The　steel　slag　and　ground　granulated　blast　furnace　slag　with　high　fineness　exhibit　synergistic　effects　on　hydration　progress　and　can　be　introduced　into　concrete　in　high　volume.　In　this　paper,　the　simulated　pore　solution　(SPS)　of　ternary　cement　incorporating　ultrafine　steel　slag　(US)　and　ultrafine　ground　granulated　blast　furnace　slag　(UG)　was　prepared　to　examine　the　passivation　behavior　of　the　HRB400　reinforcing　steel　under　SPS.　It　is　found　that　the　rapid　growth　of　passive　films　occurs　primarily　within　the　initial　2–3　days　after　immersion　in　the　SPS,　while　the　compaction　of　passive　films　takes　place　mainly　at　the　later　stages.　The　open　circuit　potential　and　charge　transfer　resistance　of　the　passive　films　initially　increase　and　then　decrease　with　the　continuous　rise　in　the　US　content.　The　Fe2O3　constitutes　a　significant　portion　of　the　passive　films　above　the　steels　treated　with　the　SPS　irrespective　of　US　dosages,　whereas　the　content　of　FeO　is　highly　influenced　by　US　dosages.　The　UG-US-C　pore　solution　could　enhance　the　passivation　behavior　because　of　lower　Ca2+　and　higher　Na+.　In　case　of　UG/US　ratio　at　2:3,　the　passive　film　exhibits　the　greatest　thickness　at　7.5　nm　and　meanwhile　with　the　highest　compaction　and　the　least　roughness.　©　2024　Elsevier　Ltd