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 °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 (ΔGeff). 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. ΔGeff was modelled by α(T) × (F/K)2 + β(T) × F/K + γ(T), α, β and γ being linear functions of temperature whose coefficients were determined by fitting the ΔGeff-theoretical to the ΔGeff-obtained from the measured phase compositions. ΔGeff is less affected by temperature than by F/K, whose increase shifts equilibrium towards glass phases. The ΔGeff-curves for ordered and disordered kaolinite intersect one another at F/K ∼0.5, a ratio close to that used in industrial practice.