Synergistic effects of cement–silica fume composite on expansive soil stabilization: Mechanisms, microstructure, and durability

• Cement-silica fume composites effectively stabilize expansive soils. • Stabilized soil retains 84.4% strength after wet-dry cycles. • Swelling and shrinkage rates are kept below 1.1% in cycles. • Free swelling rate reduced to below 14% with composite blends. • Synergistic reaction forms a denser gel network in the soil. This study systematically investigates the stabilization of expansive soils using cement-silica fume composites, focusing on the role of calcium silicate hydrate (C-S-H) gel formation in enhancing soil durability under wetting-drying cycles. Expansive soil collected from Gaochun District, Nanjing, was stabilized through an experimental program incorporating single-component treatments (cement or silica fume at 1%, 3%, and 5%) and composite mixtures with cement-to-silica fume ratios of 1:1 (C1S1) and 1:3 (C1S3). The results demonstrate that while cement alone is most effective for initial plasticity reduction, the composite system provides an optimized balance of properties, excelling in compaction and long-term durability. Specifically, the 5% C1S1 mixture achieves an optimal plasticity index of 12.5, maximum dry density of 1.68 g/cm³, and effectively reduces the free swelling rate to 13.80% after 28 days. Under cyclic wetting-drying conditions, the 3% C1S1 mixture exhibits exceptional durability, retaining 84.4% of its initial unconfined compressive strength and over 70% cohesion retention, with controlled absolute swelling and shrinkage rates below 1.1%. Microstructural characterization indicates that this enhanced performance is attributed to the formation of a synergistic gel network: cement hydration provides structural support, while silica fume accelerates hydration kinetics and generates additional C-S-H gel via pozzolanic reactions, creating a refined cementitious matrix with optimized pore structure (total pore volume of 0.166 ml/g). These comprehensive findings demonstrate the crucial role of gel formation in stabilizing expansive soils, offering superior environmental durability and promoting economic feasibility and sustainability through cement-silica fume composite stabilization at moderate dosages (3–5%).