Non-Cement-Based Soil Stabilization Material: A Review of Biochar, Nanocellulose, and Recycled Polyethylene Terephthalate (PET) Powder Composite for Sustainable Geotechnics.

Soil stabilizers using conventional cement and lime binders incur high environmental costs owing to CO2 emissions associated with their excavation, production, and processing. This has motivated research on low-carbon, waste-derived alternatives. The review shows that: biochar increases unconfined compressive strength (UCS) by 15-40% with a 2-5% dosage through pore filling and particle binding; nanocellulose promotes soil cohesion by 25-60% through fibrous network development and tensile bridging; recycled PET powder at 5-10% increases shear strength by 20-35% promoting mechanical interlocking, increasing stiffness, crack resistance and durability. Biochar provides direct carbon sequestration with a carbon transfer capacity of up to 2.5 tons CO2-eq/ton. Recycled PET introduces waste valorization, with the potential to divert millions of tons of annual PET waste, while nanocellulose provides indirect carbon savings by avoiding emissions from cement and lime replacement. This review's objectives are as follows: providing a comprehensive comparison of biochar, nanocellulose, and PET powder as promising non-cement composite stabilizers; identifying optimal dosage ranges and stabilization mechanisms for each material across different soil types; and outlining knowledge gaps and future research directions in sustainable geotechnical practices. The review assessed the individual and synergistic effects of the additives on critical geotechnical properties, including unconfined compressive strength (UCS), California bearing ratio (CBR), resilient resistance, swelling resistance, and the durability of the treated soil. Findings provide actionable guidance for practitioners seeking to reduce construction carbon footprints while maintaining geotechnical performance standards. Research gaps were identified, and future directions for integrating high-performance, low-carbon soil composites into sustainable construction solutions are proposed.