Effect of Magnesium Incorporation in Enzyme-Induced Carbonate Precipitation (EICP) to Improve Shear Strength of Soil

Abstract

Enzyme-induced carbonate precipitation (EICP) is a novel, bioinspired soil stabilization technique in which calcium carbonate ($${\text{CaCO}}:{3}$$) crystals are enzymatically precipitated to cement and link the soil grains, thereby improving the shear strength of the soil. This work aims to analyze the effect of incorporating $${\text{Mg}}^{{2+}}$$ ions on crystal morphology and their direct influence on the mechanical properties of the soil. A beaker experiment conducted by mixing urea, urease enzyme and $${\text{MgCl}}:{2}$$/$${\text{CaCl}}:{2}$$ in different molar ratios revealed that the increase in the $${{{\text{Mg}}^{{2+}}} \mathord{\left/{\vphantom {{{\text{Mg}}^{{2+}}} {{\text{Ca}}^{{2+}}}}} \right. \kern-0pt} {{\text{Ca}}^{2 + } }}$$ molar ratio decreases the amount of precipitated mass. The soil specimens for unconfined compressive strength (UCS) test were prepared as per its maximum dry unit weight ($$\gamma:{\text{dmax}}$$), and an optimum solution content ($${\text{w}}:{\text{opt}}$$) consisting of urea, urease enzyme and $${\text{MgCl}}:{2}$$/$${\text{CaCl}}:{2}$$ at various $${{{\text{Mg}}^{{2+}} }\mathord{\left/{\vphantom {{{\text{Mg}}^{2+}} {{\text{Ca}}^{2 + } }}} \right. \kern-0pt} {{\text{Ca}}^{2 + } }}$$ molar ratio. Field-emission scanning electron microscopy (FESEM) and X-ray powder diffraction (XRD) tests performed on precipitated mass verify the influence of $${\text{Mg}}^{{2+}}$$ ions on crystal morphology and the occurrence of other carbonates (dolomite) and polymorphs of $${\text{CaCO}}:{3}$$. The results of the UCS tests show that the lower molar ratio of $${\text{Mg}}^{{2+}}$$/$${\text{Ca}}^{2 + }$$ can significantly improve the undrained shear strength of the soil.