It has been an established fact that finely ground calcium carbonate represents to a certain extent an active component during the hydration of Portland cement and the formation of calcium monocarboaluminate has been confirmed many times. Additionally, the formation of calcium hemicarboaluminate, as another possible compound, has been mentioned. There is, however, lack of specific information regarding these two compounds: there has been no experimental data on their hydration time-dependent formation, their interrelations, and their amount in a hardened cement paste. This paper describes the time-related formation of hemicarboaluminate and monocarboaluminate and reports for the first time in the literature the conversion of hemicarboaluminate into monocarboaluminate on the basis of X-ray diffraction by following the content of the latter using the Rietveld method. Hemicarboaluminate appears at early hydration times in calcite-containing Portland cement, even in the presence of large amounts of calcium carbonate. As the hydration progresses, a gradual conversion of hemicarboaluminate into monocarboaluminate occurs. The only detectable AFm-type compound present in well-hydrated cement is calcium monocarboaluminate.