Mineralogical characterization and reactivity test of common clays suitable as supplementary cementitious material

Abstract The most abundant clays in the northern hemisphere are dominated by 2:1 clay minerals. However, compared to kaolinite-rich clays, these common clays have been less addressed by recent research on calcined clays. This study investigated 13 different clay-rich raw materials, focussing on available common clays from German deposits, with respect to their mineralogical composition and pozzolanic reactivity after calcination. It has been shown, that besides kaolinite content, for which X-ray diffraction (XRD) and thermogravimetry (TGA) yielded similar results, other parameters as type and quantity of 2:1 clay minerals and degree of kaolinite disorder must be considered in order to assess the potential of the raw materials. Therefore, structure models incorporating mixed layering and disorder were applied during raw clay quantification. Regarding the assessment of pozzolanic reactivity, the R3 calorimetry test and solubility of silicon and aluminum ions in alkaline solution provided comparable results. In the range of kaolinite contents below 30 wt%, the influence of 2:1 clay minerals on the reactivity becomes significant, which makes a reliable quantification of illite, smectite or mixed layer minerals mandatory for the classification of common clays as raw materials for supplementary cementitious material (SCM).