A new calorimetry-based method for calculating the activation energy of cement hydration is described. The method requires a scanning calorimeter that can change the sample temperature relatively quickly and has a stable baseline, but the technique is straightforward to perform. Activation energy values can be calculated at multiple hydration times from a single specimen, allowing any changes with time to be assessed. With this method, the measured activation energy at a given time does not depend on the previous microstructural development, as is the case with traditional methods of calculating the activation energy from parallel specimens hydrated isothermally at different temperatures. For tricalcium silicate (C3S) hydrated in pure water or in a calcium chloride solution, the activation energy remains statistically constant from the earliest times until the degree of hydration exceeds 0.65, with an overall average value of (51.1 ± 1.8) kJ/mol. For cement hydration, the activation energy is also constant until well past the main hydration peak, with slightly lower average values. These results suggest that the rate-controlling step in the hydration process does not change during the early hydration period (the first few days after mixing), as different reaction steps tend to have different activation energies.