Calcareous sand, characterized by numerous pore spaces, easy fragmentation, and low strength, is commonly used as fill material in island construction projects. Due to these limitations, it often fails to meet the requirements of actual engineering applications. This paper uses oxidized graphene in combination with fly ash cement to modify calcareous sand. The effects of oxidized graphene, fly ash cement, and curing time on the modification effect were investigated through triaxial tests and numerical simulations. The experimental results show the following: (1) Both the extension of curing age and the increase in the dosage of fly ash cement can improve the shear performance of calcareous sand, with the increase in the dosage of fly ash cement able to ensure thorough bonding between calcareous sand particles. (2) Graphene oxide can significantly improve the shear performance of calcareous sand cement mortar, with the optimal dosage being 0.06%. Excess amounts result in a reduced performance improvement, which is related to the degree of the catalysis of oxidized graphene on hydration reactions. (3) The numerical simulation shows that when the maximum shear stress reached 3437 kPa, cracks began appearing on the specimen, consistent with the experimental results. Meanwhile, the numerical simulation results reveal the crack propagation pattern in the specimens, showing that the stress at crack initiation is lower than the peak stress.

Light pozzolanic soft clay has high compressibility and relatively poor structural stability, which limits its direct application in subgrade and foundation engineering. This study develops a sustainable combined stabilization–reinforcement system for lightweight volcanic ash soil using a steel slag–fly ash-based composite cementitious binder (GS) and recycled polyester fiber (RPF). Unconfined compressive strength tests, unconsolidated undrained triaxial tests, and microstructural characterization were conducted to evaluate the mechanical behavior and microstructural features of the treated soil. The results show that the GS binder markedly increased the soil strength, whereas RPF mainly improved specimen integrity and the post-peak response. Based on the single-additive and orthogonal test results, 24% GS, 0.6% RPF, and 9 mm fibers were identified as an appropriate mixture within the investigated factor levels for further mechanical evaluation. With the increase in confining pressure, the stress–strain response changes from strain softening to strain hardening. Scanning electron microscopy (SEM) observations showed fine particulate material at local particle-contact and fiber–matrix regions, while energy-dispersive X-ray spectroscopy (EDS) analysis identified a representative Ca-rich microregion containing Si and Al. The combined use of GS binder and RPF effectively improved the strength and deformation resistance of lightweight volcanic ash soft clay.