Improvement and Soil Consistency of Sand–Clay Mixtures Treated with Enzymatic-Induced Carbonate Precipitation

Recently, microbially induced carbonate precipitation (MICP) has been studied as an alternative for the improvement of sand–clay mixtures. However, the cementing uniformity of MICP-treated sand–clay mixtures cannot be guaranteed. In this present study, enzymatic-induced carbonate precipitation (EICP) was used to deal with it. The ions used in kaolin clay was predicted to affect the production rate for calcium carbonate (CaCO3), which was studied using the calcification test. The solidification test was conducted using two different methods (the premixing method and the diffusion method). The permeability, unconfined compressive strength and the content of CaCO3 of treated samples were obtained to evaluate the solidification effect of the EICP method. Moreover, in EICP treatment, the particle aggregation decreased the liquid limit, but the addition of solution increased it. Therefore, there were contrary effects to the soil consistency. In this study, the two types of liquid limits of treated samples were measured with deionized water and 2M-NaCl brine, respectively. The results show that the Al2O3, NaCl and MgCl2 in the kaolin clay had a slight impact on the production rate for CaCO3, while FeCl3 significantly inhibited it. The EICP method can improve sand–clay mixtures and decrease their permeability. Different from MICP, the EICP method can guarantee the uniformity of treated samples. Moreover, the liquid limit of the sample treated with the premixing method decreased, while that of the sample treated with the diffusion method increased firstly and then decreased with the increasing treatment cycles. Different from the deionized water, the pore-fluid chemistry had a larger effect on the liquid limit with 2M-NaCl brine.


Introduction
In the engineering practices, several materials (e.g., cement, lime) are often injected to improve the strength or stiffness of soils to meet the requirements of the application because the soil particles are cemented together. However, the commonly used materials may cause a negative influence on the environment [1]. Over recent years, a novel alternative method based on induced calcium carbonate (CaCO 3 ) precipitation has attracted the extensive attention of researchers in the material engineering field and geotechnical engineering field [2][3][4]. The method was named microbially induced calcium carbonate precipitation (MICP) or enzymatic calcium carbonate precipitation (EICP). In MICP or EICP, the produced acid radical ions can bind with metal ions to form minerals with cementation properties, such as CaCO 3 [5][6][7]. The obtained CaCO 3 precipitation forms a bridge between soil particles or fills in the soil pores, eventually leading to strength improvement and permeability reduction [5,8].
Previous studies have demonstrated that the MICP technique can improve the properties of sands and silty sands [9][10][11][12]. There were several studies focused on the sands because the physical properties of sandy soils are easily studied. However, there were only few studies concerning the solidification of sand-clay mixtures. To replace chemical stabilizer, Morales et al. [13] used the MICP technique to produce CaCO 3

precipitation
The urease enzyme is an important part of the EICP, which can decompose urea (CO (NH 2 ) 2 ; Equation (1)) to obtain carbonate ions. CaCO 3 precipitation can be obtained when the carbonate ions bind with calcium ions (Equation (2)). In nature, the urease enzyme can be found in plants, algae, and some types of bacteria [22]. Previous researchers often used the urease enzyme extracted from jack beans for the application of EICP (Sumner, 1926). The urease enzyme in powder was bought from Sigma Aldrich Company Ltd. (St. Louis, MO, USA) for experiments. To control the initial urease activity, the urease enzyme with an activity of 1030 U/g was used for all tests. In addition, the calcium acetate was used to provide calcium ions.

