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. In this work, we have analyzed clays from quarries in the Cerro de Pasco region, Peru, to evaluate their characteristics and the structural changes produced by heating, particularly in the interlaminar region. The samples were thermally treated between 150 oC and 800 oC with intervals of 50 oC. To evaluate the structural changes produced by temperature, X-ray diffraction were carried out before and after each heat treatment. The qualitative analysis of the measurements allowed to identify 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 the kaolinite until the collapse of its structure between 450 °C and 500 °C. The illite presented greater thermal stability, with slight variations in its weight percentage during heat treatment, without compromising its structure. Although the quartz phase did not show relevant structure changes, it slightly increased its weight percentage with increasing temperature.