Performance　evolution　of　different　types　of　interfacial　transition　zones　(ITZs)　contained　in　recycled　aggregate　concrete　(RAC)　under　external　sulfate　attacks　coupled　with　dry-wet　cycling　is　experimentally　studied　in　the　labotrary.　Comparisons　are　made　between　different　ITZs　in　the　erosion　products,　the　morphology　changes,　and　the　microhardness　evolution.　Changes　in　the　pore　structure　and　microhardness　of　the　old　and　new　mortars　are　also　discussed.　Test　results　show　that　sulfate　ions　accumulate　faster　in　RAC　than　in　natural　aggregate　concrete　(NAC),　while　the　erosion　products　are　the　same.　Erosion　occurs　in　all　ITZs　but　the　predominant　erosion　prodcuts　are　not　identical:　AFt　and　gypsum　are　found　in　new　ITZs　(i.e.,　ITZ(1)　between　the　unwrapped　old　virgin　aggregate　and　the　new　mortar,　and　ITZ(3)　between　the　old　and　the　new　mortars),　while　in　the　old　ITZ(2)　(embedded　inside　RA　between　the　old　virgin　aggregate　and　the　adhering　old　mortar),　fewer　gypsum　crystals　can　be　seen;　due　to　the　dry-wet　cycling,　thenardite　crystals　are　generated　but　mainly　in　ITZ(1)　near　the　exposure　surface.　It　is　also　found　that　the　size　of　AFt　in　ITZ(1)　and　ITZ(3)　is　larger　than　that　in　ITZ(2).　The　microhardness　of　ITZs　and　cement　mortars　is　found　to　first　increase　and　then　decrease　with　the　growth　of　the　erosion　age.　The　old　ITZ(2)　presents　the　most　stabilized　performance　thereby　manifesting　itself　better　to　resist　the　sodium　sulfate　attacks　compared　to　the　new　ITZ(1)　and　ITZ(3).　The　old　mortar　is　inferior　to　both　the　new　mortar　and　the　ITZs　in　resisting　sulfate　attacks　at　a　long　erosion　age.　The　new　ITZ(1)　and　ITZ(3),　as　well　as　the　old　cement　mortar,　should　be　thereby　identified　to　be　the　predominant　weak　points　in　RAC　under　external　sulfate　attacks,　on　the　micro　scale.　This　paper　also　suggests　enhancing　these　weak　points　for　the　purpose　of　improving　RAC＇s　resistance　to　sulfate　attacks　in　dry-wet　cyclic　environments.　(C)　2019　Elsevier　Ltd.　All　rights　reserved.