The hydration process of cementitious materials involves a thermally activated reaction that depends on the composition of the mixture and the curing temperature. The main parameter affecting the temperature variation of cast-in-place concrete is the apparent activation energy, which can be used for the efficient prediction of the temperature evolution and maturity index of hardening concrete. This paper discusses two methods to determine the activation energy of mortar specimens, whose mixture proportions are based on standards. The first approach is based on isothermal calorimetry measurements, and the second involves compression tests of mortar samples stored under four different temperature conditions. Mortar mixtures with ordinary portland cement and two rates of cement substitution with siliceous fly ash (10% and 20%) are investigated. The values of the activation energy obtained using the two approaches are compared. Finally, the effectiveness of different tests in determining the activation energy, and thus, maturity index is highlighted.