Coir-reinforced low-cement soil stabilization: a systematic review of mechanisms, performance, durability, and sustainability

Coir fiber has gained increasing attention as a bio-based reinforcement for low-cement soil stabilization, particularly in problematic soils such as expansive clay, lateritic soil, peat, soft clay, sandy soil, and pavement subgrade materials. This systematic review synthesizes evidence on coir-reinforced low-cement stabilization through a fiber-to-function framework linking coir form, binder interaction, microstructural mechanisms, mechanical performance, durability, and sustainability readiness. A PRISMA-oriented review was conducted using Scopus as the formal database for record identification. The search identified 124 records; 60 full-text articles were assessed, and 33 studies were included in the final synthesis after eligibility screening, quality appraisal, and evidence weighting. The findings show that coir primarily acts as mechanical reinforcement rather than a chemical substitute for cement or lime. Its primary functions include crack bridging, frictional interlocking, tensile restraint, ductility improvement, and residual load transfer. Hybrid coir–binder systems generally provide broader performance benefits than fiber-only systems because binders and waste-derived stabilizers improve matrix bonding, pore filling, cementitious reaction, and densification. Effective coir content commonly ranges from 0.5% to 1.5%, with values near 1% often preferred. However, no universal optimum can be defined because performance depends on soil type, fiber geometry, binder chemistry, curing condition, moisture state, compaction, and target function. Durability remains the main constraint on field adoption because most evidence is still based on laboratory exposure tests. Sustainability claims also remain conditional because lifecycle assessment, techno-economic analysis, leaching evaluation, and field monitoring are limited. This review concludes that coir-reinforced low-cement stabilization should be advanced as a performance-based hybrid strategy, with practical value determined by verified mechanical performance, durability retention, environmental safety, and field applicability.