The possibility of applying steel slag as mineral admixture in steam-cured concrete was investigated. The compressive strength, chloride permeability, carbonation depth and drying shrinkage of concrete and the microstructure characteristics of hardened paste were studied. Results show that the steam-cured concrete containing steel slag at a high water-to-binder ratio has low compressive strength, coarse pore structure, moderate permeability, large carbonation depth and great drying shrinkage. The negative effects of steel slag on the mechanical and durability performance of steam-cured concrete become smaller at a low water-to-binder ratio or by adding ultrafine ground granulated blast furnace slag (UGGBS). The later-age hydration of cement is significantly promoted by adding steel slag. The pozzolanic reaction and micro-filler effect of UGGBS result in dense structure, which makes great contribution to the strength development and durability performance of steam-cured concrete. These effects of steel slag and UGGBS are significant at a low water to binder ratio. The steam-cured concrete containing steel slag-UGGBS blended mineral admixture at a low water-to-binder ratio has the best performance with high compressive strength, fine pore structure and good durability.