This study explores the effects of replacing up to 30% of ground granulated blast furnace slag with recycled concrete fines (RCF), a secondary fine powder waste (<63 µm) from recycled concrete aggregate production, while maintaining a 50% cement replacement ratio. A limestone-slag cement benchmark group with 10% slag replaced by limestone powder was also prepared. The results indicate that the incorporation of RCF can enhance the early-age hydration reactions. Isothermal calorimetry results show a notable increase in heat release and emergency of a distinct secondary aluminate peak with the increased RCF fraction, while this is not observed in limestone-slag cement. This could be attributed to the presence of additional gypsum and calcium carbonate phases in RCF. XRD results reveal that a formation of Hemicarboaluminate (Hc) was already presented at 1 day in RCF blends. TGA results indicated an increased chemically bounded water content yet reduced portlandite content in RCF blended paste at 1 day, which confirmed RCF’s synergetic acceleration effect on slag early reaction. The initial setting time of slag-blended cement was significantly advanced from 446 min to 163 min by replacing up to 30% slag with RCF. This research proves that the incorporation of RCF can be a promising method to overcome the prolonged setting time and inefficient slag use issue in the high slag volume binder system.