Recycling Waste Concrete into Alkali-Activated Geo-Binders for Sand Stabilization

This study examines the potential of waste concrete powder (WCP) to act as an alkali-activated binder (AAB) for environmentally responsible soil stabilization. Sodium hydroxide (NaOH) and sodium silicate (SS) served as alkaline activators, and the effects of WCP dosage, activator formulation, and the water-to-solution ratio (W/S) on unconfined compressive strength (UCS) were investigated through controlled laboratory testing. The stabilized specimens achieved UCS values of up to 3.1 MPa under ambient curing. Direct shear results demonstrated that even the weakest mixture improved the friction angle and cohesion by 24.7% and nearly threefold relative to conventional compaction. Microstructural analyses (SEM, EDS, XRD) revealed the development of calcium-aluminosilicate-hydrate (C–A–S–H) and sodium-aluminosilicate-hydrate (N–A–S–H) gels, confirming the formation of a chemically bonded matrix. A machine-learning model was also constructed to predict UCS from key experimental parameters, achieving a high coefficient of determination (R² = 0.99). Furthermore, life cycle assessment (LCA) showed that stabilization using WCP-based AAB emits 46.91 kg CO₂-equivalent per ton of treated soil, compared to 58.08 kg for ordinary Portland cement (OPC). Overall, the findings demonstrate that repurposing construction and demolition waste as an alkali-activated binder provides a viable pathway for sustainable soil improvement, aligning with circular economy objectives and reducing environmental impacts in geotechnical applications.