The role of NaOH molarity on the strength performance of geopolymer mortar prepared by a combination of fly ash, rice husk ash, and kaolin at heat-cured condition

Abstract The use of fly ash as geopolymer primary material has been widely used, however, the availability of this material is decreasing along with the closure of coal-based power plants to address the global warming issue. Therefore, finding an alternative material to substitute the role of fly ash in geopolymer manufacturing is necessary This paper reports the effect of NaOH molarity variations on the strength properties of fly ash-rice husk ash-kaolin-based geopolymer. The geopolymer mortars were prepared by mixing 0.7 fly ash, 0.21 rice husk ash, and 0.09 kaolin with sodium silicate and NaOH as alkaline activators. The ratio of total fly ash, rice husk ash, and kaolin to fine aggregate was kept at 1:2.75. While the sodium silicate to NaOH ratio and water to solid ratio were maintained at 2.0 and 0.39-0.40, respectively The variations of NaOH molarity were 4 M, 6 M, 8 M, 10 M, and 12 M. Heat curing treatment was applied to all specimens. The compressive strength and porosity tests were carried out at 28 days. The results showed that the NaOH molarity significantly affects the strength performance and porosity of fly ash-rice husk ash-kaolin-based geopolymer. The highest strength was achieved by a 10-molar NaOH geopolymer mix with a strength of 32.11 MPa at 28 days. This 10 Molar mix also demonstrated the lowest porosity with a value of 6.3%, indicating a better specimen density. However, increasing the molarity of NaOH by more than 10 Molar tends to decrease the geopolymer strength and improve its pore area. Thus, it can be concluded that a combination of fly ash, rice husk ash, and kaolin at a specific molarity can be used as alternative materials for fly ash-based geopolymers to address global warming and climate change issues.