Mixing　foamed　lightweight　soil　(FLS)　with　marine　sand　(MS)　can　be　a　sustainable　approach　for　maximizing　waste　resources,　although　the　fundamental　properties　(the　physical　performance,　durability,　and　microstructural　behaviors)　of　MS-mixed　FLS　(MS–FLS)　have　not　been　investigated,　and　this　significantly　restricts　its　engineering　application.　Thus,　in　this　study,　macroscopic　and　microscopic　experiments　are　conducted　to　comprehensively　explore　the　properties　of　MS–FLS　with　different　MS,　chloride　(Cl−),　and　shell　contents.　The　results　of　the　macroscopic　tests　reveal　that　MS–FLS　exhibits　a　fluidity　of　160–229　mm,　a　volumetric　water-absorption　rate　of　13.1　%–19.5　%,　and　an　unconfined　compressive　strength　(UCS)　of　0.95–2.64　MPa.　Further,　the　flow　of　bubbles　is　promoted　as　the　fluidity　and　water-absorption　rate　change,　thus　accumulating　the　bubbles　with　relatively　large　pore　sizes　and　causing　an　increase　in　the　overall　relative　pore　size.　Although　the　durability　is　reduced　to　varying　degrees　via　wet　cycling,　freeze–thaw　cycling,　sulfate　(SO2　−　4)　erosion,　and　long-term　outdoor　exposure,　most　of　the　UCS　values　can　still　satisfy　the　requirement　for　subgrade　engineering　applications.　Moreover,　the　microscopic　tests　indicate　that　the　pore　size　increases　with　the　increasing　MS　content,　whereas　the　pore-wall　thickness,　cement　density,　mechanical　properties,　and　durability　decrease.　As　the　shell　content　increases,　the　UCS　and　durability　decrease　considerably　owing　to　the　smooth,　surface-induced　weak　points　and　low　water-absorption　capacity　of　the　shells;　however,　the　UCS　can　still　satisfy　engineering　requirements.　Finally,　the　increased　Cl−　content　can　improve　the　SO2　−　4　resistance　due　to　the　generated　Friedel＇s　salt,　which　fills　the　micropores　in　cement-hydration　products,　and　this　facilitates　strengthening.　Therefore,　the　UCS　values　of　most　MS–FLS　specimens　are　>　0.3　MPa　before　and　after　the　durability　tests,　revealing　their　promise　for　subgrade　engineering　applications.　©　2023　Elsevier　Ltd