Effect of inorganic salt composition on strength, microstructure and leaching toxicity of coal-based solid waste backfill materials

This study explores the potential of coal-fired slag, desulfurized gypsum and modified magnesium slag as cementing agents for coal-based solid waste backfill materials. Inorganic salts like CaCl2, Na2SO4 and Na2SiO3 were used as excitants to investigate their combined impact on the mechanical properties, microstructure and leaching risk of hazardous elements from the coal-based solid waste backfill materials. The findings show that the addition of inorganic salt boosts the alkalinity within the liquid-phase reaction system. This disrupted the hydration-blocking membrane, accelerating the dissolution and reaction rate of the silica-alumina mineral components within the cementitious material. Concurrently, the inorganic salt ions consumed the early alkaline hydration products, destroying the hydration reaction equilibrium and facilitating the formation of hydration products like AFt, C-S-H gel, C-A-S-H gel, and Friedel's salt. This increase in the solid-phase volume content and microscopic densities in the reaction system improved the early mechanical properties of the coal-based solid waste backfill materials. Additionally, after a 28-day curing period, the leaching concentration of deleterious elements in the coal-based solid waste backfill material containing inorganic salt components was below the permissible concentration limit of the Class III groundwater quality standard. This eco-friendly backfill material is promising for mine backfilling, a potential substitute for traditional cement-based backfill material.