Study on mechanical properties and microscopic mechanism of expansive soil improved by eggshell ash-silica fume composite

To reduce the high carbon emissions of traditional modifiers (cement/lime) and address gaps in ESA–SF studies—unclear dosage-age-performance relations and insufficient micro-mechanism and durability analysis. Weak expansive soil from Huainan City, China, was stabilized using ESA and SF—both obtained from solid wastes. Macroscopic tests (free swelling ratio, unconfined compressive strength, direct shear strength) and microscopic analyses (XRD, SEM, dry–wet cycling) were conducted to evaluate the effects of ESA content (0 %, 3 %, 6 %, 9 %), SF content (0 %, 4 %, 8 %, 12 %), and curing age (1 d, 7 d, 14 d, 28 d). The results demonstrated that the composite of 9 % ESA + 8 % SF effectively reduced the free swelling ratio to below 20 % after 28 days, converting the soil to non-expansive. An unconfined compressive strength of 1.90 MPa was achieved at 28 days, with the stress–strain curve showing an excellent fit. Under 300 kPa vertical pressure, the shear strength increased to 279.49 kPa, accompanied by a 292.15 % rise in cohesion and a 43.6 % increase in the internal friction angle compared to plain soil. Microstructural analysis indicated that Ca2 + from ESA compressed the double layer, while SF reacted with Ca(OH) 2 to form C-S-H gels. After seven dry-wet cycles, the stabilized soil retained 40 % of its strength, whereas plain soil collapsed after only three cycles. Overall, the optimal 9 % ESA + 8 % SF formulation effectively mitigates expansion-contraction hazards, enhances long-term soil stability, and exemplifies waste-to-resource utilization consistent with carbon neutrality objectives, offering an economically viable, environmentally sustainable, and technically robust solution for expansive soil engineering applications.