini SEM 500 NanoVP microscope, Oberkochen, react with phosphate species [20], while for a higher dosage, it Germany). The device used is equipped with a backscattered rather delays the dissolution reaction (delay of the final setting electron detector (BSD) that operates at low-vacuum mode time) [22]. In this work, the initial and final setting times were with 15 kV acceleration voltage. delayed with the presence of boric acid. Therefore, one can Fig. 1 X-ray patterns of a volcanic ash and b ground blast furnace slag 1148 J Aust Ceram Soc (2021) 57:1145–1154 Table 2 Details of the different VA (wt%) GGBFS (wt%) Mass ratio Boric acid/ Initial setting Final setting mix designs and setting times (VA+GGBFS) (wt%) time (h) time (h) Phosphoric acid/ (VA+GGBFS) 100 0 0.45 0 6.50 24 96 4 0.45 0 0.17 0.70 94 6 0.45 0 flash 0.16 96 4 0.45 5 4.50 18 94 6 0.45 5 0.08 0.92 say that the inhibition of both dissolution and precipitation of temperature of up to 95°C depending on the content of the volcanic ash slag during the reaction is responsible for the reagents [27]. Knowing that the dissolution of silica and alu- delaying of the setting time. mina is lower at room temperature, it is evident that the addi- tional heat supplied by slag dissolution has improved the dis- solution rate of volcanic ash [5, 12, 18, 28]. Reaction kinetics The addition of boric acid significantly decreased the height of the exothermic peak (Fig. 2b). A similar decrease The effects of slag and boric acid on the reaction kinetics of is also observed in the total heat released after 24 h. The total the formation of volcanic ash phosphate geopolymer binder heat achieved after 24 h (Fig. 3a) by the slag-blended volcanic are depicted in Figs. 2 and 3. The heat flow curves (Fig. 2) ash phosphate geopolymer binder is 83 J/g, 107 J/g, and 114 J/ display a single exothermic peak whose maximum is reached g for samples with 0, 4, and 6 wt% of slag, respectively. With within 6 min after mixing. Similar behavior was also observed the addition of boric acid, the total heat released after 24 h in an acid phosphate-activated high-calcium fly ash where the (Fig. 3b) is now 74 J/g, 91 J/g, and 101 J/g for samples with 0, maximum peak of the heat evolution was reached within 2– 4, and 6 wt% respectively. This trend translates that the addi- 4min [24]. In the case of volcanic ash phosphate geopolymer tion of boric acid has decreased the rate and extent of the binder, the exothermic peak corresponds to an increase in the dissolution-precipitation reaction. This corroborates the re- reaction temperature [25]. This corresponds to the dissolution sults of the initial and final setting times. Such behavior can of the initial raw materials and the precipitation of the dis- be explained by the fact that when all components are mixed, solved species to give the reaction products [3]. When slag there is a competition between the dissolution of boric acid is added to the mix, the heat of the reaction rises significantly (endothermic process) and the dissolution of volcanic ash slag with the increase of the slag content. That change is related to (exothermic process). It is worth noting that the reaction be- the additional heat provided by the dissolution of slag, which tween boric acid (H BO ) and phosphoric acid (H PO )gives 3 3 3 4 is rich in calcium. The latter was reported as other divalent rise to boron phosphate (BPO )and water [29]. This means in metal oxides to generate excessive heat when mixed with an our case that part of the phosphoric acid was consumed for the acid phosphate [26]. This sometimes leads to a reaction Fig. 2 Effect of blast furnace slag (a)and boric acid (b) on the heat flow evolution of slag-volcanic ash phosphate geopolymer binder J Aust Ceram Soc (2021) 57:1145–1154 1149 Fig. 3 Effect of blast furnace slag (a)and boric acid (b) on the total heat evolution of slag-volcanic ash-based phosphate geopolymer binder dissolution of boric acid, which limits the availability of pro- shear rate in the non-Newtonian fluid. It is worth noting tons (H ) that requires for the subsequent dissolution of vol- that for a non-Newtonian fluid, when n < 1, it means that canic ash and slag. Therefore, it contributes to the decrease in the paste behaves like a shear-thinning fluid and when reaction temperature or heat [19, 21, 22]. However, no delay n > 1, it is a shear-thickening fluid [30, 31]. In Table 3, was observed in the time for reaching the peak maximum of the fluidity index is lower than 1, indicating that the the heat released during the reaction. This indicates that the blended slag-volcanic ash-based phosphate geopolymer boric acid in the current system does not retard the reaction pastes behave like a shear-thinning fluid. This implies that process but rather inhibits the dissolution-precipitation. This the viscosity of the pastes increases with the decrease of occurs because, in addition to the limited availability of pro- the