Experimental investigation and comparison of soak and flash calcined kaolinite and montmorillonite

Calcined clay holds great potential as a Supplementary Cementitious Material (SCM). Despite ample literature on using calcined clays as SCMs, there is a shortage of studies comparing clay calcination methods, especially on 2:1 clays. This research aims to address this gap by evaluating the properties of laboratory-grade kaolinite and montmorillonite under different calcination processes and conditions at laboratory-scale, specifically: a) a rotary kiln for soak and b) an entrained flow reactor for flash calcination. The results show that soak and flash calcination deliver similar pozzolanic reactivity when compared at equivalent dehydroxylation, and flash calcination requiring slightly higher temperatures with minimal mineralogical differences. Additionally, flash calcination influenced the structural and morphological properties of the clay particles, with kaolinite exhibiting significant agglomeration and formation of pseudo-hexagonal plates, while montmorillonite had reduced surface area and led to deposit formations which mainly consisted of mullite and quartz. This study was conducted in laboratory-scale conditions and did not account for heat-transfer limitations present in industrial systems. However, it aims to provide a solid foundation for future studies to expand to an industrial scale, potentially leading to a more substantial impact of the calcination method on clay properties. Moreover, the choice of materials in this study represents pure clays. Although this creates a fundamental background, it does not directly apply to the complexity of natural clays. As such, this work purposely serves as an initial step toward upcoming studies that compare natural clays with various compositions to further evaluate the observations.