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Abstract
Heating transformations of calcareous chloritic-illitic and carbonate-free kaolinitic-illitic raw clays, and their mixes were investigated in the range of 700°-1060°C using X-ray diffraction, thermal analyses and scanning electron microscopy. Moreover, changes of bending strength, shrinkage and water absorption versus temperature and clay content were followed. The results showed that plagioclase and sanidine together with spinel formed and persisted in fired calcareous and non-calcareous clays respectively. In fired clay mixtures only plagioclase occurred. Formation of the latter phase was discussed using CaO-SiO2-Al2O3 equilibrium diagram. The intensification of the neoformation processes due to the increase of temperature together with melt formation mainly in carbonate-free clay improved the ceramic properties to certain extent. Improvement of the mechanical strength was also observed for mixture of clays. The reported diagrams, involving grain size distribution and chemical composition, showed that both clays and their mixtures could be suitable for porous tiles or lightweight ceramic blocks manufacturing.