MICP-treated soft mudstone in open-pit coal mines combined with fly ash and CaO

Mudstone found in open-pit coal mines is prone to softening, expansion, and disintegration when exposed to water, leading to deterioration of its mechanical properties and causing slope instability. To address this issue, this study introduced a novel slope stabilization method that integrates coal-based solid waste (fly ash) with microbially induced carbonate precipitation. The effects of fly ash (FA) and CaO (FC) and Bacillus subtilis (BS) contents on the cohesion ( c ), internal friction angle ( φ ), unconfined compressive strength ( q u ), elastic modulus ( E ), Poisson's ratio ( μ ), disintegration resistance, and microstructural characteristics of mudstone were evaluated through unconfined compressive strength tests, direct shear tests, disintegration test, X-ray diffraction, scanning electron microscopy, and Fourier transform infrared spectroscopy. The results showed that under optimal curing conditions, the growth rates of c , φ , q u , E , and μ of the treated mudstone sample improved by +251.46%, +53.86%, +134.1%, +66.3%, and −65.61%, respectively, compared with those of plain mudstone samples. Moreover, its resistance to disintegration upon contact with water could be extended from 72 h to 10 d. The curing mechanism primarily involved the activation of FA into calcium silicate hydrate (C-S-H), calcium aluminate hydrate (C-A-H), Calcium Aluminum Silicate Hydrate (C-A-S-H), and microbially induced calcite (CaCO 3 ) precipitates. Under the action of an appropriate amount of curing agent, FC can provide an alkaline environment and nucleation sites for BS, making the calcium carbonate distribution more uniform. BS accelerates the catalysis of FC to form gelation products. Meanwhile, due to the continuous consumption of Ca 2+ and OH − , the equilibrium of the CaO dissolution shifts toward generating more Ca(OH) 2 , achieving a “three-way promotion” effect. On the contrary, these factors will inhibit each other due to the different rates of reaction output. The findings offer a viable solution for slope stability in open-pit coal mines and hold significance for safe mining, environmental protection, and sustainable use of coal-based solid waste.