Characterizing the engineering properties of Marine Sand-Amended foamed lightweight Soil: Macroscopic and microscopic perspectives

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 Clcontent 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.