The impact of associated minerals in kaolinite clays on their physico-chemical characteristics for use as supplementary cementitious material

Clays in their natural state are complex and often occur as mixtures of clay and non-clay minerals. Medium-grade kaolinite clays that are feasible for supplementary cementitious material (SCM) applications contain various associated minerals that can affect their behavior. This study explores how associated minerals viz., iron, illite and quartz impact the physical and chemical properties both raw and after calcination at 600 °C, 700 °C, 800 °C, 900 °C, and 1000 °C using thermal analysis, XRD, FTIR and pozzolanic reactivity techniques. The results show that associated minerals influence the crystallinity, specific surface area and water demand of kaolinite clays. Iron rich kaolinite clays are found to be highly disordered and show higher specific surface area and water demand than quartz rich clays. The effect of associated minerals on the formation temperature of metakaolin was also observed as iron rich kaolinite showed conversion at lower temperatures when compared to quartz rich kaolinite. The catalytic role of iron in the metakaolin formation is confirmed by its interaction with the kaolinite structure in model iron rich clay system using EPR technique. The optimal calcination temperature of kaolinite varies depending on the associated minerals and a positive effect of illite impurities on the pozzolanic reactivity of low-grade kaolinite clay has been observed.