Effect of alumina source on the densification, phase evolution, and strengthening of sintered mullite-based ceramics from milled coal fly ash

•Boehmite was preferable to Al(OH)3 and Al2O3 for the mullite-based ceramics.•The flexural strength of 155 MPa was achieved with 11.8% boehmite at 1100 °C.•The particle refinement facilitated the densification and mullitization.•The strengthening of sintered ceramics was related to multiple factors. Different alumina sources (Al(OH)3, Al2O3, and boehmite) were used to prepare mullite-based ceramics from milled coal fly ash. The effect of alumina source and sintering temperature on the densification and phase evolution of these ceramics was investigated. Among the three alumina sources, boehmite yielded the greatest densification and crystallization of the ceramics, and an optimal sintering temperature of 1100 °C was identified. The crystallinity and mullite phase increased with increasing boehmite addition, whereas the cristobalite phase decreased. Bulk density and flexural strength values of 2.72 g/cm3 and 155 MPa, respectively, were realized at a boehmite content of 11.8%. The sintered ceramic consisted of mullite and anorthite phases, and was characterized by a crystallinity of 82.8%. Furthermore, particle refinement of raw materials subjected to prolonged ball milling promoted densification and mullitization at relatively low temperatures. The strengthening of the sintered ceramics was attributed to the improved densification, enhanced crystallization, and increased mullite phase. These results will enable the successful reutilization of coal fly ash in the high-strength ceramic tiles.