High　incorporation　levels　of　circulating　fluidized　bed　combustion　(CFBC)　fly　ash　in　concrete　raise　concerns　due　to　its　high　SO3　content.　This　study　investigates　the　effects　of　CO2　curing　on　the　microstructure　and　hardening　performance　of　CFBC　fly　ash　based　cementitious　materials,　focusing　on　high　dosage　utilization　(≥70　%　CFBC　fly　ash).　The　compressive　strength,　volume　stability,　phase　evolution,　and　pore　characteristics　were　systematically　investigated　through　comprehensive　analyses　including　macro-performance　tests,　XRD,　TG-DTG,　SEM,　NMR,　and　MIP.　Results　indicate　that　can　yield　considerable　compressive　strength　(40–55　MPa　for　higher　CaO　content　CFBC　fly　ash　and　15–20　MPa　for　lower　CaO　content　CFBC　fly　ash)　through　the　formation　of　CaCO3　and　silica　gel,　which　densify　the　microstructure.　CO2　curing　maintains　excellent　volume　stability,　with　dimensional　changes　remaining　below　0.3　%　after　24　h　of　CO2　curing　followed　by　300　days　of　water　curing.　CO2　cured　CFBC　fly　ash　based　cementitious　materials　exhibit　pH　thresholds　(10.5–12.0)　and　SO3　content　(4.0–14.0　%)　that　fall　within　the　characteristic　range　of　supersulfated　cement　with　excellent　volume　stability.　These　findings　demonstrate　that　CO2　curing　effectively　improves　mechanical　performances　and　volume　stability.　The　CO2　cured　CFBC　fly　ash　based　cementitious　materials　could　be　used　to　produce　non-fired　bricks　and　precast　concrete　blocks,　providing　a　sustainable　pathway　for　the　large-scale　utilization　of　high　SO3　CFBC　fly　ash.　©　2025　Elsevier　Ltd