Mechanical stabilization of high-plasticity clay subgrades using CDW-geopolymer composites for differential settlement mitigation in highway widening: Laboratory evaluation and numerical modeling

• CDW-geopolymer composite boosts clay strength by 750 %, enhancing subgrade stability. • CDW provides skeletal reinforcement while geopolymers generate cementitious gels, collectively densifying the soil matrix. • Optimized composite reduces differential settlement in highway widening projects, achieving uniform stress redistribution. Highway widening projects face persistent challenges from the low bearing capacity of high-plasticity clay and differential settlement at new-old subgrade interfaces. Here, we present a sustainable solution leveraging the synergistic effects of construction and demolition waste (CDW) and geopolymer to transform high-plasticity clay into high-performance subgrade fill. Through systematic laboratory experiments and finite difference modeling, we demonstrate that a composite stabilization strategy (25 % CDW and 15 % geopolymer) enhances unconfined compressive strength, elastic modulus, and compression modulus to 3.71 MPa, 286.1 MPa, and 78.62 MPa, respectively—representing 7.5- to 8.6-fold increases over untreated clay. Mechanistic analysis reveals that CDW provides granular skeleton reinforcement, while geopolymer-induced pozzolanic reactions generate cementitious gels, collectively densifying the soil matrix. Numerical simulations confirm that the optimized composite reduces differential settlement in the newly widened subgrade region compared to conventional fills, achieving uniform stress redistribution. This approach not only addresses a critical geotechnical bottleneck but also repurposes CDW, reducing landfill disposal by ∼1.2 tons per kilometer of highway. Our findings establish a scalable, eco-conscious framework for infrastructure resilience, aligning material innovation with circular economy principles in transportation engineering.