Hydraulic　lime　has　been　successfully　applied　to　repair　masonry/geotechnical　ancient　architecture.　However,　chemical　pollutants　caused　by　modern　industrial　development　have　enriched　the　building　base　for　a　long　time,　and　the　mechanism　by　which　corrosive　media　destroy　the　hydraulic　lime　repair　material　is　unclear.　In　this　study,　the　properties　and　structural　failure　of　natural　hydraulic　lime　(NHL)　and　metakaolin-air　lime　(MK-AL)　due　to　NaOH,　NaCl　and　Na2SO4　erosion　were　studied　in　terms　of　macroscopic　variation,　mechanical　property　deterioration　and　internal　three-dimensional　(3D)　structure　evolution.　The　results　obtained　by　XRD,　TG-DTA　and　MIP　tests　show　that　the　hardening　products　and　pore　structure　composition　of　NHL　and　MK20AL　pastes　were　significantly　different,　which　led　to　different　erosion　resistances.　Based　on　the　apparent　morphology　and　X-CT　test,　no　cracks　were　formed　in　the　macroscopic　structure　of　NHL　and　MK20AL　mortar　by　NaOH　and　NaCl　erosion　after　different　periods.　The　mechanical　strength　of　NHL　mortar　showed　little　change　after　erosion　by　the　above　two　media.　For　the　MK20AL　mortar,　the　compressive　strength　increased　by　NaOH　erosion　and　the　flexural　strength　decreased　significantly　by　NaCl　erosion.　No　clear　expansion　cracks　formed　in　the　surface　and　internal　structure　of　NHL　mortar　by　Na2SO4　erosion,　and　the　compressive　strength　increased　by　∼20%.　Gypsum　erosion　and　secondary　erosion　(gypsum　and　C-A-H　reaction　to　generate　ettringite)　formed　in　MK20AL　mortar　by　a　Na2SO4　medium.　Additionally,　volume　expansion　of　solid　phase　(corrosion　product)　caused　crack　to　form　in　the　internal　mortar　structure,　the　cracks　gradually　became　wider,　and　a　crack　network　formed　with　the　extension　of　erosion;　thus,　the　internal　structure　and　mechanical　properties　were　seriously　damaged.　Overall,　compared　to　the　MK20AL　mortar,　the　NHL　mortar　shows　better　resistance　to　erosion　by　environmental　media,　and　NHL　is　the　better　choice　for　restoring　ancient　buildings　and　plastering　mortar　projects.　The　resistance　of　MK20AL　to　erosion　by　mortar　environmental　media　needs　to　be　optimized　and　modified.　©　2023