Durability and Pore Structure Evolution of Foamed Lightweight Soil for Backfilling Under Wetting and Drying Cycles: Effects of Stabilization Systems

Converting waste slurry from underground construction into foamed lightweight soil (FLS) offers a route to waste valorization, but its durability under repeated moisture changes requires evaluation. This study compared FLS prepared with ordinary Portland cement (OPC), alkali-activated slag–fly ash (AASF), and hybrid OPC-AASF. Engineering properties and resistance to 18 wetting and drying (W-D) cycles were evaluated alongside pore structure evolution, microstructural changes, and environmental and economic indicators. Increasing soil content reduced unconfined compressive strength (UCS), with OPC-AASF showing a more gradual decline than OPC. All systems exhibited non-monotonic strength evolution during cycling. After 18 cycles, the UCS losses relative to the 28 d baseline were 1.9%–5.8% for OPC-AASF and 12.3%–17.6% for AASF. In selected specimens, X-ray computed tomography showed that lower macroporosity did not necessarily correspond to better strength retention. The greater strength loss in AASF was accompanied by spatial pore enrichment, coarse low-sphericity pores, and local interfacial damage. X-ray diffraction indicated retention of the main crystalline phases, while scanning electron microscopy showed better local pore wall and interfacial continuity in OPC-AASF. On a common dry-solids mass basis, the hybrid mixture containing 40% soil required 76.0% less OPC than a theoretical OPC foam concrete without waste soil. The estimated carbon emissions, energy intensity, and material cost associated with raw material inputs were 72.2%, 68.0%, and 48.1% lower, respectively. These findings support OPC-AASF as a cement-reduced stabilization system for lightweight backfill, combining waste slurry reuse with strength retention under repeated moisture fluctuations.