Complementary Improvement in Dispersivity Resistance of Ionic Rare Earth Tailings Soil Treated by RSF-MICP

The dispersive behavior of ionic rare earth tailings restricts their potential reuse as engineering fill materials due to particle detachment and hydraulic erosion. This study investigated the effectiveness of rice straw fiber (RSF)-reinforced microbially induced calcium carbonate precipitation (MICP) for improving the dispersivity resistance of ionic rare earth tailings soil. RSF with a length of 10–15 mm, diameter of 50–150 μm, and an aspect ratio of approximately 67–300 was incorporated at dosages of 0.25%, 0.50%, 0.75%, and 1.00%. MICP treatment was conducted using Sporosarcina pasteurii with a controlled bacterial density of OD600 = 1.20 ± 0.05. The improvement effect was evaluated through pinhole, crumb, and double hydrometer tests, together with calcium carbonate content measurement and microstructural analyses. The untreated tailings were classified as D2 in both the pinhole and crumb tests. Following MICP treatment, the pinhole-test classification improved to ND2, while the crumb-test classification improved to D3. The RSF-MICP-treated specimens further reached ND2 in the crumb test, indicating enhanced resistance to water-induced disintegration. Meanwhile, the dispersion ratio obtained from the double hydrometer test decreased from 64.3% to 46.8%, indicating enhanced particle aggregation and reduced susceptibility to water-induced detachment. Microstructural analyses revealed that MICP-induced CaCO3 precipitation bonded soil particles, whereas RSF provided additional physical bridging and structural restraint. Among the investigated treatment conditions, the combination of 0.75% RSF and MICP exhibited the most favorable overall improvement in dispersivity resistance, suggesting its potential for the stabilization and engineering reuse of ionic rare earth tailings.