Effect of the Ions in Kaolin Clay on Production of CaCO 3
The components in different clays might be different. To ensure the repeatability of the results, a commercial kaolin clay was used in this study. In the kaolin clay, there were several minerals (kaolin, quartz and muscovite), whose formula are Al 2 O 3 ·2SiO 2 ·2H 2 O [23]. According to the particle analysis, the percentage of particles with a size smaller than 2 µm reached 90%. The effect of Si on the production rate for CaCO 3 can be ignored. As for Na and Mg, the effect of them on MICP has been already studied [20,21]. However, there were few previous studies about the effect of the ions in kaolin clay on the production of CaCO 3 in EICP. Therefore, Al 2 O 3 , Na, Mg and Fe with different contents were added to the urease solution to study it. Al 2 O 3 at different weight fractions (0 g, 1 g, 2 g or 3 g) was added to the 100 mL of urease solution to study the effect of Al 2 O 3 content on the production rate for CaCO 3 . For the production of CaCO 3 , 100 mL of the cementation solution was mixed with the urease solution. The cementation solution was the mixture of 0.75 M of urea and 0.5 M of calcium acetate. After 48 h, the production rate for CaCO 3 was measured.
In nature, NaCl is a common material. The elements of Mg and Fe are always present in a bivalent from (Mg 2+ ) and trivalent form (Fe 3+ ), respectively. In the present study, NaCl, MgCl 2 and FeCl 3 at different weight fractions (0, 0.5, 1.0, 1.5 and 2.0 g/L) were used to study the effect of them on the production rate for CaCO 3 . Similarly, 100 mL of urease solution was added to 100 mL of cementation solution, and the production rate for CaCO 3 was measured after 48 h.
The tests were conducted in fluid with a temperature of 30 • C and initial pH of 8.0 in the test tube. The actual produced amount of CaCO 3 was measured via the acid washing method, described in Sun et al. [24].

Sand-Clay Mixture Preparations
Sands from the Yangtze River were used for the solidification test in polyvinyl chloride (PVC) cylinders with an inner diameter of 4.5 cm and height of 10.0 cm. The sand had poor gradation. The sands with a median diameter (D 50 ) of 0.3 mm are classified as SP based on the USCS classification system. In this study, the sand-clay mixtures were divided into four groups because of different mass percentages of kaolin clay added: 2.5%, 5%, 7.5% and 10%. In addition, a sample without clay was also prepared for comparison. For the sand-clay mixtures, the sand and kaolin clay were sterilized before being added into the PVC cylinders. To ensure the sands and clays were thoroughly mixed, the premixing method was used (Table 1). The density had an impact on the solidification effects [25]; therefore, all samples were prepared with an identical initial dry density (1.89 g/cm 3 ). Because of the same total mass (300 g), the samples with a larger amounts of kaolin clay had a higher porosity. In the MICP or EICP treatment, the added kaolin clay expanded, leading to the decrease in porosity. This was why in the study of [16], the maximum mass percentage of added kaolin clays was only 7.5%. In the present study, the smaller space for solution to pass through allowed for a larger mass percentage of kaolin clays (10%).

EICP Treatment
In the study of [14], three different MICP methods (injection, premixing and diffusion) were used to treat clayey sand. The clay percentages were controlled, and they drew the conclusion that different methods were suitable to treat clayey sands with different clay percentages. The maximum mass percentage of kaolin clay was 10% in the present study; therefore, the injection method was not suitable due to the low porosity of samples. The other two different methods, premixing and diffusion, were used to comparably investigate the solidification effect. It was noted that with the premixing method limited the treatment cycle of the urease solution and cementation solution. A total of 50 mL of urease solution (1000 U/L) and 50 mL of cementation solution were premixed with sand-clay mixtures. As for the diffusion method, the urease solution and of cementation solution with equal volume were mixed and then added into the PVC cylinders. In order to achieve a significant improvement, the urease solution and cementation solution were added with the diffusion method every two days. During the 12-day treatment period, the permeability of samples was measured every two days. The solidification test was conducted at 30 • C, and the mixture solution was set with an initial pH of 7.0.

