in the literature that the initial dissolution rate of C 3 S is significantly affected by the Ca 2+ activity . That is because C 3 S dissolution rate is strongly dependent on the ion activity product (Π) (equation 15) of the C 3 S dissolution reaction (equation 16). We calculated the ion activities with PHREEQC based on the measured ion concentrations of calcium and silicon . The charge balance was used to calculate the activities of OH -and H + .