The present research investigated the hydration characteristics of model (C₃S-C₃A-gypsum-blast furnace slag) and real cement slag pastes at an early age, and emphasis was laid on the interaction of gypsum, aluminate phases dissolved from both C₃A and slag. Two additional peaks arose during the decelerating stage of calorimetric measurement. The first one was originated from the renewed aluminate reaction between C₃A and gypsum, while the second one was attributed to the reaction between aluminate phase dissolved from slag and gypsum. High alumina content in slag promoted the aluminate reactions between gypsum and C₃A. On the other hand, the reaction between slag and gypsum was suppressed because of the addition of C₃A. Besides, factors affecting the optimal sulfate requirement of cement slag paste, e.g., C₃A content in cement clinker, slag substitution level, and slag chemistry, etc. were analyzed. The authors believed that the results found in this paper provided essential insights to quantitatively understand the early-age aluminate interactions of gypsum, C₃A as well as blast furnace slag in cement slag paste.