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

Abstract

During the early hydration of ordinary Portland cement (OPC), tricalcium aluminate (C 3 A) exhibits the highest reactivity among the clinker phases. Consequently, C 3 A significantly influences the early rheological properties of OPC-based materials, thereby linking rheology with C 3 A reactivity. The reactivity of C 3 A 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 C 3 A. This work investigates the impact of three sodium salts — sodium chloride (NaCl), sodium nitrate (NaNO 3 ), and sodium sulfate (Na 2 SO 4 ) — on the hydration of C 3 A. We study model suspensions composed of 10% cubic C 3 A and 90% quartz by weight with in-situ isothermal calorimetry . The C 3 A 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 C 3 A reactivities. Combined with small amplitude oscillatory shear (SAOS) rheology experiments, we show that the addition of Na 2 SO 4 significantly reduces the heat flow and the initial storage modulus . In contrast, NaNO 3 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, Na 2 SO 4 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 C 3 A hydration and alter the reaction pathway, forming different hydrate phases.