Elastic Properties of Illite-Based Ceramics at Low Temperatures of Firing

Samples made from illitic clay were investigated using thermogravimetry (TG), thermodilatometry (TD) and dynamic mechanical analysis (DMA) during heating from room temperature to 300 °C. TG revealed three steps of mass loss: (a) the release of weakly bound H2O (with the maximum rate at ~120 °C) from the pores, (b) a small mass loss event around 215 °C, (c) a small mass loss event near ~300 °C related to dehydration when H2O molecules located in K-free sites of the illite interlayers are removed. TD indicated very small dimension changes for 20 °C → 300 °C. This behavior may result from two competing mechanisms, where the first one is regular thermal expansion and the second one is particle rearrangement caused by the removal of physically bound water. Young’s modulus initially decreases during heating up to approximately 70 °C. Young’s modulus subsequently increases exponentially, which may be explained by mechanisms analogous to those observed in the TD measurements. The activation energies derived from the exponential dependence E(t) are 5.66 kJ/mol for the temperature interval 130–200 °C and 10.96 kJ/mol for the 200–280 °C range.