An industrial by-product of steel slag has a huge potential as an alternative binder and new supplementary cementitious material (SCM). In both pure steel slag and composite systems, the amount of sulfate can be a critical factor to control early-age performance. This study aims to improve the hydration of steel slag stimulated by using a small amount of Na2SO4. The results showed that 1 % Na2SO4 enhanced both C2(A,F) and C2S hydration, leading to improved strength and denser microstructures with the formation of ettringite, AFm, and siliceous hydrogarnet. Initial thermal treatment at 40 °C with 3 % Na2SO4 provided a fast development of compressive strength at 3 days over 15 MPa. Meanwhile, an excessive amount of alkali supplied by Na2SO4 affects the relative stability between SO4-rich AFm and ettringite and has a detrimental effect on mechanical performance. The result implies that Na2SO4 content should be precisely controlled when used as a stimulator in a composite system with steel slag.