Mullite-based ceramic tiles produced solely from high-alumina fly ash: Preparation and sintering mechanism

Abstract

Mullite-based ceramic tiles were successfully prepared using only high-alumina fly ash (HAFA) at relatively low sintering temperatures (1100–1400 °C). The effects of alkali activation on the chemical composition, mineral phase, and morphology of the HAFA were respectively characterised by ICP-OES, XRD, and SEM methods. The alkali-activation pretreatment not only modifies the chemical composition of HAFA to match that of mullite, but also can introduce Na-rich compounds to promote sintering. The macroscopic and microscopic characteristics of the fully ash-based ceramic tiles formed at different sintering temperatures were examined. The ceramic samples sintered at 1300 °C exhibited optimal post-sintering properties (relative density: 90.85%, water absorption: 0.10%, rupture modulus: 109.67 MPa, linear shrinkage: 15.70%, and apparent porosity: 0.68%) and a well-developed cubic puncheon-shaped mullite morphology. Finally, through the isothermal sintering method, the melt viscosities and the sintering kinetics were calculated theoretically, and the results show that the alkali activation could dramatically reduce the melt viscosity and the apparent activation energy. This research may provide a new method in utilising the vast amounts of HAFA waste to produce mullite-based ceramic tiles at low costs.