Effects of high temperature treatment on physical-thermal properties of clay

High temperature changes the internal microstructure of clay and consequently affects its physical and thermal properties. In this paper, the influence of firing temperature (up to 900 degrees C) on the thermo-physical properties of clay is analyzed through laboratory tests and the relationship between thermal conductivity and density is revealed. Furthermore, the variation of surface characteristics including clay color and cracking is reported. The results obtained indicate that the thermal conductivity and diffusivity as well as the bulk density of clay decrease rapidly as the temperature increases from room temperature to 200 degrees C, and then become approximately constant. From the Thermogravimetric Analysis (TG) and Differential Thermal Analysis (DTA) tests it is also shown that there are two significant stages for the clay exposed to high temperature: 25-200 degrees C and 400-700 degrees C. The evaporation of adhered water and bound water are proved to be the main reason for the first stage of clay mass loss at 25-200 degrees C, while the decomposition of minerals is considered as the second stage of mass loss in the temperature range of 400-700 degrees C. In addition, the combustion of organic matter in clay is the main reason for its darkening at 300-400 degrees C.