Structural changes in clays subjected to heat treatment: an analysis by structural refinement using the Rietveld method

Structural factors in clays influence their physical properties. Therefore, it is particularly important to understand the effects of heat treatment on the structure of the material during the ceramic process. This work analyzed clays from quarries located in the Cerro de Pasco region, Peru, evaluating their characteristics and the structural changes produced by heating, particularly in the interlaminar region. The samples were thermally treated between 150 degrees C and 800 degrees C at intervals of 50 degrees C. In order to evaluate the structural changes produced by temperature, X-ray diffraction was carried out before and after each heat treatment. The qualitative analysis of the measurements allowed identifying the mineralogical composition of the samples, finding phases of calcium montmorillonite, kaolinite, illite, and quartz. The quantitative analysis by the Rietveld method found structural changes, particularly in the Ca-montmorillonite expansive clay. It was also possible to determine the decrease in the weight percentage of kaolinite till the collapse of its structure between 450 degrees C and 500 degrees C. Illite presented greater thermal stability with slight variations in its weight percentage during heat treatment but without compromising its structure. Although the quartz phase did not show relevant structure changes, it slightly increased its weight percentage with increasing temperature.