In order to improve the utilization of the steelmaking byproduct steel slag in the concrete industry, this paper mainly studied the cementitious and hydration properties of converter steel slag. X-ray diffraction (XRD), non-evaporable water amount, strength, and particle size distribution were measured and analyzed. The paste with steel slag and water can be slowly hardened, which indicates the weak cementitious capacity of steel slag. The hydration of steel slag is similar to that of cement. The hydration rate of steel slag is much slower than that of cement at the early age, while its rate is higher than cement at the curing age of 90 days. The hydration rate of steel slag increases as its specific surface area (SSA) increases, and the degree of hydration becomes similar in steel slag with varying SSA at the curing age of 180 days. The early-age hydration of cement can be inhibited to some extent when steel slag is added. And the inhibition effect increases as the replacement level of steel slag increases. There is a slow strength development of steel slag blend concrete with a low water-to-binder ratio at the early age, while this influence decreases as age increases. The mineral phases of steel-slag-blend hardened paste mainly includes hydration products (Ca(OH)₂ and C-S-H), inert components (C₂F, magnetite, and RO phase), and unhydrated phases (C₃S, C₂S, gehlenite, and C₁₂A₇).