shear rate. This observation also holds for the mixes tons (H ) for the dissolution of the aluminosilicate, there is a with boric acid. However, the fluidity index of blended lack of sufficient phosphate species for undergoing the precip- slag-volcanic ash-based phosphate geopolymer pates re- itation, so there is a reduction in the yield of the reaction mains lower than those with boric acid and decreases with products formed at the end of the process. That can be as- the slag content, so the addition of boric acid is beneficial sumed as the main mechanism for controlling the setting re- for lowering the viscosity of the paste. This agrees with action and the reaction kinetics of volcanic ash-slag phosphate the literature where an improvement of the flowability of geopolymer binder with boric acid. the paste of magnesium potassium phosphate cement with increasing dosage of boric acid was observed [21]. Rheological properties Compressive strength development Figure 4 shows one hysteresis cycle (shear stress vs. shear rate) of different mixes. The shear stress increases non- The results of the compressive strength evolution in dry con- linearly with the shear rate, so the rheological properties were ditions and after immersion in water for 24 h are depicted in computerized by applying the non-linear fitting on the down Fig. 5. The compressive strength increases over time and is ramp curve using the Herschel-Bulkley function. The latter optimal when volcanic ash is replaced by 4 wt% of slag. consists of τ = τ +Kɣ (where τ is the shear stress (Pa), ɣ is Further increase in the replacement rate leads to a decrease ̇ ̇ the shear rate (1/s), τ is the yield stress (Pa), n is the fluidity in the compressive strength. After immersion into water for index, and K is the consistency coefficient (Pa s)). The yield 24 h, the compressive strength drops sharply by up to 40.3%. stress (τ ) is the minimum force that requires to apply on the However, the mixes with slag show the maximum 28 d paste to flow [30]. In Table 3, reporting the rheological prop- strength loss of 36.4% and 24.7% respectively with 4 wt% erties, the yield stress increases with slag content even after the and 6 wt% of volcanic ash replacement. This demonstrates addition of boric acid. This illustrates the high interaction that the addition of slag is beneficial for improving the water among dissolved species in the mixes and the acceleration of stability of the volcanic ash phosphate geopolymer binder the hardening. when compared with the reference sample. That is because The flow index (n) decreases with increasing the slag slag also takes part in the reaction by consuming some proton content. It describes the dependence of the viscosity to (H ) for its dissolution and the dissolved calcium condenses 1150 J Aust Ceram Soc (2021) 57:1145–1154 Table 3 Rheological properties VA GGBFS Boric acid/ Yield stress, Fluidity of slag-blended volcanic ash (wt%) (wt%) (VA+GGBFS) τ (Pa) index, n phosphate cement 0 (wt%) 100 0 0 0.393 0.856 96 4 0 0.664 0.843 94 6 0 1.426 0.834 96 4 5 1.032 0.896 94 6 5 1.358 0.847 with the phosphate ions to form water-stable reaction phases replacement. This result correlates with the inhibitor role [12, 32–34], whereas the flash reaction of calcium with acid played by boric acid in the setting time and reaction phosphate does not allow a good subsequent dissolution of kinetics of the slag-blended volcanic ash-based phos- volcanic ash when replaced by a higher content of slag. It is phate geopolymer as shown in this work (see Figs. 2 worth noting that the calcium-containing phases are preferen- and 3). Indeed, the competition between the dissolution tially dissolved in phosphoric acid at the early reaction stage of boric acid and slag-volcanic ash hinders the forma- and precipitate with the phosphate species to form amorphous tion of high-volume reaction products. Consequently, calcium phosphate phases [9, 24], so the addition of slag will the compressive strength of the slag blended is also induce the formation of a high amount of calcium phosphate diminished. Moreover, the water stability seems to be phase. The latter was described as responsible of the flash better than the mixes without boric acid, since the max- initial setting time reported in Table 2 for phosphate binder imum strength loss here is around 22%. Thus, it can be with 6 wt% of slag. Furthermore, it was reported that alumi- suggested that boric acid has taken part in the reaction num and iron are the main elements contributing to the formation of the binder and that a new phase including strength development of volcanic ash phosphate binder [3], boron would have been formed. This will be ascertained so the limitations in the availability of these elements during and discussed further in the next sections of this paper. the reaction process will hinder the compressive strength development. Mineralogy The effect of the addition of boric