Clay Structural Transformations during Firing

Silicate ceramics with clays are some of the most complicated ceramic systems because of the very complex relationship between the behavior of mineral materials during the ceramic processing and the transformations during heating. A major challenge is to predict the phase transformations in silicate ceramics, since complex relationship occur between the microstructural and structural characteristics of fired product and the physical properties. Clay minerals undergo strong structural transformations during heating, simultaneously to a complex path of thermal transformations. Individual reactions cannot simply identify since they are closely related and overlapped. At temperature above 800°C, new phases are recrystallized and many of the reactions are strongly topotactic. Mullite is the most important phase, which recrystallizes with a range of morphology and stoichiometry. Variables affecting the mullite formation include the clay mineral type and behavior during heating, the possible occurrence of liquid and impurities as Fe. It results in large variations of the stoichiometry and shape of mullite crystals, which are embedded in a low ordered phase to form a micro-composite microstructure. This presentation will review recent research, looking at structural transformations in some typically used phyllosilicate systems : (i) structural transformation of kaolinite and mica phases were identified at temperature up to 1100°C. They evidence a residual structural order of high temperature phases which is favorable to the topotactic recrystallization of mullite; (ii) from the high temperature form of phyllosilicates, an organized network of mullite can be obtained; (iii) the composition of a local and transient liquid and the presence of minor elements as Fe has a significant influence on the mullite morphology; (iv) mechanical properties are closely related to size and organization degree of the mullite network; (v) the process itself influence the kinetic of structural transformation and particularly the powder compact density and the thermal cycle. These research in silicate ceramics evidence multiple and complex challenges, providing opportunities for future development.