Recycled　aggregate　concrete　(RAC)　prepared　with　recycled　coarse　aggregate　(RCA)　has　more　complex　material　phases　on　the　meso　scale　compared　to　natural　aggregate　concrete　(NAC),　including　the　old　virgin　aggregate,　the　new/old　mortar,　and　the　three　types　of　interfacial　transition　zone　(ITZ).　These　mesoscopic　material　phases　would　influence　the　RAC　to　external　sulfate　attacks　in　different　ways.　This　paper　provides　a　new　look　at　the　performance　of　RAC　subjected　to　external　sulfate　attacks　by　considering　such　influence.　The　quality　and　quantity　of　the　multiple　mesoscopic　phases　were　changed　by　taking　varying　water-to-binder　ratios　(i.e.,　w/b)　and　volumetric　fractions　of　RCA　(i.e.,　VRCA),　respectively.　The　changes　in　properties　of　both　the　material　phases　on　the　meso　scale　and　concrete　on　the　macro　scale　subjected　to　external　sulfate　attacks　were　experimentally　investigated.　The　old　mortar　and　ITZ　phases　(regardless　of　the　ITZ　type)　were　identified　as　the　weaker　phases　against　external　sulfate　attacks　as　their　microhardness　deteriorated　faster　than　that　of　the　new　mortar.　Improving　the　quality　of　the　new　mortar/new　ITZ　phases　by　taking　a　lower　w/b　can　greatly　enhance　RAC＇s　resistance　to　external　sulfate　attacks:　the　ingress　of　sulfate　ions　was　retarded,　meanwhile　the　degradation　in　compressive　strength　and　dynamic　elastic　modulus　were　postponed　obviously.　Increasing　the　volumetric　fraction　of　old　virgin　aggregates　by　taking　a　higher　VRCA　can　also　help　RAC　sustain　better　under　sulfate　attacks,　even　though　it　was　accompanied　by　the　simultaneously　increasing　quantity　of　the　old　mortar　and　ITZ　which　were　identified　as　the　weaker　phases　against　sulfate　attacks.　Basically,　changing　the　quality　of　the　multiple　mesoscopic　material　phases　had　more　remarkable　effects　on　RAC＇s　sulfate　resistance　than　changing　the　relative　quantity　ratios.　This　paper　thereby　suggests　that　RAC＇s　sulfate　resistance　should　be　better　enhanced　by　the　strengthening　or　removal　of　the　old　mortar　phase,　or　by　the　improvement　of　the　new　mortar　and　ITZ　phases,　rather　than　adjusting　their　quantity　ratios　in　concrete.　©　2022　Elsevier　Ltd