Performance and hydration mechanism of fly ash coal-based solid waste backfill material affected by multiple factors

Masses of coal-based solid waste like fly ash are emitted during coal combustion, with low utilization rate. Preparing fly ash into backfill material can not only improve waste utilization efficiency, but also repair the cracks in mining areas. In this research, the performance and hydration mechanism of fly ash coal-based solid waste backfill material were investigated through slurry performance experiment, mechanical strength test and microstructure analysis. Besides, the grouting effect of the backfill material was verified by grouting experiment. The research demonstrated that the increase of fly ash to cement ratio (from 0.5 to 1.5) improved the fluidity, viscosity and bleeding rate of the slurry, but prolonged the setting time. The adjustability of backfill material was good, and the compressive strength of specimen reached its highest value of 27.1 Mpa through changing multiple factors. The grouting effect of fine-grained macadam was better than that of coarse-grained macadam. The compressive strength and Young's modulus of the backfill material specimens containing 0.5-1 cm macadam were 12.2 % and 17.4 % higher, respectively, than those with 1-2 cm macadam. The primary hydration product of backfill material was gel, which structural elements were Si, Al and Ca. As the curing age increased, the distribution of the three elements became denser, and more gel was formed, corresponding to the changes in the compressive strength of the backfill material. The research offers material source and selection basis for back- filling cracks in mining areas and enhances utilization efficiency of coal-based solid waste.