UCS and Content of CaCO 3
After solidification, the samples were oven dried at 110 • C for 24 h before the unconfined compressive strength (UCS) tests. During the UCS tests, the loading speed was controlled at 1 mm/min. The added kaolin clays also provided a cementation effect; therefore, the untreated samples with different mass percentages of kaolin clay (2.5%, 5%, 7.5% and 10%) were prepared for comparison. The UCS results of these samples were obtained.
After the UCS test, the solidified specimens were divided into five parts along their length to measure the content of CaCO 3 of each part. The content of CaCO 3 was the ratio of the mass of produced CaCO 3 precipitation to the mass of treated samples at this part. The clay soils contained metal and minerals, which might affect the results of CaCO 3 contents measured by the acid pickling method. However, the contents of metal and minerals in clay soils were similar for samples. Therefore, the CaCO 3 contents were comparable, and the acid pickling method was used in this study to obtain CaCO 3 contents. Firstly, the samples were dried and weighed to obtain the total mass. After that, the samples were washed with 0.1 mol/L of HCl and then dried and weighed again. The difference of the two weights because of the acid leaching was the weight of the precipitated CaCO 3 . In addition, the solidification uniformity of EICP-treated sand-clay mixtures was evaluated by the contents of CaCO 3 at different parts.

Scanning Electron Microscope Test
After the UCS test and CaCO 3 content measurement, the sample D3 was subjected to a scanning electron microscope (SEM) test to obtain the microscopic characteristics. The SEM photo was obtained using the following apparatus: JSM-6300, JEOL company, Akishima, Japan. In nature, CaCO 3 crystals have three different types of crystal forms: calcite, vaterite and aragonite. Compared with the other two types of crystal forms, calcite is the most stable; vaterite is relatively unstable [28]. According to previous studies, the concentration of urease used in EICP would affect the type of CaCO 3 crystals [29]. When the concentration of urease was lower, the CaCO 3 crystals were mainly in the calcite form. Therefore, a lower urease concentration of 1000 U/L was chosen for the production of CaCO 3 with the stable calcite form. In the kaolin clay used, silicon dioxide (SiO 2 ) and Al 2 O 3 are main ingredients. However, in nature, SiO 2 is very stable [30,31]. Consequently, the effect of SiO 2 on the production rate for CaCO 3 was not considered in this study.

Results and Discussions
The reaction period was only 48 h; different from the MICP reaction, urease for EICP was consumed continuously and no new urease was produced, and so the reaction time could not be any longer. Al 2 O 3 was added to the urease solution to study its influence on the production rate for CaCO 3 at different contents, as shown in Figure 1.

Scanning Electron Microscope Test
After the UCS test and CaCO3 content measurement, the sample D3 was subjected to a scanning electron microscope (SEM) test to obtain the microscopic characteristics. The SEM photo was obtained using the following apparatus: JSM-6300, JEOL company, Akishima, Japan.

Effect of Al2O3 on the Production Rate for CaCO3
In nature, CaCO3 crystals have three different types of crystal forms: calcite, vaterite and aragonite. Compared with the other two types of crystal forms, calcite is the most stable; vaterite is relatively unstable [28]. According to previous studies, the concentration of urease used in EICP would affect the type of CaCO3 crystals [29]. When the concentration of urease was lower, the CaCO3 crystals were mainly in the calcite form. Therefore, a lower urease concentration of 1000 U/L was chosen for the production of CaCO3 with the stable calcite form. In the kaolin clay used, silicon dioxide (SiO2) and Al2O3 are main ingredients. However, in nature, SiO2 is very stable [30,31]. Consequently, the effect of SiO2 on the production rate for CaCO3 was not considered in this study.
The reaction period was only 48 h; different from the MICP reaction, urease for EICP was consumed continuously and no new urease was produced, and so the reaction time could not be any longer. Al2O3 was added to the urease solution to study its influence on the production rate for CaCO3 at different contents, as shown in Figure 1. EICP is a complex biochemical process, and the production of CaCO3 precipitation depends on the concentration of calcium ions, dissolved inorganic carbon, and the pH of the solution [32]. The addition of Al2O3 did not have an impact on the production rate for CaCO3. The phenomenon was different from that for MICP [20,33]. It was because in MICP, the change of pH affected bacterial growth and urease activity, while it only had an effect on urease activity in EICP, which indicated there was a smaller inhabitation impact. The initial pH of the urease solution increased after being mixed with the cementation solution due to the reaction of Equation (3). The pH changed again after adding Al2O3, EICP is a complex biochemical process, and the production of CaCO 3 precipitation depends on the concentration of calcium ions, dissolved inorganic carbon, and the pH of the solution [32]. The addition of Al 2 O 3 did not have an impact on the production rate for CaCO 3 . The phenomenon was different from that for MICP [20,33]. It was because in MICP, the change of pH affected bacterial growth and urease activity, while it only had an effect on urease activity in EICP, which indicated there was a smaller inhabitation impact. The initial pH of the urease solution increased after being mixed with the cementation solution due to the reaction of Equation (3). The pH changed again after adding Al 2 O 3 , as shown in Equation (4). When adding 3 g of Al 2 O 3 , the pH decreased to around 7, which indicated that the pH was between 7 and 8.5 in the test; the impact of pH in this range could be ignored.

