The influence of powder mixture of steel slag and blast furnace slag on the physical properties of cement paste was investigated. Besides, the effect of chemical activator on powder mixture was also studied. For these purposes, the powder mixtures with different ratio of steel slag to blast furnace slag were blended with Portland cement. The water requirement of normal consistency, setting time, and soundness of cement were measured. Water glass and anhydrite were used to stimulate the activity of powder mixture. And the microstructure of hydration products was observed using scanning electron microscope (SEM). It was concluded that powder mixture could improve the fluidity of cement paste and prolong setting time. The effect of compound activators was better than a single one. Under the action of activators, the activity of powder mixture reached to the level of S95, when the proportions of steel slag were from 10% to 30%.