Effects of amorphous Si/Al molar ratio on reaction kinetics and microstructures of slag-rich geopolymer

To more accurately reflect the potential reactivity of aluminosilicate precursors and minimize the interference of inert crystalline phases in geopolymer mix design, this study quantified the amorphous SiO 2 and Al 2 O 3 contents of raw materials and used the resulting amorphous Si/Al molar ratio as a refined compositional index. On this basis, the mechanistic influence of the initial amorphous Si/Al molar ratio on the reaction kinetics, microstructural development, and mechanical performance of slag-rich geopolymer was investigated. Results show that the lower ratio inhibits the polycondensation reaction due to insufficient silicate supply, leading to reduced gel yield and increased defect density within the geopolymer matrix. As this ratio increases, the cumulative heat release during geopolymerization intensifies, the proportion of highly connected Q 2 -Q 4 units increases, and the matrix becomes denser, resulting in the peak mechanical strength at a ratio of 2.6. Beyond this threshold, excess silicate disrupts the reaction equilibrium, which reduces the three-dimensional crosslinking of gel phases and introduces microstructural defects, thus lowering the mechanical strength. These findings elucidate the pivotal role of the amorphous Si/Al molar ratio and provide a solid basis for optimizing geopolymer design toward superior performance. • Amorphous Si/Al ratio was introduced as a refined mix design index for geopolymers. • Effects of amorphous Si/Al ratio on kinetics and microstructures were clarified. • Amorphous Si/Al ratio governs the dissolution-polycondensation balance. • Excess silicate causes incomplete polymerization and pore coarsening.