Stabilization of carbonate sand with filamentous fungi: Optimization and mechanical evaluation

Carbonate sand, commonly used as fill material in island and reef engineering, is characterized by high porosity, angular grains, and brittle structure, which makes it vulnerable to erosion and instability in tropical and subtropical environments. This study explored a filamentous fungi-based stabilization method for carbonate sand. Mycelium growth conditions were optimized through single-factor tests and response surface methodology. Triaxial shear, disintegration, and leachate tests were conducted to evaluate mechanical behavior, water stability, and environmental impact. Results showed that temperature primarily influenced early-stage growth, while nutrient concentration played a greater role in later stages; water content exerted a consistent effect throughout. A specific combination of 25 °C, 10% water content, and 10 mg/L nutrient solution was identified as optimal, leading to dense mycelial networks and high reinforcement efficacy. Fungal reinforcement significantly suppressed excess pore water pressure and enhanced the mechanical response of the sand, increasing peak strength by up to 79% and initial stiffness by up to 369%, with the largest gains observed at low confining pressures. Microscopic analysis showed that the fungal hyphae formed an interconnected three-dimensional network and chemically interacted with mineral surfaces, strengthening interparticle cohesion. The treated sand maintained structural integrity after 14 d of immersion, with a disintegration ratio of zero. Unlike microbial-induced calcite precipitation or biopolymer treatments, which relies on brittle mineral precipitates or viscous gels, fungal mycelia form a flexible, fiber-like network within the sand matrix. This mycelial skeleton reduces the inherent brittleness of carbonate sand and sustains a more stable post-peak response, offering a mechanistically distinct bio-based reinforcement pathway compared with existing techniques.