Engineered　cementitious　composites　(ECC)　achieve　exceptional　ductility　and　durability,　but　the　heavy　reliance　on　cement　has　notable　environmental　impacts.　In　this　work,　the　steel　slag　(SS)　and　ground　granulated　blast　furnace　slag　(GGBFS)　are　introduced　into　ECC　by　largely　substituting　cement.　The　impact　of　SS　and　GGBFS　content　on　ECC＇s　mechanical　performance　is　analyzed　at　macro　and　micro　scales　in　terms　of　fiber　dispersion,　mechanical　properties,　cracking　patterns,　cement　hydration,　and　microstructure.　The　results　demonstrate　that　SS　can　enhance　fiber　dispersion　due　to　the　low　water　demand　and　the　strong　electronic　double　layer　formed　on　the　surface　of　SS　particles.　While　ECC　with　a　high　volume　of　SS　alone　(60　wt%　SS)　lacks　the　necessary　strength　for　structural　applications,　the　combination　of　GGBFS　and　SS　exhibits　favorable　hydration　properties　and　can　effectively　refine　the　pore　structure.　Thus,　ECC　with　GGBFS　and　SS　attains　ideal　toughness.　ASTM　C　1018　based　toughness　indices　reveal　that　the　Imax　of　ECC　with　30　wt%　GGBFS　and　30　wt%　SS　is　2.30　times　that　of　OPC　based　ECC.　A　sustainability　analysis　demonstrates　that　substituting　30　wt%　SS　and　30　wt%　GGBFS　for　cement　leads　to　a　33%　reduction　in　energy　consumption　and　a　53%　decrease　in　CO2　emissions　of　ECC.