In some blended cements, the optimum amount of sulfate addition differs from that observed in OPC. This study aims to understand the mechanism behind the impact of two SCMs, namely metakaolin and limestone, on the sulfate balance of blended cements. No significant impact of the aluminum content of these SCMs was observed. Instead, it is observed that the filler effect of the SCM, which accelerates the reaction of alite, is the main factor impacting the sulfate balance. As the rate of precipitation of C-S-H is increased, more sulfate is adsorbed by the C-S-H and consequently, the depletion of gypsum is reached earlier in time during the hydration process. A relationship between heat release at the onset of the aluminate peak and the gypsum content of the system was established.