Reactivity and Characterization of Activated Clays for Low-Carbon Cement Applications

With the planned phase-out of coal in Germany by 2030, fly ash—a widely used supplementary cementitious material (SCM)—will no longer be available. Among potential alternatives, activated clays have emerged as one of the most promising candidates due to their high pozzolanic potential upon activation. Structural damage enhances the reactivity of clays, enabling the silicon and aluminum 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 composition and chemical properties of various clays were characterized via X-ray diffraction (XRD) and X-ray fluorescence spectroscopy (XRF) to determine the phase content and the silicon-to-aluminum (Si:Al) ratio, a key parameter governing pozzolanic reactivity. While most previous studies have focused on high-purity clays^[1], this work also investigates clay mixtures, comparing their behavior in cementitious systems to pure clays.

The activation process includes thermal treatment in a muffle furnace or mechanical treatment via grinding, both aimed at destroying the clay mineral structure. To optimize the thermal activation, decomposition temperatures were determined using differential scanning calorimetry and thermogravimetric analysis (DSC-TG). The pozzolanic reactivity of the activated materials was evaluated through isoperibolic heat flow calorimetry and the Frattini test according to DIN EN 196-5.

The results indicate that when the Si:Al ratio is nearly 2:1, the reactivity and hydration behavior of clay mixtures closely match those of pure clays, highlighting the potential of using mixed or impure clays as viable SCMs.

[1] Lagier et.al, Cement and Concrete Research 2007