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 micromol per g, 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.