Effects of densification and mullitization on the evolution of the elastic properties of a clay-based material during firing

The microstructural evolution of an illito-kaolinitic raw material has been characterized during firing at 10 °C/min up to 1200 °C. The strong evolution of porosity and amount of mullite formed from the viscous flux (from 1000 °C) has been quantified using dilatometric measurement, image analysis and X-ray diffraction. In situ ultrasonic echography has been used in order to determine the Youngʹs modulus (E) evolution associated to the microstructural changes. This technique is highly sensitive to porosity elimination and mullite development even though an abundant viscous flux is present. For low amount of mullite (<24.7 vol.%), the E evolution observed can be easily related to porosity and mullite volume proportion changes by applying the Hashin & Shtrikman approach (lower bound). For higher mullite content, the strong experimental E increase observed between 25.9 and 51.2 GPa has been related to the transition from isolated rigid inclusions (mullite and quartz) in soft matrix (viscous flux) toward a percolating network.