Development of Young's modulus of natural illitic clay during the heating and cooling stages of firing

Illitic clay from the locality of Liepa, Latvia, was investigated using dynamic thermomechanical analysis during the heating and cooling stages of firing. Differential thermal analysis, thermogravimetry, thermodilatometry, X-ray diffraction and porosimetry were also performed to shed light on the processes influencing the elastic properties of clay. The increase in the Young's modulus (YM) at low temperatures was linked to the release of physically bound water. Above 850°C, the bulk density and YM both increased as a consequence of sintering. The YM was more sensitive to the progress of sintering compared to dimension changes. The YM values continued to increase during cooling until the glass-transition temperature was reached. At this temperature, the first microcracks caused by the differences in thermal expansion coefficients of the present phases were expected to appear. The YM showed a sharp V-shaped minimum at the β → α transition of quartz, which was a result of alternation of the mechanical radial stresses around the quartz grains. When the transition of quartz was completed, the YM continued to decrease because microcracks were still being created at the boundaries between the different phases. The decrease of the YM during cooling from the glass-transition temperature down to room temperature was ~50% for all of the firing temperatures and isothermal periods applied.