Strength Characteristics and Micro-Mechanism of Coral Sand Reinforced by EICP Combined with Aluminum Ions

Abstract

To overcome the high cost, marine ecological risks of traditional coral sand reinforcement, and the insufficient mechanical performance of standalone Enzyme-Induced Carbonate Precipitation (EICP), this study proposes a novel soil improvement method integrating EICP with aluminum chloride hexahydrate (AlCl₃·6H₂O). The objectives are to identify optimal EICP curing parameters, evaluate AlCl₃·6H₂O’s enhancement effect, and reveal the synergistic micro-mechanism. Through aqueous solution, unconfined compressive strength, permeability, X-ray diffraction (XRD), nuclear magnetic resonance (NMR), and Scanning Electron Microscope (SEM) tests, this study systematically investigated the reaction conditions, mechanical properties, anti-seepage performance, mineral composition, and pore structure. The results demonstrate that EICP achieves the best curing effect under specific conditions: temperature of 30 °C, pH of 8, and cementing solution concentration of 1 mol/L. Under these optimal conditions, the unconfined compressive strength of EICP-solidified coral sand columns reaches 761.6 kPa, and the permeability coefficient is reduced by one order of magnitude compared to unsolidified samples. Notably, AlCl₃·6H₂O incorporation yields a significant synergistic effect, boosting the UCS to 2389.1 kPa (3.14 times standalone EICP) and further reducing permeability by 26%. Micro-mechanism analysis reveals that AlCl₃·6H₂O acts both by generating cementitious aggregates that provide nucleation sites for uniform calcite deposition and by accelerating the transformation of metastable aragonite and vaterite to stable calcite, thereby enhancing cementation stability. This study delivers a cost-effective, eco-friendly solution for coral sand reinforcement, providing practical technical support for marine engineering in environments like the South China Sea. By addressing the core limitations of conventional bio-cementation, it opens new avenues for advancing soil improvement science and applications.

1. Introduction

As an important node of China’s “Belt and Road”, the South China Sea is not only China’s precious territorial resource, but also a key strategic base for marine resource development and national defense construction [1,2]. Coral sands are widely distributed in the South China Sea islands and reefs, which often need to be further strengthened for subsequent construction [3,4]. However, the traditional reinforcement technology is highly dependent on cement, lime, and other building materials [5]. In view of the remote distance between the South China Sea reefs and the mainland [6], the transportation cost of related materials is high [7,8]; at the same time, conventional reinforcement methods may also have a potential negative impact on the surrounding marine ecological environment [9,10].

In recent years, bio-induced calcium carbonate precipitation technology has attracted increasing attention in the engineering field as a novel soil solidification and anti-seepage technology. Bio-induced calcium carbonate deposition technology is mainly divided into two categories: Microbially Induced Carbonate Precipitation (MICP) and Enzyme-Induced Carbonate Precipitation (EICP) [11]. At present, a large number of studies have investigated the application of MICP technology in calcareous sand, but there are few studies on the application of EICP. Zheng Junjie et al. conducted a strength test of MICP cemented calcareous sand and verified the feasibility of MICP-reinforced calcareous sand [12]; Liu Hanlong, Zhang dong et al. discussed the influence of grouting method, particle size, particle gradation, compactness, and other factors on the strength and permeability of microbial-solidified calcareous sand [13]. Wu, L et al. studied the effect of calcium source and Ca²⁺ concentration on the solidification of calcareous sand by EICP [14,15].

However, traditional EICP and MICP still have inherent limitations [16,17], which hinder their practical application in coral sand reinforcement. It is worth noting that traditional EICP and MICP generally face the problems of uneven calcium carbonate deposition and insufficient long-term mechanical stability. MICP relies on a complex bacterial culture system and is sensitive to marine environmental conditions, resulting in an unstable curing effect [18,19]; however, the weak bonding force between aragonite and vaterite metastable crystal particles generated by single EICP technology makes the unconfined compressive strength (UCS) limited and the durability poor [20,21]. In order to solve these defects, a variety of modification technologies have been developed to optimize the curing effect by adding biopolymers, alum, metal ions, and other additives. For example, Liu et al. [22] found that xanthan gum could improve the reinforcement effect of EICP sand by increasing the content of calcium carbonate, but the biopolymer could only enhance the bonding between particles and could not change the crystal morphology and stability. Tian Wei et al. [23] confirmed that biopolymers can reduce wind erosion of bare soil solidified by EICP, but its high cost and biodegradability limit its large-scale engineering application. Lai et al. believed that alum (KAl(SO₄)₂·12H₂O) could reduce the turbidity of a bean enzyme and improve the purity of urease, so as to improve the curing effect of EICP [24]. Hou et al. [25] revealed that chitosan can promote the deposition of MICP calcium carbonate, but its effect is highly dependent on pH conditions and has limited adaptability to complex marine environments. Wei et al. proposed adding AlCl₃·6H₂O to the cementing solution to improve the MICP reinforcement rate and reinforcement effect, and carried out MICP sand column test, aqueous solution test, and SEM test to explore the effect of aluminum ion flocculant on the composition and morphology of deposited calcium carbonate [26]. However, their study did not systematically analyze the synergistic mechanism and optimal parameter design under the coral sand system.

