Microstructure and alkali attack resistance of novel mullite materials prepared from low-grade raw bauxite and kyanite tailings

The widespread use of alternative fuels in cement kilns requires improvement of the alkali attack resistance of mullite raw materials for the kiln linings. In this study, a novel mullite raw material with excellent alkali resistance was prepared by introducing kyanite tailings into low-grade raw bauxite. The effects of the microstructure, glass phase composition and phase distribution on the alkali attack resistance of such raw material were investigated. As the amount of kyanite tailings introduced increased, the glass-phase content of the specimens had little change, while the silica content in the glass phase increased obviously. Additionally, with increasing kyanite tailings content, the mullite phase gradually transformed from fine needle-like structures into columnar or tabular structures, allowing mullite to be better enclosed within the glass phase. The alkali attack test indicated that adding kyanite tailings significantly improved the alkali attack resistance of the specimens. This was explained as follows. (i) The silica-rich glass would be transformed into liquid phase during alkali attack, changing the corrosion process from the direct gas–solid reactions to dissolution–precipitation process. The formed liquid phase also densified the corrosion layer, effectively preventing further penetration of the alkali vapor. (ii) The mullite phase was wrapped in the silica-rich glass phase, preventing its direct reaction with alkali vapor and the resulting volume expansion. Besides, alkali attack resistance of the mullite phase was enhanced due to its particle coarsening.