Characteristic properties of fly ash-based self-compacting geopolymer mortars with synthetic wollastonite microfiber produced from silica and calcite

2.1. Materials

SCGs were designed using Class F fly ash (FA) and alkaline activator solutions as binder materials in addition to SWM. Wollastonite, manufactured synthetically using lime (CaO) and silica (SiO₂) raw materials, was used as a substitute for FA at certain ratios in the production of SCGs. The production methods of SWM consist of mechanochemical treatment, hydrothermal treatment, and a sintering process. First, CaO and SiO₂ materials were mixed in a 1:1 mole ratio. The amount of pure water was calculated to be equal to the mixture’s weight. The materials in the container were placed in the mill in such a way to prevent them from reacting. The mill was stirred at 250 rpm for 30 minutes to ensure mechanochemical interaction. After the mixture was added to Teflon, it was placed in an autoclave. The material was kept in the oven at 200 °C for 72 hours. The autoclave was removed from the oven after 72 hours and left to cool. After the hardened tobermorite (calcium silicate hydrate mineral) exploded in the microwave oven, it was removed from the Teflon by breaking with the help of a hammer. To vaporize the moisture from the material, it was kept in the oven at 100 ^oC for 20 hours. Then, the first grinding process of tobermorite was applied. After this process, the material was placed in porcelain containers, and sintering (the solid state reaction of the production) was carried out at 1000 ^oC for 1 hour. As a result of the final grinding process, SWM with different aspect ratios was obtained. The aspect ratio can be defined as an image projection describing the proportional relationship between the width of an image and its height. At the end of the trial error methods, SWM with a high aspect ratio (the ratio of length to diameter) of 30:1 was fabricated for the design of SCGs under laboratory conditions using a special method. After scanning electron microscope (SEM) analysis, the acicular particle structure of SWM is presented in Figure 1 . Chemical compositions of FA and SWM were examined in Table 1 . The geopolymer binder was chemically activated by the alkaline solutions of NaOH and Na₂SiO₃. The ratio of Na₂SiO₃ solution to NaOH solution was 1.5, 2.0, and 2.5 by weight of the binder. The composition of Na₂SiO₃ solution comprised 25.4% SiO₂, 14.7% Na₂O, and 59.9% water by mass. Additionally, 13 M and 97% pure NaOH solution was utilized in the design. The alkali activators used are available locally. Alkaline solutions and water were the liquid components in all mixtures. Quartz aggregates with a 2.65 specific gravity (0-0.4 mm) were used as fine aggregates in the production of SCGs.

2.2. Mix proportions and casting

This study aimed to produce SCGs incorporating SWM by considering the alkali activator ratio and curing temperature. To this end, a total of 24 SCG mixtures were designed with different alkali ratios such as Na₂SiO₃/NaOH = 1.5, 2.0, and 2.5 and SWM replacement percentages specified as 0%, 4%, 8%, and 12% by weight of FA. Furthermore, the designed samples were cured at 80 ^oC and 100 ^oC, respectively. As seen in Table 2 , the ratio of the alkaline solutions (Na₂SiO₃+NaOH) to the binder was designated as 0.5, as well as the binder content (1000 kg/m³) found by the sum of FA and alkaline solution. The sand/binder ratio was defined as 1.60. SWM with the aspect ratio of 30:1 obtained at the first stage of the study was utilized at the percentages of 0%, 4%, 8%, and 12% by weight of FA. Additionally, quartz aggregates (0-0.4 mm) were used in designing SCGs. The workability of fresh SCGs basically depends on the fundamental engineering properties of fresh SCGs with high performance characteristics. Therefore, the desired workability is a very important parameter for the design of SCGs. Thus, the slump flow test was performed according to EFNARC guidelines (26), and the slump flow diameter of all SCGs was kept within the range of 250 ± 10 mm. Hence, while producing the samples (considering the workability-reducing effect of SWM), the targeted slump flow diameter of SCGs was reached using different amounts of water without adding a high-range water-reducing admixture. First, a control mixture (SCG0) containing FA and quartz aggregates was produced without SWM. Then, SWM was used at the percentages of 4%, 8%, and 12% instead of FA and the different alkali activator ratios of Na₂SiO₃/NaOH determined as 1.5, 2.0, and 2.5 by weight, respectively. SCGs were named according to the percentage of SWM and alkali contents. For example, SCGs containing 4% SWM and 1.5 Na₂SiO₃/NaOH alkali ratio were called SCG1.5-4. In this way, SCG1.5-0, SCG1.5-4, SCG1.5-8, SCG1.5-12, SCG2-0, SCG2-4, SCG2-8, SCG2-12, SCG2.5-0, SCG2.5-4, SCG2.5-8, and SCG2.5-12 mixtures were obtained at different curing temperatures.

