Comparative evaluation of ramie, hemp, and hollow fibers on the mechanical properties of MICP-treated calcareous sand

A combination of fiber reinforcement and microbially induced carbonate precipitation (MICP) has emerged as a highly promising, sustainable method for improving the mechanical properties of calcareous sands. This study investigates the effect of three fiber types, hemp (HF), ramie (RF), and hollow fiber (HoF), on the mechanical properties of MICP-treated calcareous sand. Cylindrical samples with fiber content of 0.2, 0.3, 0.4, 0.5, and 0.6% by weight of sand were prepared and tested for water absorption, bacterial retention, unconfined compressive strength (UCS), and calcium carbonate content. The results indicate that MICP effectively cemented the calcareous sand, and the inclusion of fibers further enhanced its strength and ductility, while mitigating brittle failure. The bacterial retention rate of hollow fiber was the highest at 30.5% (114.8% increase over the control), followed by ramie fiber at 26.0% (83.1% increase), and hemp fiber at 21.0% (47.9% increase). This trend was attributed to the microporous structure of the hollow fiber, which provides abundant attachment sites for bacterial adhesion. The optimum UCS was achieved at 0.3% fiber content for hemp and ramie fibers and 0.5% for hollow fibers. At these contents, the UCS increased by 12.82, 46.83, and 59.54%, respectively, compared to the control. The change in UCS closely followed the trend of calcium carbonate precipitation, with the hollow fiber reinforced samples showing the highest CaCO₃ content due to improved bacterial retention and nucleation. Scanning electron microscopy (SEM) observations revealed different interfacial bonding mechanisms between the materials. The hollow and ramie fibers formed dense calcite coatings, which bonded strongly to the sand matrix, whereas the hemp fibers showed limited calcite formation, resulting in poor interfacial strength.