Targeting the characteristics of coral sand in South China Sea islands and reefs and the limitations of conventional reinforcement methods, this study takes coral sand as the research object to explore the reinforcement effect of EICP technology. By introducing an aluminum ion flocculant (AlCl₃·6H₂O) to flocculate free calcium carbonate and enhance reinforcement efficiency, X-ray diffraction (XRD) tests, nuclear magnetic resonance (NMR) tests, and Scanning Electron Microscope (SEM) tests were conducted to investigate the mechanism of EICP reinforcement and its influence on soil pore structure. Through the aforementioned research, the scientific research in the field of EICP-reinforced coral sand is further improved. In practical applications, it provides environmentally friendly standardized design parameters for coral sand improvement in South China Sea islands and reefs, promoting the industrialization of this technology.

2. Materials and Methods

2.1. Testing Material

2.1.1. Test Sand

The coral sand sample was taken from Weizhou Island, Guangxi, near the Beibu Gulf, and its geographical location is shown in Figure 1a. The main component of coral sand on Weizhou Island is calcium carbonate. An image of the coral sand sample is shown in Figure 1b, which shows that the particles are irregular in shape and porous on the surface.

Table 1. Properties of coral sand.

Property	Value
Specific gravity, GS	2.75
Average grain size, d50: mm	0.86
Coefficient of uniformity, Cu	4.65
Curvature coefficient, Cc	0.89
Maximum void ratio, emax	1.43
Minimum void ratio, emin	0.94
Particle size range, mm	0.1–2

lists the physical properties of the silica sand. The particle size distribution curve is presented in Figure 1c, with the logarithm of particle diameter on the abscissa and the percentage finer by mass on the ordinate. Based on this curve, the characteristic diameters were determined as follows: d₆₀ = 1.07 mm, d₃₀ = 0.47 mm, and d₁₀ = 0.23 mm. The coefficient of uniformity (Cu) and the coefficient of curvature (Cc) were calculated to be 4.65 and 0.89, respectively. According to the Chinese “Standard for Geotechnical Testing Methods” (GB/T 50123-2019) [27], the coral sand is classified as poorly graded (SP).

2.1.2. Soybean Urease Solution Extraction

Commercial soybeans were used as the source of urease in this experiment. The extraction procedure was conducted as follows: First, the soybeans were dried in an oven at 40 °C for 8 h. The dried soybeans were then ground into a fine powder using a grinder. This grinding process was repeated several times, with care taken to ensure the temperature did not exceed 60 °C during grinding. The resulting powder was sieved through a 100-mesh sieve, and the fine fraction passing through the sieve was sealed and stored in a cool, dry environment.

To prepare the urease solution, a predetermined amount of the soybean powder was mixed with deionized water. The mixture was stirred vigorously for 30 min using a magnetic stirrer. Subsequently, it was allowed to stand at room temperature for 4 h. Finally, the mixture was centrifuged at 3000× g rpm for 15 min at 25 °C. The supernatant was collected as the crude soybean urease solution. The extraction process is shown in Figure 2. Urease activity is susceptible to factors such as temperature, pH, and urease concentration. To determine the optimal conditions for soil improvement, the urease activity under various influencing factors was investigated.

2.1.3. Determination Method of Soybean Urease Activity

Soybean urease activity was monitored using the conductivity method. The principle of this method is that the hydrolysis of urea, catalyzed by urease, leads to an increase in the electrical conductivity of the solution. The rate of change in conductivity is proportional to the urease activity.

The measurement procedure was as follows: First, 3 mL of the soybean powder solution was mixed with 27 mL of a 1 mol/L urea solution. The mixture was then maintained at a constant temperature of 25 °C. The conductivity of the solution was measured at 5 min intervals using a conductivity meter, and the change in conductivity was recorded over a 15 min period.

Urease activity (A, in mM urea hydrolyzed/min) was quantified according to the following relationship. A standard calibration defines that a conductivity change rate (E, in mS/cm/min) of 1 mS/cm/min corresponds to a urea hydrolysis rate of 11.1 mM/min. The urease activity (A) was then calculated by multiplying this calibration factor by the measured conductivity change rate (E) and the solution dilution factor (m). The resulting value (A = 11.1 × E × m) [28] represents the quantitative measure of urease activity.

$$A={\displaystyle \frac{\Delta E}{t}}\times 11.1\times m,$$

(1)

2.1.4. Cementing Liquid and Aluminum Ion Flocculant

In this study, the cementing solution was composed of urea and CaCl₂, and the reagents used were analytical grade.

