Effect of specimen geometry on kinetics of thermal decomposition of minerals in porous ceramic tiles

Porous ceramic tiles are used as covering for indoor vertical surfaces in buildings. Clay and limestone are the starting raw materials. The firing step consolidates the product's properties in the manufacturing process. In this step several reactions occur, among them are the dehydroxylation of the clay minerals and decomposition of the carbonates. This study determined the kinetics parameters associated with the decomposition of these 2 minerals. Two different methods of samples preparation were investigated. In the first method, 60 mg of a spray‐dried powder was submitted to analysis in traditional thermogravimetric equipment. In the second method, compacted specimens with dimensions of 80 × 20 × 2.5 mm³ and a mass of 7 g, compacted at 25 MPa, were submitted to analysis using customized thermogravimetric apparatus. The curves were obtained for 6 different heating rates between 2.5 and 20 K min−1. The activation energy, the pre‐exponential factor of the specific velocity equation, and the reaction mechanism were determined. Depending on the methods and compaction degree the activation energy varies from 177.0 to 224.7 kJ mol−1 and from 188.5 to 230.2 kJ mol−1 for kaolinite and calcium carbonate, respectively. For accurate measurement of the activation energy, the heat transfer needs to be considered for each particular experimental configuration.