In　this　work,　steel　slag　(SS),　blast　furnace　slag　(BFS),　ultrafine　SS　(USS)　and　ultrafine　BFS　(UBFS),　with　fineness　of　407,　450,　748　and　760　m2/kg　respectively,　were　prepared.　The　influences　of　SS　and　BFS　fineness　on　cementitious　systems　were　studied　from　perspectives　of　the　hydration　process,　microstructure　formation　as　well　as　fluidity　and　strength.　The　results　show　that　the　BFS　with　a　high　fineness　exhibits　a　high　reactivity　while　an　adverse　effect　on　the　fluidity.　A　high　fineness　of　SS　could　have　a　negative　impact　on　the　hydration　of　blended　cement.　The　addition　of　BFS,　regardless　of　the　fineness,　substantially　increases　the　number　of　small　pores　below　20　nm　and　enhances　the　compressive　strength　of　cementitious　systems.　The　combination　of　SS　and　BFS　can　decrease　the　Ca/Si　ratio　and　raise　the　Al/Si　ratio　of　the　main　hydrates　C(-A)-S-H,　while　the　fineness　does　not　have　a　great　impact　on　the　chemical　composition　of　C(-A)-S-H.　Compared　to　plain　cement,　the　gap-graded　ternary　cement　prepared　with　25　wt%　cement,　39　wt%　SS　and　36　wt%　UBFS　is　featured　with　a　lower　embodied　energy　and　CO2　emission　without　compromising　the　early　and　late　performances.　©　2023　Elsevier　Ltd