Various ingredients and production processes can be used to produce geopolymers. Various synthesis routes are mixed and reacted with an aluminosilicate precursor to commence the polymerization process. Therefore, there are no standard procedures for geopolymer production ( 27 27. Matsimbe, J.; Dinka, M.; Olukanni, D.; Musonda, I. (2022) Geopolymer: a systematic review of methodologies. Materials. 15, 6852. https://doi.org/10.3390/ma15196852 .
). The geopolymer mixing procedure consisted of dry and wet mixing. The solid components of SCGs (FA, quartz aggregates, and SWM) were in the mixer for about 30 sec as a dry condition. The liquid ingredients of the mixture, i.e., the sodium silicate solution, the sodium hydroxide solution, and water, had been previously mixed with each other in a container before adding to the dry mix. Then the dry mixing and the wet mixing continued for another 4 min.

Slump flow diameters were measured on fresh SCGs. According to the specifications of EFNARC ( 26 26. EFNARC F. (2002) Specification and guidelines for self-compacting concrete, European federation of specialist construction chemicals and concrete system.
), the slump flow diameter for SCGs should change between 24-26 cm. Therefore, many trial mixtures were produced until the target slump flow diameter was reached without using a high-range water-reducing admixture. Then, SCGs were cured in the oven at 80 ^oC and 100 ^oC for 24 h. After the curing process, the samples were demolded and kept at room temperature (23±2 ^oC) to be tested until the 28th day. Compressive strength, flexural strength, UPV, water sorptivity coefficient, and various physical properties were measured for SCGs incorporating SWM at the end of the 28th day.

2.3. Test procedures

3.1. Slump flow diameters

As observed from Figure 2 , SCGs were produced with a slump flow diameter determined as 25.0±1.0 cm. All SCGs designed in this study were manufactured without any segregation, as shown in Figure 3 . The slump flow diameters of SCGs were very close to each other. The desired workability was obtained by adding water determined as 83.8, 107.9, 107.9, and 128.8 kg/m³ for SCG1.5-0, SCG1.5-4, SCG1.5-8, and SCG1.5-12, respectively. Therefore, as seen in Figure 2 and Table 2 , the slump flow diameter for the desired workability was increased by increasing the amount of SWM due to the excessive amount of water. The slump flow diameters of SCGs decreased with the increasing amount of SWM. For example, it was evident from Figure 2 that although the same amount of water was used in the mixtures such as SCG-2.5-4 and SCG-2.5-12, the slump flow diameter decreased from 25.6 to 24.8 cm due to the needle-like particle structure of SWM. The decreased workability of geopolymer mortars with the addition of SWM can be explained by the increased bonding of SWM microfibers. Therefore, as the usage percentage of SWM was increased, the required amount of water for the aimed slump flow diameter of SCGs also increased. Similarly, to these results, Tatnall ( 34 34. Tatnall, P.C. (2006) Fiber-reinforced concrete. E-Publishing Inc. 578-590.
) indicated that the increased interlocking effect due to the needle-like structure of SWM reduced the workability of the mixture and thus, additional water and/or plasticizers should be used. Moreover, the amount of water needed for the same workability decreased due to the increment of water in SCGs with the increasing Na₂SiO₃/NaOH ratio. For example, as seen in Table 2 , the amount of water was changed to 128.85, 93.3, and 92.5 kg/m³ for SCG1.5-12, SCG2.0-12, and SCG2.5-12 mixtures, respectively. The reason for this is thought to be the amount of water for a 2.5 alkali ratio in SCGs higher than a 1.5 alkali ratio due to the 59.9% H₂O contained in Na₂SiO₃ ( 6 6. Sanni, S.H.; Khadiranaikar, R.B. (2013) Performance of alkaline solutions on grades of geopolymer concrete. IJRET. 366-371. http://doi.org/10.15623/ijret.2013.0213069 .
).

3.2. Physical properties

SWM, the alkali activator ratio, and the curing temperature of geopolymer mortars substantially affected the physical properties of SCGs. As shown in Tables 3 and 4 , the highest hardened bulk density value cured at 80 ^oC (1905 kg/m³) and the lowest hardened bulk density value (1637 kg/m³) cured at 100 ^oC were obtained from SCG-2-0 and SCG-2.5-12, respectively. When certain percentages of SWM were used instead of FA with high-dimensional particles serving as a filler, the void ratio increased due to the water leaving the sample, and the hardened bulk density values of the groups generally decreased. This was also supported by Patankar et al. ( 35 35. Patankar, S.V.; Jamkar, S.S.; Ghugal, Y.M. (2012) Effect of sodium hydroxide on flow and strength of fly ash based geopolymer mortar. J. Struct. Eng. 39 [1], 7-12.
). Upon comparing the curing temperatures, it was revealed that the hardened bulk density values of the samples cured at higher curing temperatures were relatively lower, which was explained by the fact that more water left the sample at higher temperatures. Additionally, it was observed that when the amount of SWM was increased for SCG 1.5 (Group I) mortars cured at 100 ^oC, the hardened bulk density values of SCGs increased on the 28th day, and this improvement proceeded to 8% of SWM (including 8%). This may be attributed to the optimum amount of FA, alkali ratio, binder, and water for a better design. Thus, substituting SWM with FA resulted in the intensified matrix structure of SCGs. However, using SWM at a 12% level reduced the physical performance of SCGs, which was clarified by the weakened bond in the matrix ( 15 15. Oz, H.O.; Güneş, M. (2021) The effects of synthetic wollastonite developed with calcite and quartz on high performance mortars. Struct. Concr. 22 [S1], E257-E272. https://doi.org/10.1002/suco.201900520 .
). Considering hardened bulk density values, the SCGs produced in this study can be classified as a structural lightweight material.