acid on the strength development of slag-blended volcanic ash- Figure 7 showsthe X-raypatternsinthe rangeof5–65° based phosphate geopolymer is depicted in Fig. 6.The 2θ of binders obtained with 4 wt% of slag and/or with the 28 d compressive strength is the most affected by the addition of boric acid. It should be noted that beyond that addition of boric acid, with a compressive strength loss range, there is no more additional information from the (as regards to the strength of the mixes without boric spectra. All the initial minerals present in volcanic ash are acid, Fig. 5) of 46.4% and 36% respectively for the still observed in the different binders. The main changes mixes with 4 wt% and 6 wt% of volcanic ash are the diminution of the peak intensity of some minerals Fig. 4 Flow curves of slag blended (a) and mixed with boric acid (b) J Aust Ceram Soc (2021) 57:1145–1154 1151 Fig. 5 Compressive strength evolution of slag-blended volcanic ash- based phosphate geopolymer (a), after immersion in water for 24 h prior Fig. 7 XRD patterns of volcanic ash (a), volcanic ash-based phosphate measurement (b) geopolymer (b), volcanic ash-based phosphate geopolymer blended with 4 wt% slag (c) volcanic ash-based phosphate geopolymer blended with or their disappearance and the formation of the broad 4 wt% slag, and boric acid (d) hump in synthesized cement paste in the range of 20– 40°. That hump is characteristic of amorphous reaction as observed in Fig. 8. Slag shows a very low mass loss products. This indicates that minerals have taken part in starting at room temperature up to 650°C corresponding the reaction, as demonstrated in previous works [3]. The to the removal of residual water both physically and reaction products were then characterized by thermal anal- chemically bonded. This is followed by mass gain, which ysis as the XRD cannot detect X-ray amorphous phases. It can be identified in the DSC curve (700–850°C) as an is worth recalling that the amorphous phase is not stable exothermic peak and is due to the oxidation process of at elevated temperatures, so the rise of temperature could sulphites. Volcanic ash and phosphate geopolymer show a trigger their crystallization which can indicate the type of single mass loss which is accompanied by an endothermic phase or its composition. peak with maximum appearing before 100°C. That corre- The thermal analysis shows the differences in the mass sponds to the removal of different types of water [4]. In change and heat flow with the increase of the temperature the phosphate cement, a very weak exothermic peak at around 650°C is identified which might be attributed to the transformation of amorphous iron phosphate phases from the binder into ferrous/ferric phosphate minerals [4]. Microstructure and phase composition The SEM images of phosphate geopolymer with 4 wt% slag and/or boric acid are depicted in Fig. 9. The micro- structure of all samples is heterogeneous with the pres- ence of a few cracks. The EDX analysis of different sam- ples reported in Table 4 helped to observe the diversity of the composition of the phases in different mixes. The asterisk put on the boron in Table 4 is to recall that the quantification of boron with EDX measurement is not accurate because it is a light element, so the molar ratio B/P presented in Table 4 is only for the qualitative pur- posetoshowhow boronreactsinthesystem.Inthesam- ples without any additives, 3 distinct phases named A, B, Fig. 6 Compressive strength evolution of blended slag-volcanic ash- and C were identified. The phase marked as A in the based phosphate geopolymer with the addition of boric acid (a), after immersion in water for 24 h prior measurement (b) micrograph is silicon-rich corresponding to unreacted 1152 J Aust Ceram Soc (2021) 57:1145–1154 Fig. 8 Thermogravimetry and differential scanning calorimetry of volcanic ash, slag, and slag-blended volcanic ash-based phosphate geopolymer particles of volcanic ash [3]. The phases B and C corre- be either physically or chemically bonded. Other new phases spond to the newly formed binder. The composition of named D and E are present. Their chemical composition re- phase B shows that it is silicon and aluminum-free phase, vealed that in phase D, apart from all elements observed in while phase C is a mix-up of all elements present in vol- phase C, there is also boron with high content of phosphorus. canic ash in which atomic ratios with phosphorus allow This phase could result from the partial reaction of boric acid its identification as the main binder phase. The latter was with the dissolved species during the formation of phase C. identified in all samples. The fact that the binders B and C The consequence of this is the lowering of the yield of reac- are shapeless confirms that the reaction product is amor- tion as discussed in the setting time and reaction kinetic in the phous, as shown in the XRD. previous section.This agrees withtheprevious