A comprehensive understanding of hydration kinetics, sulfate requirement and phase assemblage of Portland cement is crucial for further optimization of early-age performance, especially when part of cement is replaced by mineral additions in blended cements. Gypsum is added to PC to control the reaction of the aluminate phase. The addition of gypsum also enhances the reaction of C3S independently of the interaction with the aluminates. In this study, the influence of gypsum in C3S and C3S/C(3)A hydration was studied in terms of the morphology and composition evolution of the C-S-H formed. The addition of gypsum modifies morphology of the C-S-H (needle length) precipitated and increase the nucleation density. The extent of the enhancement is dependent on the specific surface area (reaction rate) of C3S. In C3S/C(3)A systems, no differences in C-S-H morphology were observed before and after the aluminate peak.