The dry specific gravity, apparent specific gravity, and saturated dry surface specific gravity values of SCG mortars obtained from the experiments performed on the 28th day under different curing conditions are also presented in Tables 3 and 4 , respectively. According to the test results, the dry specific gravity values of SCGs were in the range of 1.73-1.89, apparent specific gravity values changed between 2.33-2.51, and saturated dry surface specific gravity values varied between 2.00-2.12. Additionally, a decrease was observed in apparent specific gravity and saturated dry surface specific gravity values with the decrease in the amount of NaOH in the mortar in the comparison of Groups I and II ( 36 36. Atabey, I.I. (2017) F sınıfı uçucu küllü geopolimer harcının durabilite özelliklerinin araştırılması. Graduate Theses and Dissertations.
). This may be attributed to the lower quality geopolymer of the matrix controlled by the lowest alkaline ratio, responsible for the insufficient dissolution of solid fly ash particles. It can be noted that using an excessive amount of alkaline activator solution for Group III resulted in a little expansion of geopolymer mortars during the reaction ( 37 37. Öz, H.Ö.; Doğan-Sağlamtimur, N.; Bilgil, A.; Tamer, A.; Günaydin, K. (2021) Process development of fly ash-based geopolymer mortars in view of the mechanical characteristics. Materials. 29, 14 [11], 1-22. https://doi.org/10.3390/ma14112935 .
). Moreover, the lower initial water content would result in a denser microstructure ( 38 38. Renumathi, M.; Mukesh, P.; Balamurugan, P. (2020) A state of art on geopolymer concrete. Adalya J. 9 [1], 372-378.
).

Figure 4 and Figure 5 indicate the measurement of water absorption and apparent porosity values for geopolymer mortars with different design parameters on the 28th day. According to the test results, the water absorption values of geopolymer mortars were in the range of 8.10-13.82%, and apparent porosity values changed between 5.90-24.00%. The highest water absorption and apparent porosity values were obtained from SCG-1.5-12 cured at 80 ^oC, whereas the lowest values were measured from SCG-1.5-8 cured at 100 ^oC. Furthermore, an increase in the porosity and water absorption rate was observed for all groups at both curing temperatures, except for SCG-1.5 cured at 100 ^oC. The increment of the liquid to solid percentage enhanced the dissolution of aluminosilicate resource by facilitating the ion transference ( 39 39. Zuhua, Z.; Xiao, Y.; Huajun, Z.; Yue, C. (2009) Role of water in the synthesis of calcined kaolin based geopolymer. Appl. Clay Sci. 43, 218-223. https://doi.org/10.1016/j.clay.2008.09.003 .
), which led to slower polycondensation reactions, resulting in the transformation of a few meshes. Additionally, a higher amount of water remaining in the geopolymer matrix caused a reduction in physical properties. Moreover, the porosity of SCGs advanced due to the formation of more than one network and the amount of water trapped in the geopolymer matrix ( 40 40. Prud’homme, E.; Joussein, E.; Peyratout, C.; Smith, A.; Rossignol, S. (2010) Consolidated geo-materials from sand or industrial waste. Ceram. Eng. Sci. 30, 314-324. https://doi.org/10.1002/9780470584262.ch29 .
). However, a decrease in water absorption and apparent porosity was observed in SCG-1.5 cured at 100 ^oC. It was thought that curing at high temperatures increased the degree of geopolymerization and contributed to the formation of a denser material, which reduced the water absorption and apparent porosity values of geopolymer mortars ( 41 41. Kovalchuk, G.; Fernandez-Jimenez, A.; Palomo, A. (2017) Alkali activated fly ash: effect of thermal curing conditions on mechanical and microstructural development-Part II. Fuel. 86, 315-22. https://doi.org/10.1016/j.fuel.2006.07.010 .
). SWM also decreased pores in the geopolymer matrix and water absorption and apparent porosity due to the filler effect by providing the condensation of the microstructure, which ensured pore discontinuity in the geopolymer matrix for Group I at high curing temperatures ( 22 22. Ransinchung, G.D.; Kumar, B. (2010) Investigations on pastes and mortars of ordinary portland cement admixed with wollastonite and microsilica, J. Mater. in Civil Eng. 22 [4]. https://doi.org/10.1061/(ASCE)MT.1943-5533.0000019 .
, 23 23. Nuruddin, M.F.; Samuel, D.; Nasir, S. (2011) Effect of mix composition on workability and compressive strength of self-compacting geopolymer concrete. Can. J. Civil Eng. NRC Res. Press. 38 [11], 1196-203. https://doi.org/10.1139/l11-077 .
, 42 42. Lin, K.; Chang, J.; Chen, G.; Ruan, M.; Ning, C. (2007) A simple method to synthesize single-crystalline β-wollastonite nanowires. Mater. Sci. 300 [2], 267-271. https://doi.org/10.1016/j.jcrysgro.2006.11.215 .
). Therefore, it was stated that SWM, defined as an inert material not reacting chemically in the pore solution, affected the formation of the pore network tortuosity of SCGs ( 43 43. Lai, M.; Binhowimal, S.; Hanžič, L.; Wang, Q.; Ho, J. (2020) Dilatancy mitigation of cement powder paste by pozzolanic and inert fillers. Struct. Concr. 21 [4], 1-17. https://doi.org/10.1002/suco.201900320 .
). However, this improvement was not observed in Groups II and III at lower and higher curing temperatures. The deterioration of water absorption and apparent porosity may be attributed to the attenuation of the matrix bond in the interfacial transition zone of SWM due to the insufficient alkali ratio and curing temperature required for the best geopolymerization for SCGs ( 15 15. Oz, H.O.; Güneş, M. (2021) The effects of synthetic wollastonite developed with calcite and quartz on high performance mortars. Struct. Concr. 22 [S1], E257-E272. https://doi.org/10.1002/suco.201900520 .
).

