Experimental study on microbial-induced mineral phase transformation and its improvement of swell-shrink properties of expansive soil

This study evaluates the effects of microbial processes in improving the swelling and shrinking properties of expansive soil. Elemental microanalysis, X-ray diffraction, Fourier-transform infrared spectroscopy and soil swelling-shrinkage tests were conducted to explore the changes in composition, content and structure of the mineral aggregate of expansive soil in bio-mediated treatment process, as well as the effects of microbial-induced mineral phase transformations on the swelling and shrinking characteristics of the soil. The experimental results indicate that cations Si⁴⁺, Al³⁺, K⁺, and Mg²⁺ in the microbial-soil reaction liquid continuously leached out over time. Notably, the concentration of K⁺ exhibited an initial increase followed by a decrease, reaching a peak at 14 days. The Si-O, Al-O and -OH bonds within montmorillonite crystals were also affected. It was suggested that K⁺ ions entered the interlayers of montmorillonite crystals, disrupting their structures. After 14 days of microbial activity, an illite-smectite mixed-layer peak appeared at 2<italic>θ</italic> = 9.882° in the expansive soil samples, and an illite peak was observed at 2<italic>θ</italic> = 48.07° after 28 days. Calculations using the K-value method proved a decrease in montmorillonite content and an increase in illite content. The free swelling rate and load swelling rate of the expansive soil decreased by 69.01% and 24.04%, respectively; the linear shrinkage rate and volumetric shrinkage rate decreased by 32.9% and 30.98%, respectively. The study reveals that <italic>Paenibacillus mucilaginosus</italic> effectively promotes the transformation of montmorillonite to illite and improves the swelling and shrinking properties of expansive soil, which provides a novel approach towards modifying some engineering properties of soils.