Abstract
Dynamical thermomechanical analysis of 5 illite-based clays from deposits in Slovakia, Estonia, Latvia, and Hungary is presented. The clays consist of illite (37–80 mass%), quartz (12–48 mass%), K-feldspar (4–13 mass%), kaolinite (0–18 mass%), and calcite (0–3 mass%). Young’s modulus is measured during the heating and cooling stages of firing (25 °C → 1100 °C → 25 °C). The liberation of the physically bound water increases Young’s modulus by ∼70% for all studied clays. By increasing the temperature, dehydroxylation and the α → β transition of quartz take place without a significant effect on Young’s modulus. Sintering, which starts at 800 °C, leads to an intensive increase in Young’s modulus up to the highest temperature (1100 °C). The increase remains also in the early stage of cooling (1100 °C → 800 °C). This increase of Young’s modulus is also the result of solidification of the glassy phase, which is finished at ∼750 °C. A sharp minimum of Young’s modulus is observed at around the β → α transition of quartz. Then, Young’s modulus still decreases its value down to the room temperature. The physical processes observed during heating and cooling do not differ in nature for the studied clays. Values of Young’s modulus vary at around 8 GPa, up to 800 °C. During sintering, Young’s modulus reaches values from 30 GPa to 70 GPa for the studied clays. The microstructure and composition given by the origin of the clay play a cardinal role for the Young’s modulus of the final ceramic body.
Highlights
Traditional ceramics made from natural mineral sources are still used in many fields
This paper intends to provide a comprehensive overview of the development of Young’s modulus, thermal expansion, mass change, and bulk density during the firing of illitic clays originating in Slovakia, Hungary, Estonia, and Latvia [7,8,9,10,11,12,13]
Illitic clays undergo several structural and microstructural changes when they were heated from the room temperature up to 1100 ◦ C
Summary
Traditional ceramics made from natural mineral sources (clays, quartz, and feldspars) are still used in many fields. A plastic mixture of these materials with water is the base of the most used forming method of ceramic bodies, which are sequentially dried and fired. Some innovative materials based on illitic clays were investigated in the past decade: ceramic hollow spheres for agriculture and civil engineering [1,2] or lightweight cenosphere–clay syntactic-foam [3]. That is why the investigation of the firing of illitic clays is important and can provide advantageous information. This paper intends to provide a comprehensive overview of the development of Young’s modulus, thermal expansion, mass change, and bulk density during the firing of illitic clays originating in Slovakia, Hungary, Estonia, and Latvia [7,8,9,10,11,12,13]. The result will contribute to understanding of the heat-induced processes through the in situ measurement of Young’s modulus
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