3.3. Compressive strength, flexural strength, UPV and dynamic modulus of elasticity

The compressive and flexural strength values of SCGs are graphically presented in Figures 6 and 7 , respectively. As seen from Figure 6 , the compressive strength values of geopolymer mortars were in the range of 12.8-28.9 MPa. The highest compressive strength was obtained from SCG-1.5-8 cured at 100 ^oC, while SCG-1.5-12 cured at 80 ^oC had the lowest compressive strength.

In the development of SCGs, curing temperature, alkali activator ratio, and the replacement percentage of SWM played an important role during the activation process. As seen from Figure 6 , the compressive strength values of the samples cured at 100 ^oC in each of Group I, Group II, and Group III were higher than those of the samples cured at 80 ^oC. Curing at high temperatures increased the degree of geopolymerization and contributed to the formation of a denser material ( 41 41. Kovalchuk, G.; Fernandez-Jimenez, A.; Palomo, A. (2017) Alkali activated fly ash: effect of thermal curing conditions on mechanical and microstructural development-Part II. Fuel. 86, 315-22. https://doi.org/10.1016/j.fuel.2006.07.010 .
). As indicated in Figure 6 , when the samples cured at 80 ^oC were compared, it was determined that the compressive strength values of Group I were lower than those of Group II and Group III, respectively. It is thought that the reason for this was the decreased amount of Na₂SiO₃, which contains 59.9% water. On the other hand, the growth rate of compressive strength at higher curing temperatures can be explained by the improvement of kinetic energy, resulting in stronger Al-Si-O networks of SCGs ( 37 37. Öz, H.Ö.; Doğan-Sağlamtimur, N.; Bilgil, A.; Tamer, A.; Günaydin, K. (2021) Process development of fly ash-based geopolymer mortars in view of the mechanical characteristics. Materials. 29, 14 [11], 1-22. https://doi.org/10.3390/ma14112935 .
). Moreover, a higher NaOH concentration promoted higher strength at the early stages of the reaction. However, the strength of the activated geopolymer at the later stages might have decreased due to the excessive OH in the solution, causing the inhomogeneous morphology of the final products ( 44 44. Khale, D.; Chaudhary, R. (2007) Mechanism of geopolymerization and factors influencing its development. J. Mater. Sci. 42, 729-746. https://doi.org/10.1007/s10853-006-0401-4 .
). SWM deteriorated the compressive strength at the highest alkali activator ratio in respect of curing temperature, which may be attributed to a weaker bond with the matrix of SWM as a result of incomplete geopolymerization controlled by the highest alkali activator ratio. Thus, SWM with the ability of microfibers to bridge micro-cracks did not help to achieve a higher load-carrying capability for SCGs. Moreover, the highest compressive strength for a lower curing temperature was obtained for SCG-2.5-0. A lower water content in the SCG series cured at 80 ^oC led to a higher alkalinity in Group II and Group III series compared to Group I series. Thus, the dissolution ratio of amorphous geopolymer would be higher in Group II and Group III series, which accelerated the geopolymerization reaction ( 47 47. Duxson, P.; Fernández-Jiménez, A.; Provis, J.L.; Lukey, G.C.; Palomo, A.; Van Deventer, J.S. (2007) Geopolymer technology: the current state of the art. J. Mater. Sci. 42 [9], 2917-2933. https://doi.org/10.1007/s10853-006-0637-z .
