Kaolinites–mullite transformation in different Al2O3–SiO2 systems: Thermo-analytical studies

Abstract

Gels of synthetic kaolinites formed from mixtures of very pure metalorganic compounds of aluminium and silicon and with seven different Al/Si ratios, have been investigated by DTA/TG/DTG up to 1200°C. The enthalpies associated with the exothermic reaction occuring at 980°C, have been determined and plotted as a function of composition. The enthalpy reaches a maximum at a composition corresponding to 2Al₂O₃·3SiO₂. This result has been discussed with reference to the well known work by Brindley and Nakahira from 1959 on the kaolinite–mullite reaction series (J. Am. Ceram. Soc., 1959, 42, 319) Good correlation between the experimental enthalpies and the enthalpies of crystallization of alumina and silica taken from the literature, have been obtained. Models for distribution of the Al atoms between the octahedral and the tetrahedral positions in the mullite structure have been discussed, and models for the incongruent melting of both 3:2 mullite and 2:1 mullite have been evaluated.