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