With the planned coal phase-out in Germany by 2030, fly ash, a widely used supplementary cementitious material (SCM), will no longer be available. will no longer be available. Among the potential alternatives, activated clays have emerged as one of the most promising candidates due to their high pozzolanic potential after activation (Scrivener etal., 2018). Structural damage increases the reactivity of clays, allowing the silicon and aluminium components to participate in the formation of calcium aluminate silicate hydrate (C-A-S-H) phases in the presence of cement and water.
In this study, the phase compositions and chemical properties of different clays were characterised using X-ray diffraction (XRD) and X-ray fluorescence analysis (XRF) to determine the phase composition and silicon-to-aluminium ratio (Si:Al ratio) of the clay minerals, a key parameter for pozzolanic reactivity. While most previous studies have focused on high-purity clays, this work also investigates clay mixtures and compares their behaviour in cementitious systems with that of pure clays. The activation process involves thermal treatment in a muffle furnace or mechanical treatment by grinding, both of which aim to destroy the structure of the clay minerals. To optimise thermal activation, the decomposition temperatures were determined using differential scanning calorimetry (DSC) and thermogravimetry (TG). The pozzolanic reactivity of the activated materials was determined using isoperibolic heat flow calorimetry and the Frattini test in accordance with German Industrial Standard (DIN) European Standard (EN) 196-5. In addition, the flexural and compressive strengths of mixtures in a ratio of 30:70 were determined. The results show that at a Si:Al ratio of almost 2:1, the reactivity and hydration behaviour of clay mixtures are very similar to those of pure clays, which underlines the potential of using mixed or impure clays as useful SCMs.
