Abstract: In this investigation, steel slag (SS) was activated by using NH₄H₂PO₄ as an activator to eliminate free calcium oxide and iron oxide in SS powder and to excite the cementitious properties of SS. The effects of NH₄H₂PO₄ content on the working performance, volume stability and mechanical properties of SS slurry activated by NH₄H₂PO₄ under different curing conditions were studied. The types and microstructure of reaction products of SS activated by NH₄H₂PO₄ were characterized by XRD and SEM. Results show that the setting time of slurry prolonged with the increase of the mass ratio of NH₄H₂PO₄ to SS. Slurry will expand under room temperature curing conditions, when the mass ratio of NH₄H₂PO₄ to SS is greater than 0.2, and the expansion rate reaches about 40% when the mass ratio is 0.6. The expansion of slurry can be effectively avoided using pressure curing. The compressive strength of the sample after 28 days reaches 5.6 MPa at room temperature while rapid curing has little effect on the compressive strength of hardened slurry. XRD and SEM results show that the reaction products of SS activited by NH₄H₂PO₄ were mainly Ca(NH₄)₂(HPO₄)₂·H₂O, Ca₃(PO₄)₂ and FePO₄. The reaction products fill in a narrow micro space and the density of the matrix increases under pressure curing.