Understanding workability of low carbon cements through advanced water detection techniques

: Limestone Calcined Clay Cement (LC3) is a sustainable alternative to Ordinary Portland Cement (OPC), capable of significantly reducing CO₂ emissions. However, the mechanisms by which calcined clay affects fresh-state workability are not yet fully understood. In this study, different cements were formulated with variable content of calcined clay to investigate its effect on workability and early hydration. Advanced techniques, including 1H Time-Domain Nuclear Magnetic Resonance (TD-NMR) and Differential Scanning Calorimetry (DSC), were applied to detect free water in fresh pastes and compared with rheological measurements. Results indicate that increasing the calcined clay content leads to rapid consumption of free water which in turn substantially raises superplasticizer demand, as shown by mortar flow-table tests, and alters paste rheology, as measured by rheometer. Analyses of the first hours of hydration reveal that the available free water can drop dramatically in the presence of calcined clay, providing a clear explanation for the observed increases in yield stress and viscosity. Despite these early rheological challenges, LC3 formulations achieved excellent mechanical performance at 28 days, in some cases surpassing that of OPC CEM I. These findings highlight the dual role of calcined clay in modifying fresh-state behavior and ensuring long-term strength.