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.