statement and The addition of slag leads to the formation of a single confirms the mechanism of the reaction of boric acid as de- binding phase besides the unreacted particles. When boric scribed in this paper. The micrographs of the binder with acid is added to the previous mix, the resulting sample dis- 4 wt% slag + boric acid clearly show segregation between plays a new unreacted phase marked as AA whose composi- phase C and other phases, with a well-defined boundary. All tion includes boron with very low content of phosphorus. these observations demonstrate that boric acid partly reacted This suggests a possible formation of an intermediate with volcanic ash and slag while another part contributes to unreacted phase of boron-volcanic ash particles which could the formation of a new binder. Table 4 EDX analysis of different phases identified in volcanic ash phosphate cement blended with 4 wt% slag and/or boric acid Samples Atomic ratios Area Phases Si/P Al/P Fe/P Ca/P Mg/P Ti/P Na/P K/P B /P 0 wt% slag 11.46 5.85 1.86 1.51 0.48 0.43 2.85 3.66 0 A Unreacted particles 0 0 1.08 0.20 2.63 0.20 0.17 0.05 0 B Binder 1 0.73 1.37 2.81 0.66 0.43 0.58 0.30 0.10 0 C Binder 2 4 wt% slag 10.28 1.16 11.57 1.35 16.58 0.15 0.162 0.16 0 A Unreacted particles 1.88 1.53 3.94 1.73 1.08 0.62 0.37 0.16 0 C Binder 2 4 wt% slag + boric acid 6.35 1.63 2.91 4.50 6.22 1.84 1.28 0.81 1.73 AA Unreacted phase 3.90 3.40 15.65 0.82 0.93 3.87 0.37 0.32 0 A Unreacted particles 1.09 1.37 2.88 1.70 0.87 0.63 0.42 0.18 0 C Binder 2 1.55 1.30 2.38 1.32 0.58 0.26 0.35 0.17 0.68 D Binder 3 0 0.87 2.59 1.44 0.21 0.33 0.19 0.06 0.04 E Binder 4 J Aust Ceram Soc (2021) 57:1145–1154 1153 Fig. 9 Backscattered SEM images at different magnifications (×450, ×1000, and ×2000) of volcanic ash-based phosphate geopolymer binder Conclusion Acknowledgements The technical assistance of Mr. Tobias Dorn and David Dahncke for collecting experimental test results is appreciated. The work has reported the role of blast furnace slag and boric acid addition on the reaction and strength development of Author contribution Jean Noel Yankwa Djobo: research design, acqui- volcanic ash phosphate cement. The main findings are sum- sition, analysis and interpretation of data, and drafting of the paper. marized as follows: Dietmar Stephan: advisor, critical revision and editing, and approval of the submitted and final versions. – The partial substitution of volcanic ash with blast furnace Funding Open Access funding enabled and organized by Projekt DEAL. slag speeds up the reaction kinetics of volcanic ash phos- The authors received financial support from the Alexander von Humboldt phate geopolymer binders. Foundation through the Georg Foster Postdoctoral Fellowship (CM- – The addition of boric acid mainly contributes to the inhi- 1201499-GF-P) awarded to Dr. Djobo. bition of the dissolution-precipitation reaction by con- suming phosphate species. Declarations – Although boric acid succeeded to delay the setting time Conflict of interest The authors declare no competing interests. and improves the fluidity of the paste, it was found detri- mental for strength development. Open Access This article is licensed under a Creative Commons – The compressive strength was significantly increased Attribution 4.0 International License, which permits use, sharing, adap- with the replacement of volcanic ash with blast furnace tation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, pro- slag, while the incorporation of boric acid slows down the vide a link to the Creative Commons licence, and indicate if changes were compressive strength development. made. The images or other third party material in this article are included – The reaction products formed with the addition of slag in the article's Creative Commons licence, unless indicated otherwise in a and/or boric acid contribute to the improvement of the credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by water stability of volcanic ash phosphate cement. statutory regulation or exceeds the permitted use, you will need to obtain – The microstructure is composed of various types of permission directly from the copyright holder. To view a copy of this binders including boron as proof of the partial reaction licence, visit http://creativecommons.org/licenses/by/4.0/. of boric acid with reacted phases from volcanic ash and blast furnace slag. 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J Am Ceram Soc. 86,1850–1855 (2003). https://doi.org/10. Publisher’snote Springer Nature remains neutral with regard to jurisdic- 1111/j.1151-2916.2003.tb03571.x tional claims in published maps and institutional affiliations.

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Journal of the Australian Ceramic SocietySpringer Journals

Published: Sep 1, 2021

Keywords: Volcanic ash; Phosphoric acid; Blast furnace slag; Boric acid; Microstructure

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