Abstract: The influence of the replacement ratio, fineness and age of superfine slag powder on the properties of cement paste was investigated by tests with XRD. The results showed that compared with the reference slurry, the diffraction peak intensity of the Ca(OH)2 crystal drops sharply in the 28-day hydrated cement sample blended with superfine slag powder, and that the consumed quantity of Ca(OH)2 is in close relation with the age of hydrated cement sample and the replaement ratio and fineness of the powder. The hydration rate of the cement paste with P1000 superfine slag powder(measured specific area: 1885 m²/kg) is so quick that the secondary hydration reaction can be completed basically just in 3 days.No significant change is found in Ca(OH)2 diffraction peak intensity in the period of 3-60 days. The hydrated cement's absorbability of Ca(OH)2 is enhanced with the increasing slag fineness and slag powder's specific area. However, the hydration processes of such clinkers as C2S and C3S are not shortened with the decreasing quantity of Ca(OH)2 crystal, which shows the inconsistancy with the general rules and the reason is to be investigated further with other testing means.