Cementitious materials was prepared with powders of steel slag (SS), blast furnace slag (BFS) and desulfurized gypsum (DG). The influence of SS content on the strength of the concrete composted of all industrial solid wastes was investigated. The hydration mechanism of cementitious materials was characterized by means of XRD, IR, TG-DTA and SEM, while the variation of pH and concentration of the specified ions of cementitious materials were acquired during the hydration process. The results show that when the mass ratio of SS, BFS and DG is m(SS):m(BFS):m(DG)=30:58:12, the concrete possesses a better compressive strength after curing for 3 d, 7 d and 28 d. With the increasing dosage of SS, the pH value and Ca²⁺ concentration decreased first and then increased. The initial concentration of solutes of silicon (aluminum) was relatively low and then increased later. In the presence of DG, SS and BFS can synergistically promote the hydration and the major hydration products are ettringite (AFt) and calcium silicate hydrate (C-S-H) gels. During the later stage of the hydration process, the quantity of hydration products increased rapidly, forming a composite of C-S-H gel as matrix, within which there existed randomly dispersed needle-like AFt crystallites as strengthening phase, thereby, the hardened cement paste became much dense and hard.