Recycled　aggregate　concrete　(RAC)　comprises　complex　meso-scale　material　phases,　including　virgin　aggregate,　multiple　interfacial　transition　zones　(ITZs),　and　both　old　and　new　mortars.　The　performance　of　RAC　is　significantly　influenced　by　the　relative　quality　and　quantity　of　these　phases.　This　study　investigated　the　effects　of　varying　volume　fractions　of　recycled　aggregate　(VRA,　20　%–44　%)　and　water-to-cement　ratios　(w/c,　0.4　to　0.6)　on　the　mechanical　properties,　failure　modes,　and　durability　of　RAC,　with　a　focus　on　identifying　and　analyzing　the　＇weaker　phases＇　and　their　quantity　fractions　changes.　The　results　indicated　that　increasing　the　VRA　improved　the　gradation　of　virgin　aggregate,　leading　to　a　more　complete　stone-to-stone　skeleton　in　RAC,　thereby　enhancing　its　compressive　strength.　However,　since　the　skeleton　had　limited　tensile　stress　transfer　capacity,　and　increasing　the　VRA　raised　the　relative　quantity　of　ITZs　(by　up　to　91.2　%),　the　splitting　tensile　strength　decreased　at　44　%　VRA.　In　terms　of　durability,　although　increasing　VRA　led　to　a　larger　quantity　of　ITZs,　the　higher　volume　fraction　of　virgin　aggregate　exerted　a　more　significant　dilution　and　tortuosity　effect　on　ion　transport,　resulting　in　reductions　of　59.0　%　in　electrical　flux　and　39.8　%　in　sulfate　diffusion　coefficient　as　VRA　increased　from　20　%　to　44　%.　Increasing　the　w/c　significantly　reduced　the　microhardness　of　the　new　mortar　and　new　ITZs,　while　old　mortar　and　old　ITZs　remained　relatively　unaffected.　When　the　w/c　increased　from　0.4　to　0.6,　the　weaker　phase　transitioned　from　being　solely　old　ITZ　to　all　three　types　of　ITZs,　with　the　microhardness　of　new　mortar　falling　below　that　of　old　mortar.　This　led　to　increases　in　the　relative　quantities　of　the　weaker　ITZ　phases　and　mortar　phase,　resulting　in　a　notable　decline　in　both　mechanical　performance　and　ion　permeability.　Specifically,　the　electrical　flux　of　RAC　at　a　w/c　of　0.6　increased　by　75.1　%–103.1　%　compared　to　RAC　at　a　w/c　of　0.4,　while　the　diffusion　coefficient　of　sulfate　ions　was　0.90–7.64　times　higher.　Furthermore,　the　ITZ　between　virgin　aggregate　and　old　mortar　exhibited　the　most　severe　microhardness　degradation　under　sulfate　attack　and　the　highest　sensitivity　to　sulfate　concentration.　These　findings　provide　valuable　insights　into　the　meso-scale　mechanisms　governing　RAC　performance　and　offer　guidance　for　optimizing　material　design　to　enhance　the　mechanical　and　durability　properties　of　RAC　in　practical　applications.　©　2025　Elsevier　Ltd