Use of noncontact dilatometry for the assessment of the sintering kinetics during mullitization of three kaolinitic clays from Cameroon

Noncontact dilatometry, compared to differential scanning calorimetry (DSC), was used together with scanning electron microscopy and densification behavior studies to investigate the parameters that govern the kinetics of transformation of kaolin to mullite during sintering.Three kaolinitc clays from Cameroon, with different SiO2/Al2O3 molar ratio, were examined. The temperatures of mullite nucleation were 973, 979, and 984 deg C at 5 deg C/min heating rate, respectively, for values of SiO2/Al2O3 molar ratio equal to 4.22, 2.22, and 2.08. At 20 deg C/min heating rate, the temperatures are shifted to higher values, 992, 997, and 1,001 deg C. The mullitization phenomenon, which includes a first step of nucleation and a second one of crystal growth, presented activation energy in the range of 650–730 kJ/mol, depending on the nature of the sample investigated. These values, obtained by noncontact dilatometer measurements, were comparable to those obtained by means of DSC and are in agreement with literature values. The difference in sintering kinetics for the three kaolinitic clays could explain the different morphologies obtained for the mullite grains.