Performance Optimization and Hardening Mechanism of Alkali-Activated Coal-Based Solid Waste Grouts

Grouting for fire prevention and extinguishing in coal mines requires slurries that combine fluidity with mechanical strength to penetrate and stably seal fractures. Meanwhile, mining generates coal gangue and fly ash that pollute the environment; converting these solid wastes into fire-prevention materials enables waste utilization and underground fire control. To this end, an alkali-activated grouting material was prepared in this study using coal gangue and fly ash as the primary raw materials, and the mechanisms by which mass concentration, fly ash content, alkali equivalent, and water glass modulus govern the fluidity, setting, and mechanical properties of the grout were investigated. The results indicate that mass concentration is the dominant factor controlling grout viscosity, whereas fly ash content markedly affects the initial setting time and uniaxial compressive strength (UCS). Multi-objective optimization yielded a recommended formulation of 25% fly ash content, 76% mass concentration, 6% alkali equivalent, and a water glass modulus of 1.348, with experimentally validated values of 406 mm for spread diameter and 5.09 MPa for 3-day UCS. The combined macroscopic and microstructural observations are consistent with a proposed hardening sequence involving particle dispersion, dissolution–activation and cementitious-phase formation, network interconnection, and progressive densification.