Calcium aluminoferrite

Calcium aluminoferrite (Ca2(Al,Fe)2O5) is a dark brown crystalline phase commonly found in cements.

Its presence in clinker is solely due to the need to obtain liquid at the peak kiln processing temperature (1400−1450 °C), facilitating the formation of the desired silicate phases.

Most of the transition metals in the cement are found in the ferrite phase, notably titanium, manganese and zinc.

There is also a substantial amount of magnesium and silicon, and because of this, oxides other than CaO, Al2O3 and Fe2O3 often make up 15% of the mass of the calcium aluminoferrite.

In the case of Portland cement, subsequent slow reaction with dissolved sulfate forms AFm phases, which have negligible strength-giving properties.

In the case of calcium aluminate cements (CAC),[2]: 726  the situation is less clear-cut, but there is little contribution to early strength.