Synergistic effect of characteristics of raw materials on controlling the mechanical properties of fly ash-based geopolymers

This study investigates the macro- and micro-characteristics of the geopolymers to verify the feasibility of prediction of compressive strength via quantitatively adjusting the amorphous phase content (APC) and particle size distribution (PSD) of raw material. The optimization of PSD through the sieving method does not reach the intended compressive strength compared to the reference, while controlling APC by partly replacing fly ash with slag achieves the expected target. The critical reasons are well explained by the micro properties of geopolymers. The sieving process leads to the reduction in the porosity but has little effect on the Si/Al molar ratio and APC of geopolymers, which is related to the heterogeneous distribution of active constituents of fly ash. By contrast, the addition of slag significantly enhances the microstructure of geopolymers, especially by the interaction between C-A-S-H and N-A-S-H gel. As a result, the compressive strength is primarily determined by the micro index MCa, proved by the mutual positive correlation. Besides, given the low reaction rate of fly ash in the geopolymerization and relevant experimental results, the modified formula that only uses the APC and specific gravity (G20°C) of powder raw materials to establish the relationship with compressive strength owns a better linear dependence and probably more appropriate for the prediction of mechanical properties of fly ash-based geopolymers.