Phase evolution in mechanically treated mixtures of kaolinite and alumina hydrates (gibbsite and boehmite)

The phase evolution behaviour of kaolinite–gibbsite mixtures ground for 5 min and 4 h has been investigated by X-ray powder diffraction (XRD). Differential scanning calorimetry (DSC) and MAS NMR. The thermal behaviour of mixtures ground for 5 min was similar to that of kaolinite-α-alumina mixtures. After grinding for 4 h, the mixture is transformed into an amorphous state. Grinding caused the acceleration of thermal reactions in these mixtures, with a degree of mechanochemical reaction being found to occur between kaolinite and α-alumina. Grinding kaolinite-boehmite mixtures produced mullite precursors which were more homogeneous than those in which the kaolinite had been dehydroxylated, suggesting the participation of the structural water from the kaolinite. Secondary mullite is formed during thermal reaction of these mixtures by a solution-precipitation mechanism. The mullite initially formed was alumina-rich, but approached the stoichiometric composition with increasing firing temperature.