Bio-inspired strengthening of fiber–matrix interface via ocean-derived MgO expansion agent

Fiber-matrix interfacial transition zone (ITZ) is generally the weakest region in fiber reinforced concrete (FRC). This study aims to tailor the anisotropic fiber–matrix interface toward an asymptotic isotropic state. Self-prestressed concrete was engineered through the synergistic coupling of fibers and expansion agents, in which autogenous matrix expansion under fiber restraint generated active confinement during hardening and passive confinement under external loading. A novel process was developed to recover the MgO expansion agent from desalination brine and alkaline wastewater from ultra-high-performance concrete (UHPC) curing. Different dosages of MgO expansion agent were incorporated into polyvinyl alcohol fiber reinforced magnesium silicate hydrates (PVA-MSH) matrix. Single fiber pull-out tests revealed a 9.4–33.0% increase in fiber bond strength at 4–8% expansion agent. Quantitative nanoindentation analysis through sophisticated techniques at microscale demonstrated that incorporating waste-derived MgO expansion agent enhanced the fiber–matrix ITZ by enhancing local stiffness and hardness, with the anisotropic index ( AI ) reduced by 50% at 4% expansion agent. This study advances low-cost, sustainable, high-performance concrete design with waste-derived materials.