Microwave heating behavior and thermal homogeneity of kaolinite-, illite-, and smectite-rich clay systems with additives

Microwave heating of clay-based materials has the potential to make ceramic processing more energy efficient. However, the influence of mineralogical composition and bulk density of the clays on heating behavior is not yet fully understood. This study investigates the microwave response at 2.45 GHz of kaolinite-, illite-, and smectite-rich clays with and without additives, as well as brick clays. The heating behavior was evaluated in relation to mineralogical, thermal (experimental), and dielectric (literature) analyses. Smectite-rich systems exhibited the strongest initial microwave coupling, followed by illite-rich clays, while kaolinite-rich systems showed comparatively weak absorption. During dehydroxylation, the net heating rate decreased in all systems due to the endothermic nature of the reaction, which was most pronounced in kaolinite. Kaolinite-rich clays exhibited significant dielectric changes during phase transitions, shifting from weakly to strongly absorbing regions. This promoted local overheating and temperature inhomogeneities. In contrast, illite- and smectite-rich clays maintained a more stable absorption over several reaction intervals, resulting in more uniform energy distribution and homogeneous temperature profiles. Additional phases altered the overall microwave interaction: iron oxides and carbonaceous components enhanced absorption, while most carbonates had only a minor influence. Increased bulk density, especially in quartz-containing systems, contributed to more uniform temperature fields. Overall, the microwave heating response of the investigated clay systems was governed by the interplay between mineralogical composition, additive content, and sample density. This provides a framework for improving the evaluation of thermal treatment strategies for ceramic raw materials using microwave heating.