Effect of NaCl, MgCl 2 and FeCl 3 on the Production Rate for Calcium Carbonate
Except for Al 2 O 3 , the effects of NaCl, MgCl 2 and FeCl 3 were also studied and the results are shown in Figure 2. However, the amounts of added NaCl, MgCl 2 and FeCl 3 were quite a bit smaller than the amount of Al 2 O 3 added. This was because the amounts of Na, Mg and Fe in the used kaolin clay were smaller. To obtain more credible results, the amount of added ions should be consistent with their contents in the used kaolin clay. From Figure 2, adding NaCl did not affect the production rate for CaCO 3 . Adding MgCl 2 had a small impact on the production rate for CaCO 3 . The reason for this might be that adding MgCl 2 could change the pH of solution, as shown in Equation (5).
as shown in Equation (4). When adding 3 g of Al2O3 , the pH decreased to around 7, which indicated that the pH was between 7 and 8.5 in the test; the impact of pH in this range could be ignored. Except for Al2O3, the effects of NaCl, MgCl2 and FeCl3 were also studied and the results are shown in Figure 2. However, the amounts of added NaCl, MgCl2 and FeCl3 were quite a bit smaller than the amount of Al2O3added. This was because the amounts of Na, Mg and Fe in the used kaolin clay were smaller. To obtain more credible results, the amount of added ions should be consistent with their contents in the used kaolin clay. From Figure 2, adding NaCl did not affect the production rate for CaCO3. Adding MgCl2 had a small impact on the production rate for CaCO3. The reason for this might be that adding MgCl2 could change the pH of solution, as shown in Equation (5).  Compared with NaCl and MgCl 2 , adding FeCl 3 significantly decreased the production rate for CaCO 3 . With increased FeCl 3 , the production rate for CaCO 3 almost decreased to 15%. This was because the hydrolysis reaction of FeCl 3 resulted in more hydrogen ions than the hydrolysis reaction of MgCl 2 , which had a larger effect on the pH of solution due to the trivalent ion, as shown in Equation (6).