The source of aluminum ions used was aluminum chloride hexahydrate (AlCl₃·6H₂O) solution. It is a widely used inorganic compound, which is often used as an efficient flocculant in industrial water treatment. The Al(OH)₃ colloid produced by the hydrolysis of aluminum chloride hexahydrate has excellent charge neutralization ability and adsorption bridging effect, which can quickly form dense flocs and significantly improve the sedimentation efficiency. In the ratio of this experiment, the amount of aluminum ion flocculant was much lower than the concentration of cementing liquid.

2.2. Specimen Method

2.2.1. Aqueous Solution Test

In order to explore the optimum conditions for the reaction between plant urease and cementing solution to produce calcium carbonate in different environments, aqueous solution experiments were carried out. The aqueous solution test was carried out under the conditions of different cementing liquid concentrations, ratios, and aluminum ion flocculant concentrations. In each test group, 50 mL of urease was prepared first, and then 50 mL of cementing liquid was prepared. The composition of each cementing liquid is shown in

Table 2. The effect of cementing solution concentration on the production of calcium carbonate.

Sample Number	Soybean Powder Concentration (g/L)	Urea Concentrations (mol/L)	Calcium Chloride Concentration (mol/L)	Amount of Carbonate Precipitation (g)	Theoretical Calcium Carbonate (g)	Calcium Carbonate Precipitation Ratio (-)
S-1	130	1	0.5	2.13	2.5	0.85
S-2	130	1	0.75	2.72	3.75	0.73
S-3	130	1	1	3.52	5	0.7
S-4	130	1	1.25	3.47	5	0.69
S-5	130	1	1.5	3.45	5	0.69
S-6	130	2	1	2.3	5	0.46
S-7	130	2	1.5	2.85	7.5	0.38
S-8	130	2	2	3.18	10	0.32
S-9	130	2	2.5	2.64	10	0.26
S-10	130	2	3	1.97	10	0.2
S-11	130	0.5	0.5	2.23	2.5	0.89
S-12	130	0.75	0.75	2.93	3.75	0.78
S-13	130	1.25	1.25	3.5	6.25	0.56
S-14	130	1.5	1.5	3.38	7.5	0.45

.

According to Formula (2) [29], the theoretical amount of calcium carbonate precipitation can be calculated. The precipitation ratio of calcium carbonate is the percentage of the formation mass to the theoretical formation mass.

$$\left[{\mathrm{CaCO}}_3\right]=C\times V\times M,$$

(2)

[CaCO₃]—Theoretical production mass of calcium carbonate crystals (g);

C—Concentration of cementing fluid;

V—Volume of cementing fluid;

M—Molar mass of calcium carbonate crystal.

The cementing liquid was poured into the urease solution, and the mixed liquid was stirred by a magnetic stirrer. Three parallel groups were selected for each group, and the average value was taken as the test value. After 12 h of reaction in the non-reaction time group, the centrifuge was used at 3000× g r/min for 5 min, and the supernatant was filtered with filter paper. After drying, the mass of the product was determined by acid washing.

2.2.2. Sand Column Experiment

The coral sand used in the test was soaked in deionized water for 12 h, and the surface impurities were washed off and dried in an oven. The dried coral sand was filled into a special cylinder model to make a standard sand column with a diameter of 5 cm and a height of 10 cm. First, a pore volume cementing solution (a mixture of calcium chloride solution and aluminum ion flocculant at preset concentrations) was injected. This process can increase the electrolyte in the sand sample and increase the urease adsorption capacity. After standing for 1 h, urease was poured upward from the bottom of the sample. After the perfusion was completed, the column was left to stand for 1 h, and then the same volume of cementing fluid was poured from the bottom to the top at the same rate. The above steps represent one grouting cycle.

After injecting the cementing fluid, the sand column was left to stand for 24 h and then inverted for the subsequent cementing fluid injection. A total of 5 grouting cycles were conducted in this experiment. The grouting speed was set at 3 mL/min in the test to prevent overly rapid injection from shortening cementing fluid–soybean urease contact time and impairing reinforcement performance. During the grouting process, the liquid state at the upper end of the sand column was continuously observed. When the turbid liquid was observed to flow out, the grouting process was terminated after 5 min of continuous grouting. The grouted sand column samples were then transferred to a constant temperature incubator for 4 days of curing treatment.

2.2.3. Unconfined Compressive Strength Test

The unconfined compressive strength test was carried out by a GDS triaxial apparatus, as shown in Figure 3a. The loading rate was set to 1 mm/min during the test. By recording the stress and strain changes in the specimen under compressive load, the axial stress–strain curve was drawn, and the peak stress was taken as the unconfined compressive strength (UCS).

