Microstuctural evolution during thermal treatment of three kaolinitic clays from Cameroon

The paper examines the influence of Al2O3/SiO2 ratio, bulk chemical composition and temperature in mullite nucleation and growth from kaolinitic clays. Samples from NTAMUKA (TAN), Mayouom (MAY) and Wabane (WAB), region of high hills of West-Cameroon, undergo series of phase changes during sintering between 1000 and 1500°C. Phases transformation included: amorphous SiO2 and Al2O3 as result of breakdown of kaolin above 700°C, nuclei of mullite that growth with temperature development to fine and then large grains at high temperature. The microstructure of the sintered products consisted on the elongated secondary mullite (types II and III) interlocking with primary (type I) mullite in a compact matrix with relative amount of liquid film for MAY and WAB. The morphology of mullite grains in TAN was more different being larger cuboid grains aggregated to form together with cristobalite a compact microstructure. The formation of TiO2 crystals and then Ti–Al (tialite: Al2TiO5) crystals influenced the microstructure of MAY and WAB. The microstructure at high temperature of the three kaolinitic clays indicated their suitability for dense refractory and fine ceramic applications.