Microstructure of Supplementary Cementitious Materials in Relation to Water Uptake and Rheology

The construction sector’s environmental impact is driven by cement production, which emits 0.8–0.9 tons of CO2 per ton produced. Consequently, researchers have sought alternative binders like Limestone Calcined Clay Cement (LC3), which reduces CO2 emissions by up to 40% by lowering clinker content. However, calcined clays and other Supplementary Cementitious Materials (SCMs) often possess high surface areas and porous particles, increasing water demand and viscosity, which impairs workability. This study investigates the relationship between the physical properties of calcined clay (CC) and other SCMs (SCM1, SCM2) and their rheological properties in pastes and mortars. Using the BET nitrogen adsorption method, Specific Surface Area (SSA) and Specific Pore Volume (SPV) were measured; SCM1 exhibited a 10% higher SSA and a finer pore sizes than SCM2. Low-temperature Differential Scanning Calorimetry (DSC) revealed that these microstructures act as water traps, reducing the Free Water Index (FWI). Rotational rheometry confirmed that this water retention directly increases dynamic yield stress and induces thixotropic behavior. The study also evaluates other clinker substitutes, including mineralized wastes for CO2 uptake (CERA range) and heated dump sludge (600°C, 1V, 2V, 3C). A fundamental cause-and-effect chain is established: microstructural SSA determines water absorption, lowering the FWI and immediately impacting rheological properties. These findings provide a scientific basis for optimizing low-carbon cementitious mixtures and offer a quantifiable understanding of how material-absorbed water influences the rheology of cementitious systems.