The formation of calcium sulfoaluminate hydrate compounds: Part I

The formation of ettringite (3CaO · Al₂O₃ · 3CaSO₄ · 32H₂O) and monosulfate (3CaO · Al₂O₃ · CaSO₄ · 12H₂O) from tricalcium aluminate (3CaO · Al₂O₃) and gypsum (CaSO₄ · 2H₂O) in sodium hydroxide (NaOH) solutions was investigated by isothermal calorimetry and X-ray diffraction analyses. Tricalcium aluminate/gypsum mixtures with a molar aluminate-to-sulfate ratio of 1:3 were hydrated at constant temperatures from 40 to 80°C in deionized water and 200 and 500 mM of NaOH solutions. Ettringite was the only crystalline phase ultimately formed between 40 and 80°C, regardless of whether hydration was carried out in deionized water or sodium hydroxide solutions. The rates of ettringite formation were retarded in sodium hydroxide solutions at all temperatures when compared to hydration in deionized water. The apparent activation energy for the conversion of the tricalcium aluminate/gypsum mixture to ettringite was observed to depend on the concentration of sodium hydroxide.