Current developments in cement chemistry increasingly rely on predictive thermodynamic modeling of the phase composition in cementitious composites with the aim of linking the performance of the material with the phase composition of the material. This approach requires identification of the cement phases that form in hydrating cementitious materials using standard techniques, such as X-ray diffraction (XRD) and thermal analysis (DTA, TGA), but also state-of-the-art synchrotron-based techniques, in particular for those cases in which the signals of solid solutions overlap in XRD and TGA measurements. In this study, two ordinary Portland cements, with different chemical compositions and subject to different hydration times (similar to 10, similar to 50yr), were investigated aiming at identifying the most stable Fe-containing cement phase in the cement pastes. The Fe-containing cement phases and their solid solutions with the Al analogues in the complex cement matrix were analyzed with X-ray absorption spectroscopy, indicating the formation of a mixed Fe-Al siliceous hydrogarnet as the major Fe-containing phase. The presence of this phase after long hydration periods and upon selective dissolution of the pastes further indicates that, independent of the chemical compositions of cements, formation of the mixed Fe-Al siliceous hydrogarnet is thermodynamically favored in aged pastes, which is supported by published thermodynamic calculations.