Abstract:The effectiveness of magnesium as a substitute material in enzyme-mediated calcite precipitation was evaluated. Magnesium sulfate was added to the injecting solution composed of urea, urease, and calcium chloride. The effect of the substitution on the amount of precipitated materials was evaluated through precipitation tests. X-ray powder diffraction and scanning electron microscopy analyses were conducted to examine the mineralogical morphology of the precipitated minerals and to determine the effect of magnesium on the composition of the precipitated materials. In addition to calcite, aragonite and gypsum were formed as the precipitated materials. The effect of the presence of aragonite and gypsum, in addition to calcite, as a soil-improvement technique was evaluated through unconfined compressive strength tests. Soil specimens were prepared in polyvinyl chloride cylinders and treated with concentration-controlled solutions, which produced calcite, aragonite, and gypsum. The mineralogical analysis revealed that the low and high concentrations of magnesium sulfate effectively promoted the formation of aragonite and gypsum, respectively. The injecting solutions which produced aragonite and calcite brought about a significant improvement in soil strength. The presence of the precipitated materials, comprising 10% of the soil mass within a treated sand, generated a strength of 0.6 MPa.

The optimization of enzyme-mediated calcite precipitation was evaluated as a soil-improvement technique. In our previous works, purified urease was utilized to bio-catalyze the hydrolysis of urea, which causes the supplied Ca2+ to precipitate with CO32− as calcium carbonate. In the present work, magnesium chloride was newly added to the injecting solutions to delay the reaction rate and to enhance the amount of carbonate precipitation. Soil specimens were prepared in PVC cylinders and treated with concentration-controlled solutions composed of urea, urease, calcium, and magnesium chloride. The mechanical properties of the treated soil specimens were examined through unconfined compressive strength (UCS) tests. A precipitation ratio of the carbonate up to 90% of the maximum theoretical precipitation was achieved by adding a small amount of magnesium chloride. Adding magnesium chloride as a delaying agent was indeed found to reduce the reaction rate of the precipitation, which may increase the volume of the treated soil if used in real fields because of the slower precipitation rate and the resulting higher injectivity. A mineralogical analysis revealed that magnesium chloride decreases the crystal size of the precipitated materials and that another carbonate of aragonite is newly formed. Mechanical test results indicated that carbonate precipitates within the soils and brings about a significant improvement in strength. A maximum UCS of 0.6 MPa was obtained from the treated samples.

The optimum condition of enzyme-mediated calcite precipitation has been evaluated for its possible application as a soil improvement technique. Magnesium chloride (MgCl 2) and magnesium sulfate (MgSO 4) were substituted to the grouting solution composed of urease, urea, and calcium chloride (CaCl 2), and its effects on the precipitation process, amount, and the mineralogical substances of the precipitated materials were investigated. The evolution of the strength of treated sand was also evaluated through unconfined compressive strength (UCS) tests. The substitution of magnesium compounds was found to be able to augment the precipitated amount and reduce the hydrolysis rate of urea. The mineralogical analysis indicated that the addition of magnesium to the grouting solution was a potential method for promoting the formation of aragonite. Furthermore, the formation of gypsum was also promoted when magnesium sulfate was substituted. The mechanical analysis showed that the producing of the high precipitated amount resulted in the significant improvement in the strength of the treated sand. The relation between the UCS of the treated soil and the precipitated mass indicated that the strength could be controlled by the precipitated mass within the soil.

