Low-carbon solid waste materials for railway sub-ballast layers:a review of mechanical performance,sustainability and regulatory gapsOA

Purpose-This paper presents a comprehensive systematic review of low-carbon solid waste materials applied in railway sub-ballast layers,aiming to critically assess their mechanical performance,durability,environmental benefits and regulatory readiness.The study addresses the growing need to decarbonize rail infrastructure while reducing dependence on natural aggregates,positioning sub-ballast as a strategic layer for circular economy implementation in ballasted track systems.Design/methodology/approach-A PRISMA-based systematic review methodology was adopted to identify,screen and analyses peer-reviewed studies published between 2000 and 2025.The final database comprises experimental,numerical and field investigations covering mining residues,steel slags,construction and demolition waste,rubberized composites,alkali-activated materials and other industrial by-products applied to railway sub-ballast.Mechanical behavior under cyclic loading,resilient modulus,permanent deformation,hydraulic performance,durability and environmental indicators were extracted and synthesized.In parallel,an international regulatory analysis was conducted to compare sub-ballast specifications across Europe,North America,Asia-Pacific and Brazil,enabling identification of performance-regulation gaps and barriers to implementation.Findings-The review demonstrates that several low-carbon waste-derived materials exhibit mechanical performance comparable to or exceeding that of conventional granular sub-ballast,particularly in terms of stiffness retention,resistance to permanent deformation and degradation under repeated loading.Steel slags,recycled concrete aggregates,slate waste and rubber-modified blends consistently show favorable resilient behavior and enhanced damping capacity,while certain mining residues and alkali-activated granular systems present promising strength and durability characteristics.Life-cycle evidence indicates substantial reductions in embodied carbon and natural aggregate consumption when these materials are adopted.However,current railway standards remain largely prescriptive and index-based,rarely incorporating cyclic performance criteria or carbon metrics,creating a structural disconnect between scientific evidence and regulatory acceptance.This gap significantly limits large-scale implementation despite growing technical maturity.Originality/value-This study provides the first integrated synthesis focused exclusively on low-carbon solid waste materials for railway sub-ballast,combining mechanical performance,environmental assessment and international regulatory comparison within a unified analytical framework.By explicitly linking laboratory evidence to policy and standardization challenges,the paper advances performance-based pathways for sustainable railway substructure design.The findings offer actionable guidance for infrastructure managers,regulators and researchers seeking to accelerate the transition toward circular,low-carbon rail systems through sub-ballast innovation.