Influence of Silicate Modulus and Alkali Content on the Performance of Fly Ash–GGBS Geopolymer Grouting Materials for Roadway Rehabilitation

Geopolymer grouting material incorporating solid wastes show considerable potential for subgrade rehabilitation in roadway engineering. This study investigates the effects of alkali dosage and silicate modulus on the setting time, flowability, compressive strength, autogenous shrinkage, chemical shrinkage, and drying shrinkage of such materials prepared with a water-to-binder ratio of 0.45. Mercury intrusion porosimetry (MIP) and scanning electron microscopy (SEM) were employed to elucidate the microstructural mechanisms underlying the observed macroscopic behaviors. The results show that the flowability, expressed in terms of flow time, first increases and then decreases with increasing silicate modulus. Higher alkali dosage and silicate modulus enhance compressive strength and accelerate setting and hardening, but also substantially intensify autogenous, chemical, and drying shrinkage. The impact of silicate modulus is greater than that of alkali dosage, since a higher silicate modulus produces a more significant reduction in setting time and a more pronounced improvement in compressive strength, while also causing a much larger rise in autogenous and drying shrinkage. At 6% alkali dosage and a silicate modulus of 2.0, the grouting material attains a compressive strength over 70 MPa, but its chemical shrinkage reaches 0.04464 mL/g, autogenous shrinkage rises to 6267 µε, and drying shrinkage exceeds 3.45%, all severely impairing its volumetric stability. SEM observations show that higher alkali dosage and silicate modulus yield a denser microstructure, explaining the enhanced compressive strength, and the simultaneous growth of microcracks confirms the observed shrinkage increases. The MIP results demonstrate that increasing silicate modulus is more effective than increasing the alkali dosage in reducing porosity and refining the pore structure. However, no direct correlation was observed between the mesopore content and shrinkage in the geopolymer systems investigated in this study. Nevertheless, the fact that mesopores account for more than 85% of the total pore volume may provide a possible mechanistic explanation for why geopolymers generally exhibit substantially higher shrinkage than conventional cement-based materials.