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This study evaluates the potential of improving an insitu calcite grouting technique. The grout is composed of an equimolar solution of ureaCaCl 2 and an enzyme named urease. We examine the distribution of the grout materials and precipitated calcite within sand columns with a diameter of 5 cm and a height of 100 cm. In the first series of experiments, the concentration distributions of the individual grout materials (i.e., urea, CaCl 2 and urease) within the sand specimen are evaluated. In the second series of experiments, an enzyme-reagent mixed solution (i.e., grout) is injected into the sand columns to evaluate the distribution of calcite. Sand samples are collected from various vertical locations within the treated columns and the amount of precipitated calcite is evaluated. Furthermore, attempts are made to achieve the uniform distribution of the injected grout and hence uniform calcite distribution throughout the specimen. The results show that a uniform distribution of the grout materials up to a distance of 1 m from the inlet is achievable. It is also observed that a relatively uniform distribution of calcite is achievable as long as the rate of calcite precipitation is well controlled.

Organic soil has a high content of water and compressibility. Besides that, it has a low specific gravity, density, and shear strength. This study evaluates the applicability of the soybean crude urease for calcite precipitation (SCU-CP) method and its effectiveness in organic soil as a soil-amelioration technique. Various soybean concentrations were mixed with a reagent composed of urea and calcium chloride to produce the treatment solution. Its effect on the hydrolysis rate, pH, and amount of precipitated calcite was evaluated through test-tube experiments. SEM-EDS tests were performed to observe the mineralogy and morphology of the untreated and treated samples. The treatment solution composed of the reagent and various concentrations of soybeans was applied to organic soil. The increasing strength of the organic soil was evaluated using direct shear (DS) and unconfined compression (UCS) tests. The test-tube results show that a hydrolysis rate of 1600 u/g was obtained when using 50 g/L of soybeans with a precipitation ratio of 100%. The mechanical tests show a significant enhancement in the parameters of the organic soil’s shear strength. A shear strength improvement of 50% was achieved in this study. A UCS of 148 kPa and cohesion of 50 kPa was obtained in the treated samples of organic soil. This research elucidates that the SCU-CP is an effective technique for improving organic soil’s shear strength.

Calcite-induced precipitation methods (CIPMs) have recently become potential techniques in geotechnical engineering for improving the shear strength of sandy soil. One of the most promising methods among them is enzyme-induced calcite precipitation (EICP). In this technique, a mixed solution composed of reagents and the urease enzyme, which produces calcite, is utilized as the grouting material. The precipitated calcite in granular soil provides ties among the grains of soil and limits their mobility, thus promoting an improvement in strength and stiffness and also a reduction in the hydraulic conductivity of sandy soil. This paper discusses the potential increase in the strength and stiffness of the soil, the additional materials for grouting, the effect of these materials on the treatment process, and the engineering properties of the soil. The possible sources of the urease enzyme and the applicability of the EICP method to other soil types are also discussed in this paper. The environmental and economic impacts of the application of EICP are also presented. The envisioned plans for application, potential advantages, and limitations of EICP for soil stabilization are discussed. Finally, the primary challenges and opportunities for development in future research are briefly addressed.

This study evaluated the applicability of natural zeolite for the removal of the NH-forms in the enzyme-mediated calcite precipitation technique. The natural zeolite of mordenite was added to prepared grouting solutions composed of urea and urease and mixed thoroughly using a rotation table for the mixing times of 0.5, 1.0, and 2.0 h. Then, the concentrations of evolving NH-forms in the solutions were measured. The effects of the presence of zeolite on the amount and the mineralogical substance of the precipitated minerals were also evaluated by X-ray powder diffraction and scanning electron microscopy analyses. Sand samples were treated with the grouting solutions containing zeolite, and the improvement in strength was assessed. It was found that utilizing zeolite in grouting solutions can reduce the concentration of NH-forms. A significant reduction in the concentration of NH-forms was obtained. The addition of 10 g natural zeolite/L solution, combined with the 2-h mixing time, resulted in removal efficiencies of 75% and 45% in reagent concentrations of 0.5 and 1.0 mol/L, respectively. Mechanical test results showed that the grouting solutions also brought about a significant improvement in the soil strength. A precipitated material, comprising 9% of the sand mass, was produced by three pore volume (PV) injections of the grouting materials, which showed an unconfined compressive strength of 300 kPa.