The large-scale and high-value utilization of industrial solid waste has become a key research area in sustainable building materials. However, ensuring effective backfilling quality in narrow or irregular spaces remains challenging in civil engineering. Developing flowable solidification materials from industrial solid waste not only resolves issues inherent in traditional backfilling techniques but also enhances efficient resource utilization. In this study, phosphogypsum was used to prepare geopolymers, which served as binders replacing cement in producing phosphogypsum-based fluidized solidified soil (PFSS). The workability, mechanical strength, and toxic substance leaching of PFSS were evaluated. Moreover, the underlying mechanisms of strength formation and toxic substance immobilization were investigated. The optimal PFSS composition was determined to have a water-to-solid ratio of 0.48–0.50 and a geopolymer content of 12–18% (by mass). Under these conditions, the material exhibited fluidity ranging from 160 to 220 mm, a 28-day compressive strength of 0.86 MPa, a California Bearing Ratio (CBR) of 8%, and a resilient modulus of 40 MPa. These parameters satisfy the performance standards required for backfilling in high-grade highways. The leaching concentrations of heavy metals (As, Pb, and Cr) complied with China’s Class III groundwater quality standards. Microstructural analyses indicated the occurrence of hydration, pozzolanic reactions, geopolymerization, and carbonation. Microstructural analyses indicated the formation of an interlocking three-dimensional network, composed of C-S-H, C-A-S-H gels, and ettringite (AFt), which contributes significantly to the strength development and immobilization of heavy metals. These products collectively formed an interlocking three-dimensional network structure, significantly contributing to PFSS strength development. Heavy metals were effectively immobilized within the matrix due to the combined effects of physical adsorption and chemical bonding.

This study investigates the strength and microstructural evolution of SRX-stabilized aeolian sand–gravel mixtures for flexible base applications in desert roads. CBR, UPS (uniaxial penetration strength), and compressive resilient modulus tests were conducted under varying SRX dosages (0.4–1.0%) and aeolian sand contents (30–50%). The results show that increasing the SRX dosage significantly improves all three indices, with the 0.5% SRX and 30% aeolian sand mixture yielding the CBR (385.89%) and UPS (0.938 MPa) and achieving a compressive resilient modulus that meets the requirements for graded aggregate base layers. XRD FTIR and SEM–EDS analyses reveal that the SRX enhances material structure primarily through physical mechanisms, forming dense films and bonding networks without inducing significant chemical reactions. Extended curing improves structural integrity, while excessive aeolian sand reduces compactness. SRX-stabilized aeolian sand gravel is a viable base and subbase material for desert highways.

The scarcity of natural aggregates and the accumulation of oil sludge in desert regions pose critical challenges for highway construction. Although aeolian sand and oil sludge pyrolysis residue have been studied individually as construction materials, their combined use in foamed lightweight soil remains unexplored. This study addresses this gap by developing a novel foamed lightweight soil termed SOFS, which is created through the synergistic modification of aeolian sand and oil sludge pyrolysis residue. A six-factor, five-level orthogonal array (L25) was employed to systematically investigate the effects of residue content, sand content, foam-to-slurry ratio, foaming agent dilution, water-to-solid ratio, and mixing time. The evaluated properties included physical properties (fluidity and wet density), mechanical properties (compressive, splitting tensile, and flexural strength), and durability (wet–dry and freeze–thaw resistance). Scanning electron microscopy was used to examine the microstructural mechanisms. Variance and range analysis identified the optimal mixture, designated H14, which achieved 28-day compressive, splitting tensile, and flexural strengths of 3.75 MPa, 2.21 MPa, and 0.9 MPa, respectively, thereby meeting desert roadbed requirements. Compared with conventional materials, H14 exhibited superior durability, with strength losses of only 16.3% in compressive strength and 19.1% in splitting tensile strength after 25 cycles. Microstructural analysis revealed a dense C-S-H gel network encapsulating the solid waste particles, with nanoscale Al- and Cl-rich crystalline phases observed at interfacial pores—a phenomenon that has rarely been documented in previous studies. These findings provide a theoretical and technical foundation for solid waste valorization and the development of sustainable desert infrastructure.

Aeolian sand is abundant in arid deserts, but its high replacement in cement-stabilized bases can reduce strength and raise cracking risk. Strain localization and crack evolution are also poorly quantified. This study aimed to optimize the early age performance of cement-fly ash stabilized aeolian sand gravel (CFSAG) and clarify its failure mechanism. A Box–Behnken response surface methodology varied the cement content, cement-to-fly ash ratio, coarse aggregate gradation, and aeolian sand content. The 7-d unconfined compressive strength (UCS) and splitting tensile strength (STS) were tested. Digital image correlation (DIC) recorded full-field strains and crack metrics in compression and splitting. SEM–EDS was used to interpret microstructural changes. The aeolian sand content dominated UCS, whereas the cement content and cement-to-fly ash ratio mainly controlled STS. Factor interactions were non-negligible and supported the joint optimization of the two strength indices. DIC identified a crack propagation threshold near 0.9 P_max in splitting. Excess aeolian sand (>50%) caused earlier localization, more cracks, and wider openings. In the appropriate amount of aeolian sand mixtures, hydration products filled voids and improved paste continuity. SEM–EDS indicated that excessive fines increased porosity and weakened the interfacial transition zone. Overall, the combined RSM–DIC–SEM approach links mix design with deformation and microstructure evidence. It provides practical guidance to balance strength and cracking resistance at early ages for cement-stabilized bases in desert highway engineering.