Formation of Ettringite from Monosubstituted Calcium Sulfoaluminate Hydrate and Gypsum

Abstract

The formation of ettringite (3CaO·Al 2 O 3 ·3CaSO 4 ·32H 2 O) from monosulfate (3CaO·Al 2 O 3 ·CaSO 4 ·12H 2 O) and gypsum (CaSO 4 ·2H 2 O) was investigated by isothermal calorimetry and X‐ray diffraction (XRD) analyses. Hydration was carried out at constant temperatures from 30° to 80°C using deionized water and 0.2 M , 0.5 M , and 1.0 M sodium hydroxide (NaOH) solutions. Ettringite was found to be the dominant crystalline phase over the entire temperature range and at all sodium hydroxide concentrations. A sodium‐substituted monosulfate phase was formed as a hydration product in the 1.0 M sodium hydroxide solution regardless of temperature. XRD and calorimetry demonstrate that hydration in increasing sodium hydroxide concentrations decreases the amount of ettringite formed and retards the rate of reaction. The apparent activation energy for the conversion of the monosulfate/gypsum mixture to ettringite was observed to vary depending on the sodium hydroxide concentration. Ettringite formation was observed to depend upon the concentration of calcium in solution; thus the formation of calcium hydroxide and sodium‐substituted monosulfate phase competes with ettringite formation.