Stabilization mechanism and strength evolution of expansive soil stabilized by calcium carbide residue-rice husk ash binder: mineralogical and microstructural evidence

Expansive soils, featuring severe swelling-shrinkage behavior and low bearing capacity, pose persistent challenges to geotechnical infrastructure. As a promising alternative to lime and cement stabilizers, calcium carbide residue-rice husk ash (CCR-RHA) blends have been widely investigated, but existing studies focus mostly on macroscopic parameter optimization and performance evaluation. The long-term mineral evolution pathway, dynamic carbonation process, and micro-macro coupling mechanism of stabilized expansive soils remain poorly understood, hindering further engineering application of this green technology. To address this gap, a multi-scale systematic study is conducted on medium-swelling expansive soil stabilized by CCR-RHA binder at the optimal mass ratio of 35:65. UCS tests, XRD and SEM-EDS are applied across seven dosages (0%–30%) and five curing ages (0–90 d) to characterize mechanical properties, mineral assemblage evolution and microstructural development. The results demonstrate that UCS increases monotonically with binder dosage up to an optimal value of 25%, beyond which a marginal decline occurs. The peak 90-day UCS reaches 8.94 MPa, which significantly outperforms the strength level of traditional lime-stabilized expansive soils. Mineralogical analysis reveals a clear time-resolved reaction sequence: portlandite is rapidly consumed within the first 7 days, swelling clay minerals undergo progressive dissolution under alkaline attack, gismondine-like calcium aluminosilicate hydrate crystallizes at 28 days, and multiple hydrated phases including ettringite and strätlingite form at 90 days, verifying the sustained long-term pozzolanic activity of the system. The most prominent original finding is the non-monotonic shift of the calcite composite diffraction peak, which directly verifies a three-stage calcium carbonate evolution trajectory: initial metastable carbonate phases, intermediate gel-carbonate composite phases, and final well-crystallized stable calcite. Qualitative microstructural observation further shows that the soil matrix transforms from an ordered face-to-face stacked clay platelet structure into a flocculated gel-cemented framework, and eventually develops into a dense three-dimensional interlocking skeleton composed of amorphous gel and multi-morphology crystalline phases. On the basis of systematic experimental evidence, a four-coupled stabilization mechanism integrating hydration, ion exchange-flocculation, pozzolanic reaction, and carbonation is established. The progressive conversion of C-S-H gel to crystalline phases is identified as the core driver for continuous long-term strength growth. These findings provide fundamental micro-mechanical support for large-scale engineering application of CCR-RHA binders, and offer new conceptual insights into the behavior of multi-component waste-based soil stabilization systems.