[en] The effect slag powder on strength development of alkali-activated recycled cement paste was investigated. The results show that alkali-activated recycled cement paste has low cementing strengths, which depend on the types of the original cement. The alkali-activated recycled cement pastes prepared from the recycled cement pastes with the original cement of fly ash cement (PF) resulted the highest strengths, followed by that from slag cement, and that from the ordinary Portland cement resulted the lowest strengths. Replacing recycled cement pastes partly by slag powder can greatly increase the strengths of alkali-activated recycled cement pastes. The 90 day flexural and compressive of alkali-activated recycled PF replaced by 30% slag powder were 6.7MPa and 51.9 MPa. Scanning electron microscopy(SEM) indicated that the microstructure of the alkali-activated recycled cement paste with 30% replacement by slag powder is much denser than that without replacement. (paper)