Similar to section 3.2, we can describe the dependency of the retardation time of the CA main reaction on the amount of added protons with an exponential fit when phosphoric acid is present. These observations agree with the results of Manninger et al. , who investigated the H 3 PO 4 influence on CAC hydration in cement pastes. Using in situ-QXRD and pore solution analysis, the authors showed that the small heat event before the main CA reaction represents the initial CA-dissolution. Thus, the addition of phosphoric acid led to the insertion of a retardation period, which was called "step 0". This initial dissolution inhibition corresponds to the effect discussed in section 3.2. During this period, a reduced phosphate concentration in the pore solution was found, and it was assigned to the precipitation of a calcium phosphate phase. According to thermodynamic calculations, the solution was supersaturated with regard to hydroxylapatite (Ca 5 (PO 4 ) 3 (OH)), which QXRD could not verify due to its amorphous, nanocrystalline character. Nevertheless, the precipitation of a poorly soluble phase with H 3 PO 4 would explain the outstanding early retardation and the proceeding of the classical hydration periods after the end of the retardation capacity. Manninger also found an exponential relation between H 3 PO 4 concentration and retardation time for CAC pastes. In the mentioned work, the small heat flow peak, representing the initial CA-dissolution, was used to describe the retardation, while here, we use the main CA reaction peak. Nevertheless, a good agreement exists since both investigations showed that the temporal distance between the small heat flow event and the main reaction stayed constant among the series.