Steel　slag　as　a　solid　waste　during　steel-making　has　potential　cementitious　activity,　and　its　use　as　supplementary　cementitious　material　is　an　effective　way　to　improve　its　resource　utilization.　Ethanol-diisopropanolamine　(EDIPA),　as　an　organic　admixture,　affects　the　hydration　of　the　steel　slag-cement　composite　system.　In　this　paper,　the　effect　of　EDIPA　on　the　hydration　and　mechanical　properties　of　the　steel　slag-cement　composite　system　was　investigated　by　using　mortar　strength　tests　and　combining　with　heat　of　hydration,　X-ray　diffraction　(XRD),　thermogravimetry/differential　thermogravimetry　(TG/DTG),　scanning　electron　microscope　(SEM)　and　other　testing　methods.　The　results　show　that　EDIPA　can　improve　both　early　and　late　compressive　strength　of　steel　slag-cement　mortar　at　low　dosing　(0.02%).　However,　at　high　dosing　(0.06%,　0.10%),　EDIPA　is　unfavorable　to　early　strength,　but　enhanced　late　strength　more　strongly　than　low　dosing.　Low　dose　of　EDIPA　can　promote　the　hydration　of　steel　slag-cement　composite　pastes,　and　can　also　improve　the　hydration　degree　in　the　early　and　late　stages.　At　low　dosing,　EDIPA　can　promote　the　formation　of　ettringite　(AFt)　and　facilitate　the　transformation　from　AFt　to　monosulfoaluminate　(AFm).　In　addition,　EDIPA　affects　the　crystal　structure　and　morphology　of　Ca(OH)2　prepared　by　co-precipitation　method.　And　the　morphology　of　Ca(OH)2　gradually　changes　from　long　hexagonal　column　to　short　hexagonal　column　and　then　to　irregularly　radiating　lamellar　with　the　increase　of　EDIPA　dosing.　This　indicates　that　EDIPA　inhibits　the　vertical　growth　of　Ca(OH)2　and　has　the　reference　significance　for　the　change　of　Ca(OH)2　morphology　in　the　composite　paste.　Copyright　©　2022　Japan　Concrete　Institute.