Na-feldspar (F) and kaolinite (K) system at high temperature: Resulting phase composition, micro-structural features and mullite-glass Gibbs energy of formation, as a function of F/K ratio and kaolinite crystallinity

The system Na-feldspar (F) and kaolinite (K) was investigated at temperatures of interest in ceramic applications (1200-1280 degrees C) to study the effects of F/K ratios by weight and crystallinity degree of kaolinite on the final product, micro-structural features and mullite-glass Gibbs energy of formation (Delta G(eff)). Mullite and glass are the dominant phases; in general, the higher the temperature, the larger the former. An F/K increase promotes the formation of glass and secondary mullite, appearing along with the primary one. Delta G(eff) was modelled by alpha(T) x (F/K)(2) + beta(T) x F/K + gamma(T), alpha, beta and gamma being linear functions of temperature whose coefficients were determined by fitting the Delta G(eff)-theoretical to the Delta G(eff)-obtained from the measured phase compositions. Delta G(eff) is less affected by temperature than by F/K, whose increase shifts equilibrium towards glass phases. The Delta G(eff)-curves for ordered and disordered kaolinite intersect one another at F/K similar to 0.5, a ratio close to that used in industrial practice.