The formation of ettringite from tricalcium aluminate and magnesium sulphate

Abstract

Isothermal calorimetry and X-ray diffraction analyses were used to investigate the hydration of tricalcium aluminate (3CaO·Al2O3) in 0·5, 1·0 and 3·0 M magnesium sulphate solutions. Hydration was carried out isothermally at temperatures ranging from 30 to 80°C. Monosulphate (Ca4Al2(OH)12·(SO4)·6H2O), ettringite ([Ca3Al(OH)6·12H2O]2·(SO4)3·2H2O), gypsum (CaSO4·2H2O) and a hydrogarnet phase (Ca3[Al(OH)6]; C3AH6) were all observed as hydration products. Monosulphate and C3AH6 were the only phases observed for hydration in 0·5 and 1·0 M magnesium sulphate solutions. Ettringite was the dominant crystalline phase after hydration in 3·0 M solution, regardless of temperature. Gypsum was the only other crystalline phase observed after hydration in 3·0 M solutions. Hydration rates are faster than those observed in previous studies using similar sources of tricalcium aluminate and calcium sulphate. This indicates the hydration rate of tricalcium aluminate is enhanced by the presence of magnesium in solution. No crystalline magnesium containing phases were detected, regardless of temperature or magnesium sulphate concentration.