Study on the influence of magnesium/calcium ratios on bio-cemented sandy soils

Microbially induced carbonate precipitation (MICP) has been extensively studied as a promising technique for soil stabilization. However, the heterogeneity in bio-cementation always hampers the application of MICP. Seawater contains more magnesium ions (Mg 2+ ) than calcium ions (Ca 2+ ) and using magnesium for bio-cementation might be more cost-effective in coastal cities. In this study, the microbially induced magnesium and calcium precipitation (MIMCP) treatment was proposed to solve the problem of heterogeneity, where the urea-magnesium-calcium solution was used as the cementation solution with various Mg 2+ /Ca 2+ ratios. The influences of Mg 2+ /Ca 2+ ratios on pH, bio-flocculation, and chemical conversion efficiency were studied. The sand bio-cementation tests were subsequently conducted to compare the treatment effects with different Mg 2+ /Ca 2+ ratios. Results showed that the increase in Mg 2+ /Ca 2+ ratios resulted in lower pH levels and smaller percentages of bio-flocculation. The higher Mg 2+ /Ca 2+ ratio also provided a longer lag period, regardless of biomass concentrations; however, the chemical conversion efficiency decreased. Furthermore, the increased Mg 2+ /Ca 2+ ratios resulted in a small difference in UCS and contents of precipitation at different parts of bio-cemented soils, achieving better homogeneity in bio-cementation. However, the strength significantly decreased at an Mg 2+ concentration over 0.8 M due to much smaller contents of precipitation. In addition, with increased Mg 2+ /Ca 2+ ratios, more aragonite in calcium precipitation was produced. The proposed MIMCP method in this study was significant to improve the homogeneity of bio-cemented soil in practical engineering applications.