Influence of PCE superplasticizers on the fresh properties of low carbon cements containing calcined clays: A comparative study of calcined clays from three different sources

The performance of low carbon cements prepared using calcined clay can be very inconsistent due to variations of the composition of the calcined clay samples. To gain a better understanding and further promote the practical application of this kind of "Green" cement, the influence of calcined clays from different sources on the behavior of the blended cement was investigated. First, cement blends holding 30 wt% of calcined clays sourced from Germany, India and China were prepared and admixed with PCE superplasticizers. Their properties in mortar regarding workability, time-dependent fluidity and strength development were evaluated and compared with that of OPC. Two commonly used individual PCEs, one for precast and one for ready-mix concrete, were employed. It was found that the performance of the blended cement significantly depends on the kind and property of calcined clay. High pozzolanic reactivity promotes early strength development (up to 7 days), but reduces fluidity and slump retention. To elucidate the mechanism behind this, surface charge (zeta potential) and PCE adsorption on the calcined clays were determined, and hence, a correlation between highly negatively charged calcined clay samples and high PCE adsorption was found.