Study of the illite-kaolinite-CaCO₃ mixtures during firing

The raw materials used in traditional ceramics include highly plastic clay minerals such as illite or kaolinite. Non-plastic materials are usually added to the mixtures, which affect their workability and sintering process. The formation of a new crystalline phase (e.g. anorthite) is often observed in mixtures that contain a large amount of CaO. The improvement of final properties of ceramic materials is attributed to the formation of anorthite. The aim of this paper is to study the firing of anorthite ceramics in the system illite-kaolinite-CaCO₃. Four sets of samples with different illite:kaolinite ratio (8:2, 6:4, 4:6, 2:8), and with a constant addition of CaCO₃ are produced. The samples were subjected to the thermal and structural analyses. A significant mass loss is observed for all studied samples during the process of dehydroxylation and decomposition of CaCO₃. An exothermic peak around 950 °C, which is connected with the formation of anorthite, is observed during differential thermal analysis. Above this temperature, no significant length changes are recorded. The presence of anorthite is confirmed by XRD analysis. The most formation of anorthite is observed in the illite-kaolinite sample with ratio 8:2.