Effects of calcium sources and magnesium ions on the mechanical behavior of MICP-treated calcareous sand: experimental evidence and precipitated crystal insights

To explore the optimized design of bio-mediated soil enhancement, the influence of different calcium sources (i.e., calcium acetate, calcium nitrate, and calcium chloride) and magnesium ion concentrations (to simulate the seawater environment) on calcareous sand treated with microbially induced carbonate precipitation (MICP) technique was investigated. Unconfined compression tests and carbonate content tests were conducted to characterize the mechanical strength and carbonate amounts, respectively. The changes of calcium ion concentration and electrical conductivity of the MICP solution were monitored in the bio-mineralization process. Scanning electron microscopy (SEM) observation reveals the carbonate crystal morphologies and X-ray diffraction (XRD) analysis quantifies the mineral compositions of carbonate precipitations. The experimental results show that the bio-cemented samples treated with calcium acetate achieve higher carbonate content, while those treated with calcium nitrate or calcium chloride give higher strength. The addition of 0.05 M magnesium ions (i.e., close to the magnesium ion concentration in seawater) enhances the mechanical strength of the bio-cemented samples treated with each calcium source. When magnesium ion concentration increased to 0.5 M, the bio-cemented samples treated with calcium chloride achieve the highest strength and the highest carbonate content. Moreover, calcium source type affects the MICP reaction rate and additional magnesium ion could slow down the reaction rate of calcium ions. SEM and XRD analysis results highlight the strong dependence of the morphological features and mineral compositions of carbonate precipitations on calcium source and magnesium ion concentration.