Recycling waste concrete into alkali-activated geo-binders for sand stabilization

This study investigates the feasibility of using waste concrete powder (WCP) as an alkali-activated binder (AAB) for sustainable soil stabilization. Sodium hydroxide (NaOH) and sodium silicate (SS) were used as activators, and the influence of WCP content, activator composition, and water-to-solution ratio (W/S) on the unconfined compressive strength (UCS) was evaluated through laboratory tests. The results showed that the UCS of the stabilized soil reached up to 3.1 MPa under ambient conditions. The direct shear test revealed that even the lowest-performing mixture design increased both the friction angle and cohesion compared to the conventional compaction method. Microstructural characterization using FE-SEM, EDS, and XRD was performed, and the observations confirmed the formation of calcium-aluminosilicate-hydrate (C-A-S-H) and sodium-aluminosilicate-hydrate (N-A-S-H) gels that contribute to soil strength enhancement. A gradient boosting regression (GBR) machine learning model was developed to predict UCS based on experimental variables, showing high accuracy (R² = 0.95). Additionally, life cycle assessment (LCA) demonstrated that WCP-based AAB and ordinary portland cement (OPC)-based stabilization emit 46.91 and 58.08 kg CO₂-equivalent, respectively, per m³ of stabilized soil. This work highlights the potential of transforming construction and demolition waste into sustainable construction materials, promoting circular economy principles, and reducing environmental impacts in geotechnical engineering.