The very early phase of cement hydration, i.e., the first minutes after mixing cement with water, is influenced by the initial dissolution of the cement and the nucleation of the first hydrate phases. We were interested in the influence of acids on the early dissolution and hydration behavior of CAC. We characterize the dissolution by adding CAC to water (or aqueous acidic solutions) at a water-to-CAC ratio of 100 and measuring the proton activity and conductivity changes in real-time with pH and conductivity electrodes. The rise in ion concentration, pH, and conductivity results from the dissolution of CAC and, more precisely, the initial dissolution of CA. Throughout the experiment, the apparent dissolution rate decreases (Figure ). The system transitions to the dormant period with very low dissolution rates. The CaO/Al 2 O 3 ratio (C/A) in the pore solution is slightly higher than one during the first 120 min (see Table ). This ratio agrees with published data, which report ratios between 1.05 and 1.16 . Among these studies, the maximum CaO and Al 2 O 3 concentrations measured during the dormant period are relatively consistent at 20 -25 mmol/L. This range is higher than our measured concentrations of 7 -8 mmol/L. We tentatively assign this finding to the significantly higher water-to-solid ratio of 100 employed in this study.