Phase compositions and equilibria in the CaO–Al2O3–Fe2O3–SO3 system, for assemblages containing ye'elimite and ferrite Ca2(Al,Fe)O5

•The equilibrium between ye’elimite and brownmillerite has been studied at 1325 °C.•Phase assemblages and solid solutions tie lines have been measured as a function of Fe2O3 content.•At 1325 °C, the two phases can coexist with a sulfate free liquid.•Fe2O3 content in brownmillerite and ye’elimite increases simultaneously.•A maximum Fe2O3 of 8.8 wt% was measured in ye’elimite.

Phase equilibria in the CaO–Al2O3–Fe2O3–SO3 system have been studied, mainly at 1325 °C. In particular the solid solution compositions of ye'elimite Ca4(Al6O12)SO4 and brownmillerite (C2(A,F)) phases have been analysed and used to estimate the composition boundaries of these phases in their co-existence with liquid at 1325 °C. Sulfate and iron oxide facilitate the formation of a liquid phase, whose amount increases with overall Fe2O3 content. However, the solubility of sulfate in this liquid phase is very low and liquid formation tends to lead to the volatilisation of sulfate in unsealed samples.