The work is interested in the study of the hydration process by means of the isothermal calorimetry method. The Portland cement hydration process led to gradual consumption of contained calcium sulfate. This sulfate depletion is observed on the calorimetry curve as a shoulder of main silicate peak. The addition of hydratable alumina to Portland cement was studied in order to determine its influence on hydration process. Experimental results showed the shifted position of this shoulder according to alumina and calcium sulfate content. It was also observed that the overall hydration process was almost unaffected by alumina addition.