The paste hydration of brownmillerite with and without gypsum: a time resolved synchrotron diffraction study at 30, 70, 100 and 150 °C

Abstract

Synchrotron energy dispersive diffraction has been used to monitor the mineral transformations which occur in the paste hydration of brownmillerite Ca₂AlFeO₅ in a closed hydrothermal environment at temperatures in the range 30 to 150 °C. In the absence of sulfate brownmillerite reacts rapidly to form a metastable hexagonal hydrate with a basal spacing of 1.07 nm. As temperature increases the metastable hydrate transforms to the stable hydrogarnet phase Ca₃(Al,Fe)₂(OH)₁₂ with an Al/(Al + Fe) ratio of about 0.4. The lack of iron in the metastable hydrate suggests a through-solution mechanism while the iron content of the hydrogarnet indicates it must form on the surface of the brownmillerite, probably in contact with the Fe-rich residue remaining after dissolution. The addition of gypsum CaSO₄·2H₂O increases the rate of brownmillerite hydration. Ettringite Ca₆Al₂(SO₄)₃(OH)₁₂·26 H₂O is formed at ambient temperature and is replaced by calcium aluminium monosulfate-14 hydrate Ca₄Al₂O₆(SO₄)·14H₂O at elevated temperatures (70, 100 and 150 °C). Transient increases in gypsum accompany this transformation and all three phases can co-exist at 70 °C. Monosulfate-14 is stable at 150 °C. Increases in brownmillerite peak intensities accompany the conversion of ettringite to monosulfate and are a result of sedimentation. This has implications for paste rheology and slurry design in oilwell cements at elevated temperatures.