Eco-friendly Flax (Linum usitatissimum) Fiber/Epoxy Resin Composites for Sludge Reinforcement: Quicklime Modification, Synergistic Effect and Mechanical Performance

Sludge, with high compressibility and low strength, often fails to meet bearing capacity and deformation resistance requirements in engineering. Natural fibers have sludge reinforcement potential, yet their corrosion susceptibility and weak interfacial bonding impair service performance. In this study, a composite system of surface-modified flax fibers and epoxy resin was employed for sludge reinforcement. Via tensile tests, unconfined compressive strength (UCS) test, and consolidated undrained triaxial compression (CUTC) test, the effects of fiber modification methods, fiber content, and epoxy resin content on the mechanical properties of sludge were investigated. Additionally, the synergistic mechanism of fibers and resin on sludge was revealed via microscopic characterization. Test results showed epoxy resin significantly inhibited fiber corrosion. Quicklime-modified flax fibers enhanced interfacial bonding with epoxy, increasing composite fiber tensile strength by 16.2% and elongation at break by 3.35 times. For sludge reinforcement, the optimum epoxy and fiber dosage were 12% and 0.6%, respectively. Under this condition, the 7d UCS reached 824.54 kPa (> 5 times untreated sludge), ductility improved to 7%, and cohesion (124.43 kPa) and internal friction angle (39.83°) peaked simultaneously. Scanning electron microscopy (SEM) and nuclear magnetic resonance (NMR) test revealed resin formed dense bonds to reduce porosity, fibers bridged for stress transfer, and their synergy achieved brittle-to-ductile transition with enhanced long-term stability.