Beyond Soil Mechanics: How Soil Chemistry Drives Microbial Calcite Precipitation in Clayey Soils

This study investigates the influence of two fundamental soil properties—Cation Exchange Capacity (CEC) and Specific Surface Area (SSA)—on the efficiency of Microbially Induced Calcite Precipitation (MICP) in mitigating the swelling potential of expansive clayey soils. While previous research has explored biological and environmental factors affecting MICP, the role of soil physico-chemical properties, particularly CEC and SSA, has not been comprehensively studied. Six clayey soils from diverse geographic locations in the United States were collected, classified, and treated with MICP solutions. The impact of MICP treatment was assessed through one-dimensional (1D) swelling tests, carbonate content determination, and microstructural analyses using Scanning Electron Microscopy (SEM) and X-ray Diffraction (XRD). The results demonstrated that MICP treatment significantly reduced the swelling potential of all tested soils, with the most substantial reductions observed in the MA and NTP soils, which have the highest clay content (96% and 73%, respectively). The analysis of CaCO₃ precipitation revealed a strong positive correlation between SSA and CEC with the amount of biogenic calcium carbonate formed, suggesting that these soil properties could serve as reliable indicators for predicting MICP efficiency. Additionally, soils with higher initial inorganic carbonate (SIC) content exhibited lower CaCO₃ precipitation post-MICP treatment, likely due to a pre-existing carbonate film that reduced available reaction sites. The study further revealed a distinct influence of Soil Organic Carbon (SOC) on the efficiency of MICP, with increased SOC levels correlating with decreased CaCO₃ precipitation, indicating competitive interactions between organic matter and carbonate formation. SEM and XRD analyses confirmed that post-MICP CaCO₃ predominantly precipitated in the form of agglomerated rhombohedral calcite crystals, with occasional vaterite formations. In high-clay soils, precipitated CaCO₃ manifested as a discontinuous film coating clay particles, a morphology that likely contributes to enhanced soil stabilization. These findings highlight the critical role of soil chemical and physical properties in optimizing MICP for expansive soil stabilization and suggest that SSA and CEC should be considered in pre-treatment assessments for field applications.