The　introduction　of　high-volume　steel　slag　into　ordinary　Portland　cement　(OPC)　has　a　negative　effect　on　the　pore　structure　due　to　its　poor　hydraulic　reactivity.　Synergistic　effects　between　steel　slag　and　ground　granulated　blast　furnace　slag　(GGBFS)　in　cementitious　system　have　drawn　increasing　attention.　In　this　study,　the　modified　Andreasen　and　Andersen　(MAA)　model　was　adopted　for　the　mixture　design　of　ternary　cementitious　system　incorporating　high-volume　ultrafine　steel　slag　(USS)　and　GGBFS　without　a　compromise　in　strength.　X-ray　computed　tomography　(X-CT)　and　mercury　intrusion　porosimetry　(MIP)　were　applied　to　characterize　the　pore　structure　of　various　cementitious　mortars　in　terms　of　porosity,　pore　size,　pore　number,　pore　morphology　and　homogeneity.　Results　manifested　that　the　adverse　impact　of　USS　addition　on　the　pore　structure　of　binary　cement　was　effectively　mitigated　in　the　ternary　cement　USS-GGBFS-OPC　after　mixture　optimization,　whereby　the　pore　size　was　shifted　toward　a　finer　distribution　and　simultaneously　with　a　higher　uniformity　and　consequently　promoting　the　compressive　strength　development.　An　increase　of　cumulative　pore　volume　as　captured　by　X-CT　was　associated　with　an　exponential　decrease　of　the　number　of　pores　while　together　with　larger　pore　sizes.　The　increase　in　the　number　of　small　pores　led　to　an　increase　in　the　pore　tortuosity　and　degree　of　sphericity.　The　fractal　dimension　of　pores　was　negatively　correlated　with　pore　inhomogeneity　and　compressive　strength.　These　findings　provide　new　insights　into　homogeneity-oriented　mixture　design　of　composite　cementitious　binders　based　on　gap-graded　particle　packing,　together　with　their　synergistic　effects　on　hydration　progress.