). The replacement percentage of SWM did not have a positive effect on compressive strength due to a lower curing temperature for SCG-1.5, SCG-2, and SCG-2.5 samples. Considering the test results, the compressive strength values of SCG-1.5 cured at 100 ^oC increased up to 8% (including 8%) SWM content and decreased at 12%. In light of such information, it was determined that substituting SWM with FA in SCG-1.5 cured at 100 ^oC was critical for an 8% replacement level on the 28th day. The positive effect of SWM on compressive strength was explained by the filler effect of SWM in the matrix, resulting in a higher density in the microstructure considering the binder content, optimum curing temperature, and alkali activator ratio of SCGs. Upon comparing Group II, Group III, and Group I mixtures cured at 100 ^oC, the reason for the improvement in compressive strength of Group I mixtures may be supported by the addition of SWM that improved the microstructure of the geopolymer matrix owing to the mechanical interlocking of unreacted SWM molecules and connecting to the geopolymeric gel. However, an increase in the SWM percentage to 12% by mass in all groups for both curing temperatures increased the proportion of unreacted SWM molecules in the geopolymer matrix. Compared to Group I series, this situation could destroy the geopolymeric gel mash and disrupt the matrix structure by decreasing compressive strength values. Another reason was that SWM partially disintegrated in the alkaline medium and the dissolution rate increased proportionally with alkalinity ( 25 25. Yip, C.K.; Lukey, G.C.; Provis, J.L.; Van Deventer, J.S. (2008) Effect of calcium silicate sources on geopolymerisation. Cem. Concr. Res. 38 [4], 554-564. https://doi.org/10.1016/j.cemconres.2007.11.001 .
). Moreover, a higher compressive strength in SCG series mixtures can be explained by the fact that dissoluble calcium caused by SWM could react quickly with Si and Al to form the geopolymeric gel ( 2 2. Lee, W.K.W; Van Deventer, J.S.J. (2002) The effect of ionic contaminants on the early-age properties of alkali-activated fly ash-based cements. Cem. Concr. Res. 32 [4], 577-584. https://doi.org/10.1016/S0008-8846(01)00724-4 .
). Additionally, it was revealed that the strength loss due to the increased percentage of SWM was related to the increase in the amount of water needed for the other groups, except for Group I cured at 100 ^oC. The high aspect ratio (30:1) and high surface area of SWM caused a loss of workability, resulting in the need for water to obtain the aimed slump flow diameter of SCGs. Likewise, Patankar et al. ( 35 35. Patankar, S.V.; Jamkar, S.S.; Ghugal, Y.M. (2012) Effect of sodium hydroxide on flow and strength of fly ash based geopolymer mortar. J. Struct. Eng. 39 [1], 7-12.
) found that the excessive amount of water used during the polymerization of the sample did not make any positive contribution to strength characteristics. In addition, the presence of dissoluble silica affected the dissolution of FA, the reaction percentage of crystallization, and kinetics for different Na₂SiO₃/NaOH ratios. Silica causes strong Si gel formation, supporting the precipitation of big molecular types. Thus, silica is an important compound for the strength increment of the material with an improved density ( 48 48. Zuda, L.; Pavlik, Z.; Rovnanikova, P.; Bayer, P.; Cerny, R. (2006) Properties of alkali activated aluminosilicate material after thermal load. Int. J. Thermophys. 27 [4], 1250-1263. https://doi.org/10.1007/s10765-006-0077-7 .
).

