Abstract: The hydration of converter steel slag was investigated by the activity test, the amount of chemically combined water, the mechanical test and X-ray diffraction analysis (XRD). The results showed that steel slag had a low activity at early ages. When partly replacing the cement in concrete, the degree of hydration cement decreased at early ages and then the hydration process of cement inhibited. The bigger the proportion of steel slag is, the greater inhibition effect is. With a low water binder ratio, the concrete blended with steel slag had a slow development of strength at early ages, and the inhibition effect of steel slag on the concrete strength was weakened as the curing time increased. The main mineral phases of hardened slurry blended with steel slag include Ca(OH)2, C-S-H gel, inert components (C2F, Fe3O4，RO phase), and incomplete hydrated gelling components (C2S, C3S, Ca2Al2Si3O12 and C12A7).

Key words: steel slag, hydration characteristics, concrete, hydration product