The combined processing of industrial wastes of titania-bearing slags with coal fly ash is an important part of the circular economy. In the present work, the effect of Al2O3 on the 1400 °C liquidus isotherms of the perovskite, rutile and tridymite primary phase fields in the CaO–SiO2–TiO2–Al2O3 system has been determined by employing a high-temperature equilibration-quenching technique, followed by X-ray Photoelectron Spectroscopy and Scanning Electron Microscopy-Energy Dispersive X-ray Spectrometry. Titanium was confirmed to be stable as TiO2 in the present equilibria. The equilibrium solid phases of perovskite CaO·TiO2, rutile TiO2 and tridymite SiO2 were confirmed to be coexisting with the liquid oxide phase. The comparison of the addition of 0–15 wt% Al2O3 was beneficial for expanding the primary phase field of perovskite to lower TiO2 concentrations. Comparisons of the experimental 1400 °C isotherm with the predictions using FactSage and MTDATA databases confirmed some differences to the present experimental data, thus demonstrating the direction for updating the present thermodynamic titania-bearing oxide databases.