The flexural strength values of geopolymer mortars changed between 3.2-6.5 MPa, as shown in Figure 7 . The highest flexural strength was obtained from SCG-2-12 cured at 80 ^oC, while SCG-2.5-12 cured at 100 ^oC had the lowest flexural strength. Similar to compressive test results, the increasing replacement percentages of SWM decreased flexural strength in Group I cured at 80 ^oC. This may be attributed to the lowest alkaline ratio controlling geopolymerization, which is responsible for the dissolution of solid fly ash particles. Thus, the degree of polymerization of the dissolved gel was affected by soluble silicates. Due to the weaker matrix of SCGs, the flexural strength of SCGs decreased with the increasing percentage ratio of SWM in Group I. Based on the observation from the test results, the improvement in the flexural strength of SCGs cured at 80 ^oC in Groups II and III can be explained by the fact that SWM could bind between microcracks ( 45 45. Nikonova, N.S.; Tikhomirova, I.N.; Belyakov, A.V.; Zakharov, A.I. (2003) Wollastonite in silicate matrices. Glass Ceramics. 60, 342-346. https://doi.org/10.1023/B:GLAC.0000008241.84600.f9 .
, 46 46. Dey, V.; Kachala, R.; Bonakdar, A.; Mobasher, B. (2015) Mechanical properties of micro and sub-micron wollastonite fibers in cementitious composites. Constr. Build. Mater. 82, 351-359. https://doi.org/10.1016/j.conbuildmat.2015.02.084 .
). Therefore, mechanical properties were improved by increasing the microfiber/matrix bond strength at the interface ( 47 47. Duxson, P.; Fernández-Jiménez, A.; Provis, J.L.; Lukey, G.C.; Palomo, A.; Van Deventer, J.S. (2007) Geopolymer technology: the current state of the art. J. Mater. Sci. 42 [9], 2917-2933. https://doi.org/10.1007/s10853-006-0637-z .
, 48 48. Zuda, L.; Pavlik, Z.; Rovnanikova, P.; Bayer, P.; Cerny, R. (2006) Properties of alkali activated aluminosilicate material after thermal load. Int. J. Thermophys. 27 [4], 1250-1263. https://doi.org/10.1007/s10765-006-0077-7 .
). Studies in the literature have shown that the bond between wollastonite within a geopolymer matrix increases the flexural strength of SCGs with the increasing replacement percentages of SWM. If the strength of the geopolymer matrix based on geopolymerization is high, SWM with an acicular structure can attach to the geopolymer matrix. Thus, large acicular SWM failed during loading, causing SCGs to be able to maintain their higher flexural strength ( 21 21. Nurjaya, D.M.; Astutiningsih, S.; Zulfia, A. (2015) Thermal effect on flexural strength of geopolymer matrix composite with alumina and wollastonite as fillers. Int. J. Technol. 6, 462-470. https://doi.org/10.14716/ijtech.v6i3.1441 .
). On the other hand, SWM partly disintegrated in the alkaline medium and finally fused to the geopolymeric gel, which strengthened the geopolymer matrix and improved the flexural properties of SCG-2-12 cured at 80 ^oC ( 21 21. Nurjaya, D.M.; Astutiningsih, S.; Zulfia, A. (2015) Thermal effect on flexural strength of geopolymer matrix composite with alumina and wollastonite as fillers. Int. J. Technol. 6, 462-470. https://doi.org/10.14716/ijtech.v6i3.1441 .
, 25 25. Yip, C.K.; Lukey, G.C.; Provis, J.L.; Van Deventer, J.S. (2008) Effect of calcium silicate sources on geopolymerisation. Cem. Concr. Res. 38 [4], 554-564. https://doi.org/10.1016/j.cemconres.2007.11.001 .
). Similar to this study, it was stated that fibers could control the crack width by increasing the amount of absorbable energy and caused a greater increase in flexural strength than compressive strength values ( 49 49. Hughes, B.P. (1981) Design of prestressed fiber reinforced concrete beams for impact. ACI Materials Journal. 78, 276-281.
). However, as shown in Figure 7 , replacing the geopolymer binder with SWM did not increase the flexural strength of Group II and III series mixtures cured at 100 ^oC. During the formation of geopolymers, the water evaporated from the matrix during the curing or drying periods left discontinuous nanopores in the matrix. Thus, it can be thought that the breakage of SWM could cause SCGs to be unable to resist their flexural strength at high curing temperatures. As a result, the crack-bridging effect was reduced by adding SWM fibers. On the other hand, SWM particles could be highly etched in SCG series mixtures due to their more alkaline medium. Thus, SWM could not fulfill its function as a fiber by reinforcing SCGs against bending, compared to mixtures cured at lower temperatures. These results were also supported by Bong and Nematollahi ( 19 19. Bong, S.H.; Nematollahi, B.; Xia, M.; Nazari, A.; Sanjayan, J. (2020) Properties of one-part geopolymer incorporating wollastonite as partial replacement of geopolymer precursor or sand. Mater. Letters. 263, 127263. https://doi.org/10.1016/j.matlet.2019.127236 .
).

