AFm phase

An AFm phase is an "alumina, ferric oxide, monosulfate" phase. AFm phases are important in the hydration of hydraulic cements.

They are crystalline hydrates with general, simplified formula 3 CaO·(Al,Fe)₂O₃·CaSO₄·nH₂O.
They form inter alia when tricalcium aluminate reacts with dissolved calcium sulfate.