Cement admixture having magnesium smelting slag, method of manufacturing the same, and cement having the same

The present invention provides a method of manufacturing a cement admixture having magnesium smelting slag, through which the magnesium smelting slag can be recycled. The method of manufacturing a cement admixture having magnesium smelting slag according to an embodiment of the present invention comprises the steps of: forming magnesium molten metal by smelting magnesium from a magnesium material; separating magnesium smelting slag from the magnesium molten metal; and forming a β-C2S crystal phase by rapidly cooling the magnesium smelting slag.