This work presents a novel way to use X-ray diffraction (XRD) to quantify the plate-like calcium-aluminate-hydrate phases C₂AH_x (2CaO·Al₂O₃·xH₂O), for which three different basal spacings are detected during early hydration at ambient temperatures. By combining Rietveld refinement and the G-factor method, C₂AH_7.5 was calibrated using a variety of syntheses and preparations for XRD. Since cell volume and X-ray density (that are necessary for the G-factor method) of C₂AH_7.5 are not known, an approach similar to [1], [2] was used for the generation of a hkl-phase model: A “F-factor” – a factor that replaces the product of these two unknowns, which was verified by the multitude of syntheses – was successfully determined within this paper. The model was applied for the absolute quantification of C₂AH_x during in-situ XRD of CA (CaO·Al₂O₃) hydration at 40 °C. The results demonstrate the potential of the hkl-phase model in practical use and reveal the accuracy of the determined F-factor.