Strength formation mechanism of mycelium bio-composites lightweight soil

Mycelium-wheat bran-sand composite lightweight soil (MBLS) is a lightweight geotechnical material composed of fungal mycelium, wheat bran, and sand. It has attracted significant attention due to its lightweight and environmentally friendly properties. At present, there have been many studies on its macroscopic mechanical properties, while relatively few studies have been conducted on its microscopic characteristics. This paper investigates the strength formation mechanism of MBLS using X-ray diffraction, Fourier-transform infrared spectroscopy, scanning electron microscopy, and X-ray energy spectroscopy. The results indicate that fungal colonization did not generate new mineral species, with only aragonite and calcite present. However, changes are observed in mineral content, crystallite size, and crystallinity, with distinct patterns of change for aragonite and calcite. During colonization, oxidation reactions occurr between the fungus and the particles, resulting in significant alterations to the functional groups on the sample surfaces. This process leads to the formation of polysaccharides, ester compounds, and inorganic products, which enhances the hydrophobicity of the particles. The mycelium adheres to and penetrates the pores and cracks on the surface of calcareous sand, dissolving existing calcium carbonate minerals and forming aragonite-type calcium carbonate. The strength of the MBLS specimens resultes from the combined effects of biochemical and biophysical processes. Polar groups from the fungal cell walls and extracellular polymers initially bond with the negatively charged surfaces of the particles and metal cations through hydrogen bonding, adsorption, co-precipitation, and crystallization, forming microaggregates. The growth of the mycelium applies pressure and traction on the particles, enlarging the aggregates. The mycelium then linkes the aggregates and fills the voids, ultimately forming the mycelium-sand composite lightweight soil. This study provides insights for the development and application of green and environmentally friendly lightweight geotechnical materials.