Permeability
The EICP method can be applied extensively because of the production of CaCO 3 precipitation [34]. In contrast to MICP, the room for the production of CaCO 3 precipitation can be smaller for EICP because no bacteria is used. Therefore, in the present study, the largest mass percentage of clay soils was 10%. In the study of [14], three different MICP methods (injection, premixing and diffusion) were used to treat clayey sand. They drew the conclusion that when the mass percentage of kaolin clay was larger than 7.5%, the space left in the sand-clay mixtures was not sufficient for the growth and reproduction of bacteria. In addition, it was hard to achieve multiple injections of the bacterial suspension and cementation solution. This was also why the maximum mass percentage of kaolin clay in Sun et al. [16] was 7.5%. Therefore, the injection method was not suitable due to the low porosity of samples.
For MICP-or EICP-solidified samples, the property of permeation was an important indicator to evaluate the solidification effect. De Muynck et al. [35] assessed the durability from the permeation properties and resistance towards degradation processes. When the samples were solidified with the diffusion method, the decreasing ranges of permeability coefficients reached about 3-4 orders of magnitude for all samples (Figure 3). In the study of DeJong et al. [1], the MICP treatment resulted in an about 2-3 orders of magnitude of decreasing range for the permeability of silica. In this study, the decreasing range of permeability coefficients was larger because of the expansion of the added kaolin clays. Adding clay reduced the permeability, and the permeability coefficient became smaller with increased amounts of added clay soils. Therefore, the permeability coefficient of the sample without clay (D1) was always the largest. This was because clay soils expanded during the EICP treatment, decreasing the size of pores. Moreover, smaller pores made it easier for CaCO 3 to remain rather than being flushed out; so, the sample D5 with the mass percentage of 10% added kaolin clays had the largest decreasing range of permeability coefficients. However, according to the study of [16], the sample with 2.5% kaolin clay had a larger decreasing range of permeability coefficients than the sample with 7.5% of kaolin clay. This was because in the MICP solidification test, smaller pores made little bacteria remain between particles, eventually leading to decreased contents of CaCO 3 ; so, the decreasing range of permeability coefficients was smaller. However, with the EICP method, the smaller pores had a smaller impact on the production of CaCO 3 . Furthermore, adding more clay soils decreased bacterial urease activity and further inhibited the production of CaCO 3 [16], which does not have to be considered in the application of EICP.

UCS and Content of Precipitation
The strengths of samples made with the premixing method were much smaller (Figure 4a), because only one treatment cycle limited the amount of precipitated CaCO 3 and the improvement of strength. Sample P5 had the highest strength (about 0.33 MPa). Moreover, sample P1 did not form a strong cemented unit. The difference between the strength of samples solidified with the two different methods demonstrated that the improvement of strength resulted from the precipitated CaCO 3 . The sample D4 had a larger strength than other samples. Small pores the cementation of CaCO 3 and meant that it could remain; however, too much kaolin clay (10%) decreased the strength of the sand-clay mixture. The reason for this was that too much clay significantly decreased the initial porosity and the space left in samples was too small, eventually leading to a worse cementation homogeneity. In general, the amount of precipitated CaCO 3 was similar for the samples with various mass percentages of clay soils. Therefore, the strength was contributed to by the cementing effect from clay soils and cementation homogeneity. The cementing effect from clay soils and better cementation homogeneity resulted in higher strengths than the results in [36,37].

UCS and Content of Precipitation
The strengths of samples made with the premixing method were much smaller (Figure 4a), because only one treatment cycle limited the amount of precipitated CaCO3 and the improvement of strength. Sample P5 had the highest strength (about 0.33 MPa). Moreover, sample P1 did not form a strong cemented unit. The difference between the strength of samples solidified with the two different methods demonstrated that the improvement of strength resulted from the precipitated CaCO3. The sample D4 had a larger strength than other samples. Small pores the cementation of CaCO3 and meant that it could remain; however, too much kaolin clay (10%) decreased the strength of the sand-clay mixture. The reason for this was that too much clay significantly decreased the initial porosity and the space left in samples was too small, eventually leading to a worse cementation homogeneity. In general, the amount of precipitated CaCO3 was similar for the samples with various mass percentages of clay soils. Therefore, the strength was contributed to by the cementing effect from clay soils and cementation homogeneity. The cementing effect from clay soils and better cementation homogeneity resulted in higher strengths than the results in [36] and [37].  The location from the top (cm) D1-0%-diffusion method D2-2.5%-diffusion method D3-5%-diffusion method D4-7.5%-diffusion method D5-10%-diffusion method P2-2.5%-premixing method P3-5%-premixing method P4-7.5%-premixing method P5-10%-premixing method (a) (b)

