Durability of fly ash/slag geopolymers: Role of OPC and silica under sulfate attack

This study evaluates the durability of fly ash/slag-based geopolymer with limited ordinary Portland cement (OPC) content up to 6 %. The raw materials used are low-calcium fly ash (class F) and ground granulated blast furnace slag (GGBFS). Variations in slag and OPC contents and the SiO₂/Na₂O ratio were employed to prepare fly ash/slag pastes. Pure OPC samples were also prepared. Physical appearance, weight, pH change, compressive strength, microstructural, and elemental composition analyses were conducted to investigate the samples under sulfate attack. The sulfate solution concentration considered in this study is fixed at 5 %. It was found that geopolymer samples showed a very minimal weight loss of up to 0.7 %, whereas OPC samples lost up to 2.5 % of their weight. Moreover, with 3 % and 6 % of OPC content in the geopolymer samples, there was an observed improvement in compressive strength by 6 %–16 % when compared to their control groups. The morphology of the hydrated products under sulfate attack at all ages with 3 % and 6 % of OPC content shows considerably denser matrices than the control groups with no OPC content. When the available soluble silica provided by sodium silicate increased, it revealed positive improvements in the compressive strength and the formation of the final products under a sulfate environment.