The calorimetry data confirms that the main hydration peak is retarded with increasing amounts of acid (see Figure ) when compared to the reference. In the presence of phosphoric acid, an additional small peak is detected between the initial heat flow peak and the main CA reaction peak. Both the additional peak and the main reaction peak are shifted to later hydration times with increasing H 3 PO 4 concentration. Simultaneously, the time between these two peaks stays constant at 6 -7 h. With a proton concentration of 400 µmol/g CAC , no heat flow event could be observed over the complete duration (72 h) of the hydration experiment. The presence of perchloric, hydrochloric, sulfuric, and nitric acid leads to different hydration characteristics: No small heat flow peak is visible between the initial heat flow signal and the main CA reaction. It cannot be excluded, however, that a similar smaller heat flow peak may happen in the first 60 min at the low w/s ratio where we can not detect it. In almost all experiments, the effect of acid addition on the main CA reaction manifests as a delay in the maximum of the main peak. Only at high HCl concentrations is the main reaction observed to occur slightly earlier.