Ettringite

Ettringite is hexacalcium aluminate trisulfate hydrate, (CaO)₆(Al₂O₃)(SO₄)₃.32H₂O.

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Ettrigite is found in hydrated Portland cement system as a result of the reaction of calcium aluminate with calcium sulfate, both present in Portland cement. Ettringite (or aft phase) can also be produced in the laboratory by reacting stoichiometric amounts of calcium, aluminium and sulfate in water. In the cement system, the presence of ettringite depends on the ratio of calcium sulfate to aluminate; when this ratio is low, ettringite forms during early hydration and then converts to the calcium aluminate monosulfate (afm phase). When the ratio is intermediate, only a portion of the ettringite converts to afm and both can coexist, while ettringite is unlikely to convert to afm at high ratios.