Parameters affecting the pozzolanic reactivity of common clays after mechanochemical activation

Mechanochemical activation (MCA) of phyllosilicates is one of the most promising solutions to produce reactive supplementary cementitious materials (SCMs). Several studies have already shown that MCA outperforms thermal activation (TA) in terms of reactivity when clays are low in kaolinite but rich in 2:1 phyllosilicates. Because reactivity depends on phyllosilicate type as well as content, and clays vary widely, a quick and simple test is needed to assess raw material suitability. Therefore, the aim of this study was to investigate the mineralogical and physical properties of common clays and their impact on reactivity after MCA, as well as their suitability as SCM. Hence, a multi-method approach was used to determine the reactivity of seven German common clays after TA and MCA and their use as SCM. The R3-test was used to determine the evolved heat after 7 days. Thermal analysis showed an altered OH binding that went along with a decrease in surface area with prolonged milling. Through Rietveld refinement of the X-ray diffraction analysis and the external standard method, the amorphous content of the phyllosilicates was determined. An increase in the amorphous content was observed after MCA. The solubility of Al and Si could also be correlated with the reactivity of the phyllosilicates. The concentration of all ions increased with prolonged milling time, and therefore also correlated linearly with the reactivity. Both the loss of dehydroxylation area and the Si:Al solubility ratio served as practical, rapid screening tools for raw-material suitability.