Belite calcium sulfoaluminate (BCSA) cements are low-CO2 building materials. However, their hydration behavior and its effect on mechanical properties have still to be clarified. Here, we report a full multitechnique study of the hydration behavior up to 120 days of nonactivated and activated BCSA laboratory-prepared clinkers, with beta- or alpha(H)-belite as main phase, respectively. The effects of the amount of gypsum added were also studied. The hydration and crystallization processes are reported and discussed in detail. Finally, shrinkage/expansion data are also given. The optimum amount of gypsum was close to 10 wt %. Our study has demonstrated that beta-belite reacts at a higher pace than alpha'(H)-belite, irrespective of the gypsum content. The hydration mechanism of belite determines the development of the mechanical strengths. These are much higher for activated BCSA cement, similar to 65 MPa at 120 days, against similar to 20 MPa for nonactivated BCSA cement, with the latter having larger amounts of stratlingite.