Early hydration and viscoelastic properties of tricalcium aluminate pastes influenced by soluble sodium salts
Daniel Axthammer, Tobias Lange, Joachim Dengler, Torben Gaedt
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.