In　this　investigation,　steel　slag　(SS)　was　activated　by　using　NH4　H2　PO4　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　NH4　H2　PO4　were　mainly　Ca(NH4　)　2　(HPO4　)　2·H2　O,　Ca3　(PO4　)　2　and　FePO4　.　The　reaction　products　fill　in　a　narrow　micro　space　and　the　density　of　the　matrix　increases　under　pressure　curing.　©　2023　Beijing　University　of　Technology.　All　rights　reserved.