In　recent　years,　construction　and　demolition　wastes　(CDW)　have　been　used　to　produce　recycled　fine　and　coarse　aggregates.　However,　the　non-sorted　presence　of　large　quantities　of　crushed　old　concrete　results　in　discontinuous　particle　gradation　along　with　the　presence　of　brick,　mud,　and　wood　in　a　portion　of　the　CDW,　renders　the　recycled　aggregates　unsuitable　for　recycled　mortar.　An　effective　optimization　method　involves　the　use　of　fly　ash　to　replace　a　portion　of　the　cement　in　recycled　mortar　to　reduce　the　water–cement　ratio　and　the　mortar　and　brick　interface　transfer　zone　thickness.　In　this　paper,　we　report　the　results　of　a　comprehensive　study　on　the　feasibility　of　CDW　as　a　fine　aggregate　for　stone　masonry.　The　effects　of　the　fly　ash　content　and　natural　sand　substitution　rate　on　the　mechanical　properties　of　the　recycled　mortar　were　quantified.　Based　on　the　experimental　results,　with　an　increase　in　the　fly　ash　substitution　rate,　the　28-day　compressive　strength　of　the　recycled　mortar　first　increased　and　then　decreased.　When　the　substitution　rate　of　fly　ash　was　20%,　the　strength　reached　a　maximum　value　of　20.8 MPa.　The　results　suggest　that　the　fly　ash　substitution　rate　used　in　recycled　mortar　should　be　maintained　at　20%,　and　the　natural　sand　substitution　rate　should　be　within　20–30%.　©　The　Author(s),　under　exclusive　license　to　Springer　Nature　Singapore　Pte　Ltd.　2024.