We have developed an open-source package to analyze isothermal heat flow calorimetry data. The package is designed to be flexible and extensible and is implemented in Python. This package can programmatically obtain characteristic times and the corresponding heat and heat flow from (potentially large amounts of) isothermal calorimetry data. To illustrate the usefulness of this approach, we have studied the influence of three different retarders, i.e., racemic tartaric acid, etidronic acid, and sucrose, on the hydration of Portland cement. The analysis of the time of the maximum heat flow t max is not ideal for analyzing the reaction kinetics of cement hydration due to its overlap with the sulfate depletion heat flow event. Additionally, we have determined the time of the maximum heat flow acceleration t a . This time is less influenced by the sulfate depletion event and is, therefore, more suitable for studying the reaction kinetics of the C 3 S reaction of cement hydration. The time t a is very similar to the time obtained by determining the 50% value of the maximum heat flow according to ASTM C1679. All characteristic times are strongly correlated. Suitable methods to determine the timing of the C 3 S reaction are unaffected by sulfate depletion. Therefore, suitable methods implemented in the CaloCem package are determining the intersection time t is , maximum acceleration time t a , or the time according to ASTM C1679 t AST M . With an increasing dosage of the retarders, we observe an exponential increase in the characteristic times. Furthermore, we determined the setting of the different cement pastes using a Vicat robot. Again, the dependence of the stiffening time on retarder dosage is exponential. Interestingly, the total heat at the initial Vicat set increases with the initial Vicat set time. The slope of the linear increase is the same for both retarders studied, which suggests that the set depends on the degree of hydration of the cement.