The magnesium slag is discarded from production of magnesium metal from dolomite. However the magnesium slag is slaked in some factories by means of sprinkling water to prevent from dust pollution. The possibility of slaked magnesium slag (SMS) to play a role of mineral admixture for concrete was investigated by experiments of mortars and concretes prepared with SMS. The results revealed that SMS was still reactive. When SMS is substituted for 30% of cement, its reactivity index is equivalent to that of grade I fly ash (FA). And when SMS is used as mineral admixture to prepare concrete, it contributes to strength of concrete no less than S95 ground granulated blast-furnace slag (GGBS). Better effect of binary or ternary blend can be obtained by combining SMS with GGBS and/or FA. Because the magnesium slag is slaked, SMS has no problem on soundness, so it can be applied in concrete as mineral admixture.