Early hydration and viscoelastic properties of tricalcium aluminate pastes influenced by soluble sodium salts

During the early hydration of ordinary Portland cement (OPC), tricalcium aluminate (C₃A) exhibits the highest reactivity among the clinker phases. Consequently, C₃A significantly influences the early rheological properties of OPC-based materials, thereby linking rheology with C₃A reactivity. The reactivity of C₃A is affected by temperature, calcium sulfates, admixtures, and ionic strength. Calcium sulfate phases such as gypsum, bassanite, or anhydrite are used in technical Portland cement to control the early reactivity of C₃A. This work investigates the impact of three sodium salts — sodium chloride (NaCl), sodium nitrate (NaNO₃), and sodium sulfate (Na₂SO₄) — on the hydration of C₃A. We study model suspensions composed of 10% cubic C₃A and 90% quartz by weight with in-situ isothermal calorimetry. The C₃A suspensions were mixed inside the calorimeter with a water-to-solid ratio of 0.8. Increasing concentrations, i.e., 400, 1000, and 2000 µmol g⁻¹, of the sodium salts mentioned above lead to characteristically decreased C₃A reactivities. Combined with small amplitude oscillatory shear (SAOS) rheology experiments, we show that the addition of Na₂SO₄ significantly reduces the heat flow and the initial storage modulus. In contrast, NaNO₃ and NaCl had less pronounced effects on both storage modulus and reaction heat. The differences in structure development are attributed to the formation of different hydrate phases. Specifically, Na₂SO₄ leads to ettringite formation, whereas the presence of nitrate and chloride ions favors the precipitation of AFm phases. The study concludes that introducing various sodium salts can modulate the kinetics of C₃A hydration and alter the reaction pathway, forming different hydrate phases.