Abstract
The formation of ettringite (3CaO • Al 2O 3 • 3CaSO 4 • 32H 2O) and monosulfate (3CaO • Al 2O 3 • CaSO 4 • 12H 2O) from tricalcium aluminate (3CaO • Al 2O 3), gypsum (CaSO 4 • 2H 2O), and sodium hydroxide (NaOH) solutions were investigated by isothermal calorimetry and X-ray diffraction analyses. Tricalcium aluminate/gypsum mixtures with a molar ratio of 1:1 sulfate-to-aluminate were hydrated at constant temperatures from 30 to 90°C in deionized water and in 200 and 500 mM of sodium hydroxide (NaOH) solutions. Hydration in deionized water produced ettringite and monosulfate as the dominant crystalline phases, regardless of temperature. Complex assemblages of phases formed in 200 and 500 mM of sodium hydroxide; these included ettringite, monosulfate, and U-phase, at all temperatures. Calorimetry and X-ray diffraction data indicated that U-phase formation becomes the dominant reaction as sodium hydroxide concentrations increase. U-phase is the only AFm phase observed after hydration in 1.0 M of NaOH. The data also indicated that the presence of sodium cations promotes AFm formation.
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