Baseline Investigation on Enzyme-Induced Calcium Carbonate Precipitation

A baseline study was conducted to evaluate the influence of the composition of the enzyme-induced carbonate precipitation (EICP) treatment solution on the efficiency of carbonate precipitation. EICP is an emerging biogeotechnical ground-improvement technique in which calcium carbonate is precipitated from an aqueous solution within the soil pores. A solution consisting of 1 M urea, 0.67 M calcium chloride (${\mathrm{CaCl}}_2$ ), and $3\mathrm{g}/\mathrm{L}$ urease enzyme (reported activity $\mathrm{3,500}\mathrm{U}/\mathrm{g}$ ) was identified as the preferred composition of the solution for EICP treatment of soil, because that concentration of ingredients results in high precipitation mass, high precipitation efficiency, and high enzyme efficiency. It was also shown that effluent from EICP treatment may be used as a source of ${\mathrm{CaCl}}_2$ and urea but cannot be used as a source of urease enzyme. Adding calcite seeds facilitates densely agglomerated calcite crystal formation. Unconfined compression tests on EICP-treated specimens rinsed with deionized water to flush organic matter and ammonium chloride from the specimens suggest that there may be a threshold carbonate content above which the strength dramatically increases. The strength of EICP-treated soil is influenced not only by the amount of carbonate precipitation and but also by the method of preparation and by particle packing. Rinsing of the EICP-treated specimens with deionized water was found to cause dissolution and flushing of organic matter and ammonium chloride salt precipitates.