This　work　proposes　that　diatomite　can　be　used　to　improve　the　performance　of　recycled　aggregate　concrete　(RAC).　A　calcined　diatomite　sample　with　an　amorphous　silica　content　over　90%　was　added　to　the　RAC　mixture　as　a　partial　replacement　of　the　ordinary　Portland　cement　(OPC)　at　varying　rates　from　0　to　20　wt%.　Test　results　show　that　both　the　strength　and　impermeability　of　RAC　can　be　obviously　enhanced　with　the　greatest　enhancement　observed　at　a　replacement　level　of　15　wt%　for　which　the　28-day　compressive/splitting　tensile　strengths　were　improved　by　29.2%　and　33.9%,　respectively,　and　the　chloride　diffusion　coefficient　was　reduced　by　86.27%.　The　microscopic　measurements　confirmed　the　nucleation　effects　and　the　high　pozzolanic　reactivity　of　diatomite　which　both　contributed　to　the　more　complete　hydration　of　cement　in　RAC.　It　was　also　found　that　the　proportion　of　large　capillary　pores　in　the　new　mortar　was　reduced　by　using　diatomite,　meanwhile　the　average　porosity　of　the　new　interfacial　transition　zone　(ITZ)　became　smaller.　This　work　suggests　that　the　calcined　diatomite　used　for　improving　RAC　performance　should　be　no　larger　than　15　wt%　of　cement,　as　the　scarcity　of　CH　at　a　higher　replacement　ratio　would　hamper　the　pozzolanic　reactions,　meanwhile　the　diluted　cement　clinkers　would　have　a　negative　effect　on　the　pore　structure　of　RAC.