Mullitization behaviour during thermal treatment of three kaolinitic clays from Cameroon: Densificaron, sintering kinetics and microstructure

Three kaolinitic clays from Cameroon were studied for their mullitization behaviour. The three clayey materials were from Ntamuka (TAN), Mayouom (MAY) and Wabane (WAB), all situated in the hills of western Cameroon. X-ray diffraction and thermal, dilatometric and SEM-EDS analyses were used to follow up the phase evolution, sintering kinetics and microstructure of the three materials as a function of temperature (1000-1500°C). Fine powders of each sample were pressed and treated in the above temperature range with the goal to correlate the phase evolution with densificaron parameters (shrinkage, porosity, density and mechanical strength). The nucleation of mullite and the increase of peak intensities were directly correlated to continuous densification and reduction of open porosity as observed under the SEM, The mullitization peak temperatures at 5°C/min were 973°C, 979.1°C, and 983.6°C respectively for TAN, MAY and WAB and - in the same order but at 20°C/min 992.1°C, 997.4°C and 1001.2°C. The mullitization phenomenon, which includes a first step of nucleation and a second of crystal growth, shows an activation energy that varies depending on the nature of sample investigated: the values ranged from 650 to 730 kJ/mol. 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 glassy phase for MAY and WAB. The morphology of mullite grains in TAN was more different being laiger cuboid grains aggregated with cristobalite to form a compact microstructure. The formation of TiO2 crystals and then Ti-Al (tialite: Al2TiO5) crystals influenced the microstructure of MAY and WAB.