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 m in and 4 h has been investigated by X-ray powder diffraction (XRD). Differe ntial scanning calorimetry (DSC) and MAS NMR. The thermal behaviour of mixt ures ground for 5 min was similar to that of kaolinite-alpha-alumina mixtur es. After grinding for 4 h, the mixture is transformed into an amorphous st ate. Grinding caused the acceleration of thermal reactions in these mixture s, with a degree of mechanochemical reaction being found to occur between k aolinite and alpha-alumina. Grinding kaolinite-boehmite mixtures produced m ullite precursors which were more homogeneous than those in which the kaoli nite had been dehydroxylated, suggesting the participation of the structura l water from the kaolinite. Secondary mullite is formed during thermal reac tion of these mixtures by a solution-precipitation mechanism. The mullite i nitially formed was alumina-rich, but approached the stoichiometric composi tion with increasing firing temperature. (C) 2000 Elsevier Science Ltd. All rights reserved.