UCS and Content of Precipitation
The strengths of samples made with the premixing method were much smaller (Figure 4a), because only one treatment cycle limited the amount of precipitated CaCO3 and the improvement of strength. Sample P5 had the highest strength (about 0.33 MPa). Moreover, sample P1 did not form a strong cemented unit. The difference between the strength of samples solidified with the two different methods demonstrated that the improvement of strength resulted from the precipitated CaCO3. The sample D4 had a larger strength than other samples. Small pores the cementation of CaCO3 and meant that it could remain; however, too much kaolin clay (10%) decreased the strength of the sand-clay mixture. The reason for this was that too much clay significantly decreased the initial porosity and the space left in samples was too small, eventually leading to a worse cementation homogeneity. In general, the amount of precipitated CaCO3 was similar for the samples with various mass percentages of clay soils. Therefore, the strength was contributed to by the cementing effect from clay soils and cementation homogeneity. The cementing effect from clay soils and better cementation homogeneity resulted in higher strengths than the results in [36] and [37]. The location from the top (cm) D1-0%-diffusion method D2-2.5%-diffusion method D3-5%-diffusion method D4-7.5%-diffusion method D5-10%-diffusion method P2-2.5%-premixing method P3-5%-premixing method P4-7.5%-premixing method P5-10%-premixing method (a) (b) The content of CaCO 3 is also an important indicator for the evaluation of treatment effects [38]. Compared with the samples treated with the diffusion method, the amount of CaCO 3 in the samples treated with the premixing method was much smaller (Figure 4b).
The results were consistent with the results from UCS (Figure 4a). Moreover, the difference in the amount of CaCO 3 was small at different location in samples, regardless of different treatment methods, which was much smaller than the difference in the amount of CaCO 3 at different locations in [38]. There were two reasons: the first one was that MICP resulted in a worse cementing homogeneity than EICP; the second reason was due to the different calcium resource used in [38]. These are based on the study of [39] who reported that the mortar treated with Ca(CH 3 COO) 2 had better solidification homogeneity than the samples treated with CaCl 2 or Ca(NO 3 ) 2 . For the samples with the diffusion method, the increasing mass percentages of added kaolin clay slightly affected the average contents of CaCO 3 . For D5, the content of CaCO 3 at the top was larger than at the bottom, because adding too much kaolin clay made pore space smaller and the diffusion direction had a significant effect on the cementing homogeneity. With smaller pore spaces, a larger amount of CaCO 3 was produced at the top, which clogged up the pores and made it harder for the fluid to flow through. However, the cementing homogeneity was still better than the sand columns solidified with the MICP method [16,38]. For the samples with a mass percentage of added clays below 7.5%, both the premixing method and the diffusion method could guarantee the cementing homogeneity.

Soil Consistency
Higher clay contents showed higher LL values ( Figure 5, which was similar to the results in [26]. For untreated samples, the counter-ions in the pore-fluid decreased the water adsorption of clay surfaces; therefore, the LL DI was larger than the LL Na [27]. The production of CaCO 3 allowed for particle aggregation; so, both the values of LL DI and LL Na of treated samples were smaller than untreated samples. Moreover, the LL DI of treated samples was also larger than LL Na . The reason for this might be that with the premixing method, the addition of solution slightly affected the LL values. The LL of treated sample P1 was not obtained because of a small mass percentage of clay soils. different calcium resource used in [38]. These are based on the study of [39] who reported that the mortar treated with Ca(CH3COO)2 had better solidification homogeneity than the samples treated with CaCl2 or Ca(NO3)2. For the samples with the diffusion method, the increasing mass percentages of added kaolin clay slightly affected the average contents of CaCO3. For D5, the content of CaCO3 at the top was larger than at the bottom, because adding too much kaolin clay made pore space smaller and the diffusion direction had a significant effect on the cementing homogeneity. With smaller pore spaces, a larger amount of CaCO3 was produced at the top, which clogged up the pores and made it harder for the fluid to flow through. However, the cementing homogeneity was still better than the sand columns solidified with the MICP method [16,38]. For the samples with a mass percentage of added clays below 7.5%, both the premixing method and the diffusion method could guarantee the cementing homogeneity.