Figure 8 present the ultrasonic pulse velocity (UPV) variations of SCGs on the 28th day for samples cured at different temperatures. The UPV values of SCGs changed in parallel with compressive strength. As seen in Figure 8 , the UPV values of geopolymer mortars changed in the range of 2299-3524 m/s. The highest UPV value cured at 100 ^oC and the lowest UPV value cured at 80 ^oC were obtained from SCG-1.5-8 and SCG-1.5-12, respectively. UPV values improved in the mixtures cured at 100 ^oC with an alkaline ratio of 1.5. On the other hand, the increasing replacement percentages of SWM improved UPV values at a ratio of 5% and 6% in SCG-1.5-4 and SCG-1.5-8 compared to SCG-1.5-0 for Group I cured at 100 ^oC (without SCG-1.5-12). In case of SCG-1.5-12, the reason for deterioration may be the attenuation of the matrix bond in the interfacial transition zone of the excessive amount of SWM ( 15 15. Oz, H.O.; Güneş, M. (2021) The effects of synthetic wollastonite developed with calcite and quartz on high performance mortars. Struct. Concr. 22 [S1], E257-E272. https://doi.org/10.1002/suco.201900520 .
). The filler effect of SWM caused an increase in density in the microstructure for the transitional zone around the synthetic fiber, which was the reason for the denser matrix bound up to an 8% replacement percentage of SWM and provided the lack of continuity in pores ( 50 50. Mathur, R.; Misra, A.K.; Goel, P. (2007) Influence of wollastonite on mechanical properties of concrete. J. Sci. Ind. Res. 66, 1029-1034.
, 51 51. Wahab, M.A. Latif, I.A.; Kohail, M.; Almasry, A. (2017) The use of wollastonite to enhance the mechanical properties of mortar mixes. Constr. Build. Mater. 152, 304-309. https://doi.org/10.1016/j.conbuildmat.2017.07.005 .
). Moreover, this development could have originated from the bond strength supported by the microfiber/matrix in the interfacial of SWM microfibers ( 52-54 52. Soliman, A.M.; Nehdi, M.L. (2014) Effects of shrinkage reducing admixture and wollastonite microfiber on early-age behaviour of ultrahigh performance concrete. Cem. Concr. Compos. 46, 81-89. https://doi.org/10.1016/j.cemconcomp.2013.11.008 .
53. Banthia, N.; Sheng, J. (1996) Fracture toughness of micro-fiber reinforced cement composites. Cem. Concr. Compos. 18 [4], 251-269. https://doi.org/10.1016/0958-9465(95)00030-5 .
54. Hameed, R.; Turatsinze, A.; Duprat, F.; Sellier, A. (2009) Metallic fiber reinforced concrete: Effect of fiber aspect ratio on the flexural properties. ARPN J. Eng. Appl. Sci. 4, 67-72.
). Upon comparing the samples cured at 80 ^oC, it was found that the UPV values of Group I were lower than those of Groups II and III. It was thought that the reason for this was the weaker geopolymer matrix due to the insufficient amount of Na₂SiO₃/NaOH. However, it was determined that the mixtures in Group II had higher UPV values than those in Group III. The alkali content and water ratio of Group II were found to be more suitable than those in Group III for sufficient geopolymer activation. Moreover, according to the classification of both, all the samples produced in this study can be considered as “medium” on the 28th day ( 55 55. Malhotra, V.M. (1976) Testing hardened concrete: nondestructive methods. ACI Monograph No.9.
). Additionally, it can be said that the UPV values of SCGs exhibited a similar trend to the hardened bulk density values rather than their compressive and flexural strength values. Likewise, Xu et al. ( 56 56. Xu, S.; Malik, M.A.; Qi, Z.; Huang, B.; Li, Q.; Sarkar, M. (2018) Influence of the PVA fibers and SiO₂ NPs on the structural properties of fly ash based sustainable geopolymer. Constr. Build. Mater. 164, 238-245. https://doi.org/10.1016/j.conbuildmat.2017.12.227 .
) reported that some of the ultrasonic wave energy was lost due to the porous structure during the test, while the UPV measurement of samples with low density and high porosity was reduced.

