Enhancing lime mortar composites with dual surface-treated natural fibers: synergistic effects of alkaline activation and graphene nanocoating

This study investigates the combined effect of alkaline activation and graphene nanocoating on the mechanical response of hydraulic lime mortars reinforced with natural fibers, addressing the limited understanding of dual surface treatments in lime-based composites. Hemp and Spanish broom fibers were treated with sodium hydroxide (NaOH) and subsequently dip-coated with graphene to modify the mechanical response of the resulting composites. Three-point bending and uniaxial compression tests were performed, while fracture energy was evaluated from the post-peak response. Alkali activation produced a marked increase in flexural stiffness, while graphene coating raised stiffness by up to 6–7 times compared to mortars reinforced with untreated fibers, with peak flexural strength remaining nearly unchanged. Fiber reinforcement also led to a substantial increase in fracture energy, indicating more effective crack-bridging and improved post-cracking behavior. Under compression, graphene-coated hemp fiber composites exhibited strength and stiffness increases of about 1.4 times and up to 2.7 times, respectively, whereas Spanish broom fiber composites showed reductions of roughly 1.3–1.5 times, highlighting a clear fiber-dependent mechanical response to surface modification. Overall, the results demonstrate how combined surface treatments govern stiffness, strength, and failure behavior in lime-based composites, supporting the development of tailored and sustainable materials for structural and heritage applications.