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 raw material and blend for portland cement was investigated by experiments of raw mix preparation, clinker calcining and property determination of cement pastes and mortars. The results revealed that SMS was still reactive. The raw mix with SMS was of excellent burnability that would contribute to energy saving. As a raw material, SMS can be used for calcining clinker of good quality; and as a blend it is suitable for production of ordinary portland cements. Because the magnesium slag is slaked, SMS has no problem on soundness. Higher strength of cement can be obtained in form of binary blends consisting of SMS and ground granulated blast-furnace slag or fly ash.