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.
References
AFm phase Wikipedia(Text) CC BY-SA