Thermal Behavior of the Raw Materials Forming Porcelain Stoneware Mixtures by Combined Optical and In Situ X‐Ray Dilatometry

The dilatometric behavior of the raw materials forming a porcelain stoneware tile mixture was investigated at a molecular/microscopical scale by combined in situ optical dilatometry and X‐ray powder diffraction. Each single raw material was investigated separately and mixed with all the others to simulate a standard production mixture. Thus, it was possible to evaluate whether the reaction path taking place during the firing of the raw materials is changed during the firing of the combined mixture. A general classification of the causes of contraction and/or expansion events has been attempted. Causes are subdivided into structural (S, intra‐mineral) and microstructural (M, inter‐mineral). Contractions can be generated by: loss of volatiles (S); structure collapse of the pseudo‐amorphous products of dehydroxylation of clay minerals (S); solid state or viscous sintering (M); crystallization from amorphous precursor (S) and related volume reduction (densification) (M); displacive phase transition with a volume decrease (S); and melting generally at T >800°C (S). Expansion can be generated by thermal expansion (S); displacive phase transition with a volume increase (S); glass transitions (S) and related volume change (M); and overfiring/pyroplastic deformation (M). Many reactions observed in the single raw materials have also been observed in the mixture. The major variations concern (i) crystallization of cristobalite is observed only in the kaolin and not in the mixture, where silica segregated from the reaction of formation of mullite from metakaolinite is readily incorporated in the alkaline amorphous phase; (ii) quartz alone is stable with temperature but tends to be partly decomposed in contact with the alkaline melt in the mixture; and (iii) in the illite‐rich clay, melting of the system and subsequent expansion caused by pyroplastic deformation of the K‐rich melt begins at relatively low temperature (1150°C). This behavior is not observed in the mixture below 1350°C where a presumably less viscous melt is formed.