Natural　hydraulic　lime　(NHL)　is　an　inorganic　building　material　with　hydraulic　and　air-hardening　properties,　having　exceptional　application　in　restoring　ancient　buildings　compared　to　Portland　cement　and　ancient　air-hardening　lime.　However,　the　characteristic　of　slow　performance　development　limits　its　application　field.　This　study　used　ground　granulated　blast　furnace　slag　(GGBFS)　to　replace　parts　of　NHL.　The　hydration　heat　release,　phase　composition,　micromorphology,　pore　structure,　and　physical　and　mechanical　properties　of　GGBFS-NHL　(S-NHL)　based　materials　in　the　hardening　process　are　investigated　in-depth.　The　influence　of　GGBFS　on　the　hardening　process　and　performance　development　of　NHL　are　systematically　evaluated.　The　NHL　is　dominated　by　the　hydration　reaction　of　C2S　to　produce　C–S–H　in　the　early　stage　(less　than　28　days).　Moreover,　C3AH6,　CASH4,　C4AH11,　and　C–S–H　gel　generated　by　the　reaction　of　GGBFS　with　Ca(OH)2　changes　the　composition　of　hardened　products,　including　the　microstructure　of　the　hardened　paste.　The　higher　the　dosage　of　GGBFS,　the　more　the　hydration　products　in　the　NHL-based　paste.　The　NHL-based　paste　gradually　hardens　with　increasing　age,　mainly　due　to　carbonation　reaction.　Moreover,　its　pore　structure　gradually　densifies　with　curing　age,　and　the　average　pore　size　and　total　pore　volume　of　S-NHL　decrease　gradually　with　increasing　GGBFS　content.　GGBFS　significantly　influences　the　mechanical　properties　and　weight　of　the　NHL-based　mortar　development.　In　addition,　the　S-NHL　mortar＇s　early　and　long-term　mechanical　strength　increased　significantly　with　increasing　GGBFS.　©　2022　Elsevier　Ltd