Abstract: The strength of a cementitious material made from steel slag, iron slag, cement clinker and flue gas desulfurization (FGD) gypsum was optimize by orthogonal tests to prepare a concrete for building high-strength artificial reefs. When the ratio of steel slag to iron slag is 5:3 and when 10% cement clinker and 10% FGD gypsum are mixed with 80% iron and steel slag powder, the mixed cementitious material has an optimized strength. A concrete with a compressive strength over 50 MPa can be prepared by using such a mixture as a cementitious material and steel slag grains as its aggregates which have been stabilized by a hot-simmering method. Hydration processes of the net slurry were analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The results show that early hydration products in the system are mainly AFt and C-S-H gel, and in the later stage the pozzolanic reaction of iron and steel slags contributes mainly to the increase in strength.