This review explores the efficacy of nanomaterials as sustainable alternatives to conventional subgrade soil stabilizers, driven by advancements in nanotechnology over the past two decades. It examines the impact of various nanoparticles on key soil engineering properties, including compaction, California Bearing Ratio (CBR), compressive and shear strength, and swelling-shrinking behaviour. The study also analyzes nanoparticle-soil interactions at micro and nano scales, highlighting their role as primary additives or secondary activators in enhancing soil strength and stiffness. Ecological and geo-environmental concerns associated with nanoparticle use are critically reviewed, alongside potential future research avenues. Findings indicate that nanoparticles significantly improve subgrade mechanical properties, reduce settlement, and absorb heavy metal pollution while maintaining pH balance. Their filling capacity and gel formation enhance bonds, adhesion, and friction angles, with effectiveness varying by nanoparticle type, dosage, and soil characteristics. This study offers valuable insights into eco-friendly nanoparticle applications in road engineering.

Problematic ground conditions constituted by weak or expansive clays are commonly encountered in con-struction projects and require some form of chemical treatment such as lime and cement to re-engineer their performance. However, in the light of the adverse effects of these traditional additives on the climate, alternative eco-friendlier materials are now sourced. In the current study, the viability of calcinated wastepaper sludge ash geopolymer in enhancing the engineering behaviour of a problematic site condition is evaluated. A highly expansive clay (HEC) constituted with a blend of kaolinite and bentonite clays is treated with calcinated wastepaper sludge ash (CPSA) geopolymer. Activation of the precursor is actualised at room temperature using a combination of NaOH and Na2SiO3 at various activator to soil + binder ratios (AL/P), and molarity (M). The mechanical, microstructural, and mineralogical characteristics of the treated clay were investigated through unconfined compressive strength (UCS), swell, water absorption, SEM, and EDX analysis. The performance of the stabilised samples was then compared with the requirements for road subgrade and subbase materials and that of OPC and lime-GGBS treatment. The results showed that CPSA-geopolymer enhanced the engineering properties of the treated clay better than traditional binders (OPC and Iime-GGBS). UCS improvement of 220 % was observed in the CPSA-stabilised soil over that of OPC-treated ones, while the swell potential and water absorption were drastically reduced by over 95 and 97 % respectively after 28-day soaking. The SEM and EDX results showed improved crystallisation of earth-metal-based cementitious flakes (NASH) with increasing CPSA, molarity, and AL/P ratios, which enhanced the inter-particle bonds with simultaneous reduction in porosity. The modified characteristics of the stabilised materials meet the requirements for pavement subgrades. Further, the equivalent carbon emission (CO2-e) from the stabilised materials were also evaluated and compared with that of traditional binders. The results also showed that CPSA-geopolymer had lower CO2-e at higher subgrade strengths than OPC, making it more eco-friendly. Therefore, wastepaper sludge, a common landfill waste from paper recycling is a viable geopolymer precursor that could be utilised in enhancing the engineering properties of subgrade and sub-base materials for road and foundation construction.