Phases formed from the hydration of tetracalcium aluminoferrite and magnesium sulfate

The phase assemblage formation resulting from hydration of a tetracalcium aluminoferrite (ferrite) powder (4CaO·Al₂O₃·Fe₂O₃) was investigated by isothermal calorimetry, X ray diffraction analyses and scanning electron microscopy (SEM). Hydration of the ferrite powder was carried out isothermally at 50°C in 1·0 M magnesium sulfate solution for 30 h. Monosubstituted calcium aluminosulfate hydrate (monosulfate) (3CaO·Al₂O₃· CaSO₄·12H₂O) was the dominant crystalline phase. Other crystalline phases present included ettringite (3CaO·Al₂O₃·3CaSO₄·32H₂O), magnesium aluminate hydroxide hydrate (Mg₄Al₂(OH)₁₄·3H₂O) and calcium aluminate carbonate hydrate (4CaO·Al₂O₃·½CO₂·l2H₂O). The amorphous solids produced include a calcium/iron-rich gel and a magnesium/aluminium/sulfate-rich phase. The calcium/iron-rich gel is the only iron-rich phase observed in the hydrated phase assemblage. SEM indicates magnesium aids in the movement of aluminate from the ferrite phase during hydration, the reason for increased ferrite reactivity. Iron in the ferrite phase exhibits little mobility, since most of the iron is found in residual ferrite grains. SEM also indicates monosulfate can be expansive; a five-fold increase in the lengths of the ferrite grains is observed.