Soil Consistency
Higher clay contents showed higher LL values ( Figure 5, which was similar to the results in [26]. For untreated samples, the counter-ions in the pore-fluid decreased the water adsorption of clay surfaces; therefore, the LLDI was larger than the LLNa [27]. The production of CaCO3 allowed for particle aggregation; so, both the values of LLDI and LLNa of treated samples were smaller than untreated samples. Moreover, the LLDI of treated samples was also larger than LLNa. The reason for this might be that with the premixing method, the addition of solution slightly affected the LL values. The LL of treated sample P1 was not obtained because of a small mass percentage of clay soils.  The LL DI of sand-clay mixtures treated with the diffusion method increased first. It reached a peak point after two treatment cycles (four days), and then decreased to a constant value with the increasing treatment cycles (Figure 6a). The variation of LL DI with time implies particle aggregation effects from EICP. Firstly, the addition of a mixed solution resulted in the increase in LL DI . All samples with different mass percentages of clay soils showed a peak LL DI . After that, EICP initiated particle aggregation via CaCO 3 bonding, eventually decreasing the values of LL DI . The change in LL DI seemed to be attributed to the equilibrium between the addition of solution and the simultaneous particle aggregation resulted from EICP. From Figure 6b, the LL Na of samples with different mass percentages of clay soils gradually decreased. The EICP treatment cemented particles of sand-clay mixtures, thus altering the USCS classification. In the study of [26], the pore-fluid chemistry governed the LL of samples treated with xanthan gum in the brine. Similar to xanthan gum, the EICP treatment also increased the soil plasticity because of the addition of solution. Electrical sensitivity changed with the amount of precipitated CaCO 3 . Therefore, it is reasonable to assume that the pore-fluid chemistry governs the LL of EICP-treated samples in the brine. centages of clay soils gradually decreased. The EICP treatment cemented particles of sand-clay mixtures, thus altering the USCS classification. In the study of [26], the porefluid chemistry governed the LL of samples treated with xanthan gum in the brine. Similar to xanthan gum, the EICP treatment also increased the soil plasticity because of the addition of solution. Electrical sensitivity changed with the amount of precipitated CaCO3. Therefore, it is reasonable to assume that the pore-fluid chemistry governs the LL of EICPtreated samples in the brine.

Scanning Electron Microscope Test
The EDS test was not conducted in this study; however, previous studies have used the EDS test to confirm the CaCO3 produced in MICP-solidified clay soils [40][41][42]. SEM testing can be used to evaluate the treatment effect from a microscopic perspective. The sample D3 was subjected to an SEM test, as shown in Figure 7. In response to MICP treatment, a large number of CaCO3 crystals were produced between sand particles. Moreover, clay particles were coated by contacted CaCO3 crystals. In addition to their bridge function between sand particles, CaCO3 crystals were also deposited on the surface of sand particles. Furthermore, most CaCO3 crystals were vaterite, with a size of about 1-2 μm. In addition to spherical crystals, few amorphous crystals were found.

Scanning Electron Microscope Test
The EDS test was not conducted in this study; however, previous studies have used the EDS test to confirm the CaCO 3 produced in MICP-solidified clay soils [40][41][42]. SEM testing can be used to evaluate the treatment effect from a microscopic perspective. The sample D3 was subjected to an SEM test, as shown in Figure 7. In response to MICP treatment, a large number of CaCO 3 crystals were produced between sand particles. Moreover, clay particles were coated by contacted CaCO 3 crystals. In addition to their bridge function between sand particles, CaCO 3 crystals were also deposited on the surface of sand particles. Furthermore, most CaCO 3 crystals were vaterite, with a size of about 1-2 µm. In addition to spherical crystals, few amorphous crystals were found.

Applications and Limitations
The strengths of sand-clay mixtures solidified with the diffusion method were much larger than the samples solidified with the premixing method, even reaching 0.9 MPa. The achieved strength with the diffusion method was more adequate for real-field applications. However, the optimum solidification conditions (e.g., reagent concentrations, urease activity, treatment cycles) of the two different methods still should be further studied for the sand-clay mixture solidification.

Applications and Limitations
The strengths of sand-clay mixtures solidified with the diffusion method were much larger than the samples solidified with the premixing method, even reaching 0.9 MPa. The achieved strength with the diffusion method was more adequate for real-field applications.