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 porcelainstoneware tile mixture was investigated at a molecular/microscopical scale by combined in situ optical dilatometry andX-ray powder diffraction. Each single raw material was investigatedseparately and mixed with all the others to simulate astandard production mixture. Thus, it was possible to evaluatewhether the reaction path taking place during the firing of theraw materials is changed during the firing of the combined mixture.A general classification of the causes of contraction and/orexpansion events has been attempted. Causes are subdivided intostructural (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 dehydroxylationof clay minerals (S); solid state or viscous sintering(M); crystallization from amorphous precursor (S) and relatedvolume reduction (densification) (M); displacive phase transitionwith a volume decrease (S); and melting generally at T48001C(S). Expansion can be generated by thermal expansion (S);displacive phase transition with a volume increase (S); glasstransitions (S) and related volume change (M); and overfiring/pyroplastic deformation (M). Many reactions observed in thesingle raw materials have also been observed in the mixture. Themajor variations concern (i) crystallization of cristobalite is observedonly in the kaolin and not in the mixture, where silicasegregated from the reaction of formation of mullite from metakaoliniteis readily incorporated in the alkaline amorphousphase; (ii) quartz alone is stable with temperature but tends tobe partly decomposed in contact with the alkaline melt in themixture; and (iii) in the illite-rich clay, melting of the system andsubsequent expansion caused by pyroplastic deformation of theK-rich melt begins at relatively low temperature (11501C). Thisbehavior is not observed in the mixture below 13501C where apresumably less viscous melt is formed.