Performance and characterization of fly ash-metakaolin-based geopolymer pastes

Geopolymers derived from various aluminosilicate materials are significantly impacted by the composition of raw materials and activators. However, existing research has not clearly elucidated the relationship and evolution laws between raw material composition, activator alkalinity, and geopolymer properties. To address this gap, this study investigates the effects of the fly ash/metakaolin ratio and activator concentration on the mechanical performance and microstructures of fly ash-metakaolin (FM) based geopolymer pastes. This comprehensive approach aims to uncover the evolutionary patterns of material properties and reveal the reaction mechanism and influencing factors of geopolymer derived from fly ash and metakaolin. The study revealed that an increase in fly ash content led to a gradual decrease in the total heat release of FM-based geopolymer pastes, indicating reduced hydration reaction degree and subsequent compressive strength decline. Additionally, higher residual alkali content in the pore solution contributed to elevated pH values. Increasing activator concentration effectively enhanced geopolymer hydration reactions, with compressive strength correlating with total heat release. Varied activator concentrations yielded similar final pH values in the pore solution. FM-based geopolymer pastes exhibited numerous nano-sized pore structures, with paste compactness decreasing as fly ash content increased, while total porosity decreased with higher activator concentrations, resulting in a denser microstructure. Hydration products of FM-based geopolymer pastes primarily comprised amorphous aluminosilicate gel, quartz, mullite, and partially unreacted sodium silicate crystals.