Mullitization

As kaolinite is heated beyond 980°C, the small fraction of mullite crystals that formed at 980°C continue to grow, albeit at a slow rate. Mullite growth is accompanied by the disappearance of the spinel phase, although the amount of mullite formed is lower than expected based on the spinel loss [33]. Mullite formation does not approach com­pletion until a second exothermic event occurs at approximately 1200°C, as recorded by differential thermal analysis [33]. When formed by solid-state reaction, mullite has a composition of 3Al2O32SiO2, approximately 72 wt% alumina and 28 wt% silica [39]. According to Brindley, the mullite formed by heating to 1200°C contains all of the alumina from the original clay, while the silica is distributed between the mullite phase and an amorphous phase [33]. Further heating alters the size of the needle-like mullite grains and can result in crystallization of the silica to crystobalite [22]. Heating to 1200°C is generally sufficient to fully densify clay-based ceramic bodies.