One　of　the　most　promising　strategies　to　manage　the　large　volume　of　construction　and　demolition　(C&D)　waste　is　recycling　and　utilizing　it　for　the　production　of　new　concrete.　However,　recycled　concrete　aggregate　(RCA)　derived　from　C&D　waste　possesses　relatively　higher　porosity　and　water　absorption　capability,　which　often　limits　its　wild　utilization.　In　this　study,　pozzolan　slurry　(includes　silica　fume,　nano-SiO2,　and　fly　ash　slurries)　and　CO2　treatments　as　enhancement　methods　for　RCA　were　investigated.　Test　results　showed　that　CO2　treatment　was　more　effective　in　reducing　water　absorption　and　enhancing　fluidity,　whereas　pozzolan　slurry　treatment　could　decrease　fluidity.　Mortars　prepared　with　treated　RCA　exhibited　better　mechanical　strength　and　higher　resistance　towards　carbonation　and　chloride-ion　diffusion　than　those　with　untreated　RCA.　Both　pozzolan　slurry　and　CO2　treatments　enhanced　not　only　the　properties　of　RCA,　but　also　the　old　and　new　interfacial　transition　zones　(ITZs)　as　demonstrated　in　the　measured　micro-hardness　and　SEM　observation.　(C)　2017　Elsevier　Ltd.　All　rights　reserved.