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.

As part of an effort to lower the cost of urease enzyme used in enzyme induced carbonate precipitation (EICP) for soil improvement, urease enzyme was extracted from watermelon seeds. EICP is a biologically-based ground improvement technique in which a solution containing calcium, urea, and urease enzyme is used to induce calcium carbonate precipitation in a granular soil, enhancing strength, and stiffness. To reduce the enzyme cost by obtaining it from a waste material, the effectiveness of urease enzyme extracted from watermelon seeds, a urease-rich agricultural waste, was evaluated. Low-tech methods were employed for extraction and purification of the enzyme. The extracted enzyme, which showed urease activity of around 611 U/ml, was used to treat Ottawa 20/30 sand. Results of scanning electron microscope imaging and energy dispersive X-ray analysis demonstrated calcium carbonate precipitation. The ratio of the precipitated carbonate content to the theoretical maximum was found to be around 64%.

Beach sands are composed of a variety of minerals including quartz and different carbonate minerals. Seawater in beach sand contains several ions such as sodium, magnesium, calcium, chloride, sulfate, and potassium. These variations in mineralogy and the presence of salts in beach sand may affect the treatment via enzyme-induced carbonate precipitation (EICP). In this study, set test tube experiments were conducted to evaluate the precipitation kinetics and mineral phase of the precipitates in the presence of zero, five, and ten percent seawater (v/v). The kinetics were studied by measuring electrical conductivity (EC), pH, ammonium concentration, and carbonate precipitation mass in EICP solution at different time intervals. A beach sand was also treated using EICP solution containing zero and ten percent seawater at one, two, and three cycles of treatment. Unconfined compressive strength (UCS), carbonate content, and mineralogy of the precipitates in the treated specimens were evaluated. The kinetics study showed that the rate of urea hydrolysis and the rate of precipitation for zero, five, and ten percent seawater were similar within the first 16 h of the reaction. After 16 h, it was observed that the rates dropped in the solution containing seawater, which might be attributed to the faster decay rate of urease enzyme when seawater is present. All the precipitates from the test tube experiments contained calcite and vaterite, with an increase in vaterite content by increasing the amount of seawater. The presence of ten percent seawater was found to not significantly affect the UCS, carbonate content, and mineralogy of the precipitates of the treated beach sand.

Treatment method can have a significant influence on the mineralogy of the precipitated calcium carbonate (CaCO₃) in soil treated by microbially-induced carbonate precipitation (MICP). Rhombohedral calcite crystals are the most desirable form of CaCO₃ for geotechnical applications due to their thermodynamically stable nature. However, not all treatment methods produce this form of CaCO₃. Soaking and injection are two methods for MICP treatment of soil commonly used in the laboratory. The injection method was found to be more efficient for obtaining rhombohedral calcite crystals. It appears that rhombohedral calcite crystal growth is hindered in the soaking treatment method due to high concentration of organic matter in the treatment solution. These results indicate that mineralogical considerations should be included when determining an appropriate method for MICP-treatment of soil in the laboratory such that the resulting specimen is representative of the in situ mechanical behavior of MICP-treated soil.

The stability (longevity of activity) of three crude urease extracts was evaluated in a laboratory study as part of an effort to reduce the cost of urease for applications that do not require high purity enzyme. A low-cost, stable source of urease will greatly facilitate engineering applications of urease such as biocementation of soil. Inexpensive crude extracts of urease have been shown to be effective at hydrolyzing urea for carbonate precipitation. However, some studies have suggested that the activity of a crude extract may decrease with time, limiting the potential for its mass production for commercial applications. The stability of crude urease extracts shown to be effective for biocementation was studied. The crude extracts were obtained from jack beans via a simple extraction process, stored at room temperature and at 4 ℃, and periodically tested to evaluate their stability. To facilitate storage and transportation of the extracted enzyme, the longevity of the enzyme following freeze drying (lyophilization) to reduce the crude extract to a powder and subsequent re-hydration into an aqueous solution was evaluated. In an attempt to improve the shelf life of the lyophilized extract, dextran and sucrose were added during lyophilization. The stability of purified commercial urease following rehydration was also investigated. Results of the laboratory tests showed that the lyophilized crude extract maintained its activity during storage more effectively than either the crude extract solution or the rehydrated commercial urease. While incorporating 2% dextran (w/v) prior to lyophilization of the crude extract increased the overall enzymatic activity, it did not enhance the stability of the urease during storage.

Sisal fibers were employed to enhance the performance of soil treated by enzyme induced carbonate precipitation (EICP). In EICP, free urease enzyme is used to catalyze the reaction between calcium chloride and urea that induces calcium carbonate precipitation in soil. The precipitation changes the mechanical properties of the treated soil through binding particles together, roughening particle surfaces, and filling pores. A laboratory study was conducted to investigate the effect of the addition of sisal fibers, including fiber ratio (as a percentage of the dry mass of fibers and sand) and the length of fiber, on the strength and ductility of EICP-improved soil. Two different lengths (10 and 20 mm) of sisal fibers were added at fiber ratios of 0 (control), 0.2, 0.3, 0.4, 0.75, and 0.85% to create specimens for unconfined compression strength tests. The optimal fiber ratio and fiber length with respect to the UCS were 0.3% and 10 mm, respectively. The UCS strength of EICP-treated sand at the optimum fiber content was four times higher than that of the control sample. The strain at failure (peak strength) was too variable to draw any conclusions regarding ductility.