Accelerated　carbonation　effectively　mitigates　the　unsoundness　issue　of　steel　slag　caused　by　free　CaO/MgO.　However,　the　long-term　impact　of　residual　uncarbonated　iron-rich　phases,　which　may　oxidize　and　lead　to　expansion　and　degradation　during　long-term　curing,　remains　unclear.　This　study　investigated　the　long-term　behavior　of　carbonated　steel　slag　blocks　with　varying　residual　iron　content　in　the　steel　slag　obtained　by　magnetic　separation.　The　carbon　sequestration　efficiency,　mechanical　properties,　volume　stability,　and　microstructure　were　thoroughly　evaluated.　Results　revealed　that　carbonated　steel　slag　blocks　with　a　higher　residual　Fe2O3　content　(38.47　%)　achieved　only　46　%　of　the　compressive　strength　and　37　%　of　the　CO2　uptake　compared　to　those　with　lower　residual　iron　content　(25.88　%).　Through　180　days　of　accelerated　degradation　and　natural　curing　tests,　it　was　observed　that　the　compressive　strength　of　blocks　exhibited　an　overall　increase　of　20–150　%,　especially　in　blocks　with　high　iron　content,　due　to　the　hydration　of　uncarbonated　steel　slag.　Despite　the　formation　of　rust　in　samples　with　elevated　residual　iron　content,　as　verified　through　XRD　and　optical　microscopy,　no　substantial　negative　impacts　were　observed　at　later　stages.　The　excessive　residual　iron　containing　phase　contributed　to　the　formation　porous　microstructure　and　enhance　the　capacity　of　accommodating　corrosion　products.　This　study　highlights　that　reducing　residual　iron　content　through　magnetic　separation　not　only　enhances　the　mechanical　properties　and　stability　of　carbonated　steel　slag　blocks,　but　also　offers　a　valuable　recycling　opportunity　for　steel　and　iron　industries.　©　2024　The　Authors