The dynamic modulus of elasticity is directly based on the time controlled by the voids in the concrete/geopolymer, which is necessary to move ultrasonic pulse waves through the sample ( 57 57. Alnahhal, A.M.; Alengaram, U.J.; Yusoff, S.; Singh, R.; Radwan, M.K.H; Deboucha, W. (2021) Synthesis of sustainable lightweight foamed concrete using palm oil fuel ash as a cement replacement material. J. Build. Eng. 35, 102047. https://doi.org/10.1016/j.jobe.2020.102047 .
). Furthermore, the deformation behavior of structural elements can be observed by DME while being subjected to a load ( 58 58. Gupta, T.; Siddique, S.; Sharma, R.K.; Chaudhary, S. (2017) Effect of elevated temperature and cooling regimes on mechanical and durability properties of concrete containing waste rubber fiber. Constr. Build. Mater. 137, 35-45. https://doi.org/10.1016/j.conbuildmat.2017.01.065 .
). It is reported that the reduction of density results in a lower time, which in turn decreases speed (km/s) ( 57 57. Alnahhal, A.M.; Alengaram, U.J.; Yusoff, S.; Singh, R.; Radwan, M.K.H; Deboucha, W. (2021) Synthesis of sustainable lightweight foamed concrete using palm oil fuel ash as a cement replacement material. J. Build. Eng. 35, 102047. https://doi.org/10.1016/j.jobe.2020.102047 .
). Figure 9 indicate DME as a function of the hardened bulk density of SCGs and ultrasonic pulse velocity (m/s). As seen in Figure 9 , the hardened bulk density of SCGs affected DME. Similar to the results of the study by Alnahhal et al. ( 57 57. Alnahhal, A.M.; Alengaram, U.J.; Yusoff, S.; Singh, R.; Radwan, M.K.H; Deboucha, W. (2021) Synthesis of sustainable lightweight foamed concrete using palm oil fuel ash as a cement replacement material. J. Build. Eng. 35, 102047. https://doi.org/10.1016/j.jobe.2020.102047 .
), the voids of the sample obstructed the movement of ultrasonic pulse waves, which increased the time required to pass the ultrasonic pulse wave. The filler effect of SWM, the formation of bond strength supported by the microfiber/matrix in the interfacial of SWM microfibers, and the sufficient alkali content and water ratio improved UPV measurements, which in turn increased DME. The highest DME values were detected in Groups I and III cured at 100 ^oC, which were 22.37 GPa and 22.01 GPa, respectively. It can be concluded that the hardened bulk density, UPV, replacement percentages of SWM, and alkali ratio determined the range of DME for SCGs.

3.4. Water sorptivity coefficient

Water sorptivity coefficient values were obtained from the experiments performed on the 28th day of SCGs produced under different curing conditions and are presented in Figure 10 . According to the test results, the water sorptivity coefficient values of geopolymer mortars changed in the range of 1.10-2.38 mm/min^0.5, respectively. The highest water sorptivity coefficient was obtained from SCG-1.5-12 cured at 80 ^oC, whereas SCG-1.5-8 cured at 100 ^oC had the lowest water sorptivity coefficient. As seen from Figure 10 , the water sorptivity coefficient increased with the increasing percentage of SWM in SGCs cured at 80 ^oC for all groups and 100 ^oC, except for Group I, respectively. Considering the test results for Group I mortars cured at 100 ^oC, when the amount of SWM increased, the water sorptivity coefficient values of SCGs decreased on the 28th day, and this development continued to 8% of SWM (including 8%). Hence, the compactness of the microstructure with curing the geopolymer at a high temperature can be attributed to a reduction in pores via the positive filler effect of SWM ( 47 47. Duxson, P.; Fernández-Jiménez, A.; Provis, J.L.; Lukey, G.C.; Palomo, A.; Van Deventer, J.S. (2007) Geopolymer technology: the current state of the art. J. Mater. Sci. 42 [9], 2917-2933. https://doi.org/10.1007/s10853-006-0637-z .
). Thus, a pore discontinuity could be provided with the formation of the pore structure of SWM in the geopolymer system, which liquids cannot attain at normal pressures ( 50 50. Mathur, R.; Misra, A.K.; Goel, P. (2007) Influence of wollastonite on mechanical properties of concrete. J. Sci. Ind. Res. 66, 1029-1034.
). However, using SWM at a 12% replacement percentage deteriorated the mechanical performance of SCGs. The impairment of SCG-1.5-12 cured 100 ^oC in Group I can be explained by the weakening of the matrix bond in the interfacial transition zone of SWM ( 51 51. Wahab, M.A. Latif, I.A.; Kohail, M.; Almasry, A. (2017) The use of wollastonite to enhance the mechanical properties of mortar mixes. Constr. Build. Mater. 152, 304-309. https://doi.org/10.1016/j.conbuildmat.2017.07.005 .
). Additionally, this value increased in Group I cured at 80 ^oC due to the presence of such pores formed as a result of the evaporation of the free water not included in geopolymerization during the curing period from the samples. Thus, higher water usage for the aimed slump flow diameter of SCGs due to the high aspect ratio (30:1) and high surface area of SWM caused a loss of workability, which adversely affected mechanical properties. As a result, the gaps created due to more water leaving the mortar after curing caused an increment in the water sorptivity coefficient values of SCGs. In addition to these, it is thought that choosing a high molarity of NaOH causes a decrease in capillarity. Furthermore, this may be related to the gel’s concentration since the increasing temperature causes the gel to bind denser. Moreover, water sorptivity coefficient values are generally related to the values of apparent porosity. Therefore, water sorptivity coefficients decreased with the reduction in porosity due to increasing unit volume weights ( 7 7. Kurklu, G.; Gorhan, G. (2019) Investigation of usability of quarry dust waste in fly ash-based geopolymer adhesive mortar production. Constr. Build. Mater. 217, 498-506. https://doi.org/10.1016/j.conbuildmat.2019.05.104 .
).