Mechanical Properties and Leaching Characteristics of BF-MICP Solidified/Stabilized Ion-Type Rare Earth Tailings

Ion-type rare earth tailings are mechanically weak and may release Pb and Zn, posing both geotechnical and environmental risks. Basalt-fiber-reinforced microbially induced carbonate precipitation (BF-MICP) was investigated as a combined solidification/stabilization treatment for these tailings. By integrating peak and post-peak mechanical responses, heavy-metal leaching, the spatial distribution of calcium carbonate (CaCO₃), and microstructural characterization, this study distinguishes the respective contributions of microbial mineralization and fiber reinforcement. Tailings specimens were treated with basalt fiber contents ranging from 0 to 0.8% and evaluated using unconfined compression tests, leaching tests, CaCO₃ measurements, X-ray diffraction, Fourier-transform infrared spectroscopy, and scanning electron microscopy with energy-dispersive spectroscopy. The unconfined compressive strength first increased and then decreased with increasing fiber content, reaching 1.72 MPa at 0.4% fiber, approximately 90% higher than that of the MICP-only group. At the same fiber content, compressive total energy absorption increased from approximately 18 to 68 kJ·m^-3, indicating a marked improvement in post-peak toughness. BF-MICP treatment increased the CaCO₃ content to approximately 2-3 times that of untreated tailings, although the deposits remained more abundant in the outer region than in the core, and the total CaCO₃ content varied little with fiber dosage. The leached concentrations of Pb and Zn decreased by 79-81% and 81-84%, respectively, with no clear additional reduction as the fiber dosage increased. Microstructural analyses showed that calcite-dominated deposits connected tailing particles and fiber surfaces. These results indicate that MICP primarily governed mineral cementation and heavy-metal immobilization, whereas basalt fibers mainly improved load transfer, crack bridging, and post-peak structural integrity. A fiber content of 0.3-0.4% provided the best overall balance between mechanical performance and leaching control.