Sintering Behavior and Environmental Safety of Ceramsites Prepared from High-Alumina Fly Ash, Coal Gangue and Waste Glass

High-alumina fly ash and coal gangue were used as the main aluminosilicate precursors, and waste glass was introduced as a fluxing additive to prepare waste-derived ceramsites. Based on composition design assisted by the CaO-SiO2-Al2O3-Na2O phase diagram, the effects of sintering temperature and holding time on phase evolution, microstructure, bulk density, compressive strength, and environmental safety were systematically investigated. The results showed that waste glass promoted early liquid-phase formation and broadened the effective sintering window. As the sintering temperature increased from 1200 to 1300 °C, the bulk density continuously decreased from 1.77 to 1.19 g/cm3, whereas the compressive strength first increased from 4.9 MPa to a maximum of 10.0 MPa at 1240 °C and then decreased to 2.8 MPa because of the competition between skeletal consolidation and excessive pore coarsening. The main crystalline phases of the ceramsites were anorthite and mullite. Prolonged holding mainly accelerated pore coalescence and pore-wall thinning, resulting in a continuous decrease in compressive strength. TCLP and long-term leaching tests conducted in freshwater, saline, and acidic media showed that the release of hazardous elements remained very low. These results demonstrate that glass-assisted sintering is an effective route for converting coal-based solid wastes into environmentally safe ceramsites.