Enhanced corrosion resistance and mechanical performance of reinforced concrete using Rhizopus-derived phospholipid biosurfactant

The corrosion of steel reinforcement remains a critical challenge affecting the durability and service life of reinforced concrete structures, particularly under aggressive environmental conditions. Although conventional chemical corrosion inhibitors are widely applied, their associated environmental and health risks necessitate the development of sustainable alternatives. This study investigates the potential of fungal-derived biosurfactants as eco-friendly corrosion inhibitors and multifunctional admixtures for reinforced concrete. Indigenous biosurfactant-producing fungi were isolated from contaminated soils, among which Rhizopus sp. demonstrated the highest corrosion inhibition efficiency. The biosurfactant produced via submerged-batch fermentation using WFO as sole carbon source was characterised as phospholipid. The corrosion inhibition performance of the biosurfactant on mild steel reinforcement was evaluated using the migrating corrosion method, supported by weight loss measurements, electrical resistivity testing, and electrochemical impedance spectroscopy. Surface morphology and elemental composition of treated steel specimens were examined using scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy, while tensile tests assessed mechanical performance retention of the steel bars. Results indicate that the phospholipid biosurfactant exhibited superior corrosion inhibition performance compared to the commercial surfactant Tween 80, achieving a maximum inhibition efficiency of 72.08% at a concentration of 20% (v/v) and reducing the corrosion rate to 0.0227 mm/year. When incorporated into fresh concrete as a water-replacement admixture, the biosurfactant significantly enhanced workability. Optimal performance was observed at a 10% (w/w) replacement level, resulting in increases of 13.8% in compressive strength (fc) and 8.7% in flexural strength (fs). Furthermore, pull-out testing confirmed a 9% improvement in steel–concrete bond strength after 120 days of curing. The findings demonstrate that the amphiphilic phospholipid biosurfactant derived from Rhizopus sp. functions as a mixed anodic–cathodic corrosion inhibitor by forming a protective barrier on steel surfaces, while simultaneously improving the mechanical and interfacial properties of reinforced concrete. Overall, this study provides new scientific insight into the dual-function role of Rhizopus-derived phospholipid biosurfactants and establishes their practical potential as sustainable green admixtures for corrosion mitigation and performance enhancement in reinforced concrete, contributing to the advancement of durable and environmentally responsible construction materials.