2.2.4. Penetrating Quality

The formation of calcium carbonate crystals in the EICP reaction can fill the pore structure of the soil, which can reduce the permeability coefficient and porosity of the EICP solidified body to a certain extent. In this study, the permeability coefficient of the EICP solidified body was determined by the constant head method.

2.2.5. Microscopic Test

The mineral composition of the samples was analyzed by an X-ray diffraction (XRD) analyzer. The Aeris X-ray diffraction analyzer used in this study was manufactured by Malvern Panalytical B.V., Almelo, The Netherlands., which can not only measure the mineral composition of the sample but also quantitatively analyze the mineral composition. The instrument is shown in Figure 3b. First, the aqueous solution test product was ground into a powder with a particle size of less than 0.075 mm. It was then loaded onto a glass diffractometer and put into a diffractometer for testing. During the experiment, the scanning angle was between 5° and 90°, and the scanning speed was 10°/min.

The nuclear magnetic resonance imaging analysis equipment was a model Meso MR mesoscale nuclear magnetic resonance imaging analyzer provided by Suzhou Newmai Technology Electronic Co, Ltd., Suzhou, China; the instrument is shown in Figure 3c. Before the test, the sample was dried first and then placed in a vacuum saturation device to vacuumize and saturate for 24 h. After saturation, the sample was taken out for nuclear magnetic resonance-related tests. The imaging system of the nuclear magnetic resonance instrument was used to characterize the distribution of pores inside the sample, and the cross-sections were used for analysis.

3. Experiment Results and Analysis

3.1. Effects of Different Factors on Urease Activity and Calcium Carbonate Production

3.1.1. Effect of Temperature on Urease Activity

To explore temperature’s effect on urease activity and CaCO₃ precipitation during EICP, the following experiments were conducted. Based on the South China Sea temperature range, a 10 °C gradient was set using a constant temperature incubator, with test tubes incubated at 20–60 °C for urea hydrolysis. The experimental results are shown in Figure 4. The initial urease activity showed a steady upward trend with the increase in temperature. The initial activity at 60 °C was nearly twice as high as that at 20 °C. Although the initial activity increased with the increase in temperature, the activity decreased rapidly during the standing process, and the higher the temperature, the faster the rate of inactivation. When the temperature exceeded 50 °C, the soybean urease activity declined rapidly within a few hours.

3.1.2. Effect of pH on Urease Activity

To systematically study pH’s effect on urease activity, seven pH gradients (4–10) were set and precisely adjusted using HCl and NaOH solutions. The experimental results are shown in Figure 5. The change in urease activity showed a typical unimodal distribution. In the range of pH 6–8, urease activity and calcium carbonate production increased synchronously with the increase in the pH value, which was mainly attributed to the improvement in the protonation degree of active sites, which promoted the formation of enzyme–substrate complexes. Urease reached optimal catalytic efficiency at pH 8, with peak activity 15% higher than at pH 7. However, activity dropped sharply above pH 8, retaining 75% of the peak at pH 9 and plummeting to 40% at pH 10, indicating urease is unstable in strong alkalinity. Based on the above findings, considering the catalytic efficiency and stability of urease, the pH value of the reaction system was uniformly controlled to 8 in the subsequent experiments, which not only met the maximum activity requirement (15% higher than that at pH 7) but also avoided the enzyme structural damage caused by the strong alkaline environment.

3.1.3. Effect of Soybean Powder Concentration on Urease Activity

The effect of soybean powder concentration on urease activity was studied. The concentration of soybean powder was 40 g/L, 70 g/L, 100 g/L, 130 g/L, and 160 g/L. The experimental results are shown in Figure 6. With the increase in soybean powder concentration, the development of urease activity showed a decreasing trend. The curve change process shown in the figure indicates that the change in urease activity has a non-linear relationship with soybean powder concentration, and the overall performance is characterized by a decreasing growth rate.

According to the shape of the curve, its evolution process can be divided into three typical stages: the initial stage shows a rapid upward trend; the medium term shows progressive growth; and finally, the reaction activity tends to be stable after entering the plateau period. Specifically, urease activity rose sharply at a 40–70 g/L soybean powder concentration, slowed at 70–100 g/L, and stabilized above 100 g/L. This phenomenon’s mechanism involves two factors: higher urease concentration increases catalytically active sites and promotes ammonia and CO₂ generation, while excessive enzyme concentration saturates substrate-binding sites, limiting catalytic efficiency improvement. Therefore, when the concentration of soybean powder exceeded the critical value of 100 g/L, the enhancement effect of urease activity was no longer significant.

3.1.4. The Influence of the Proportion of Cementation Liquid on the Content of Calcium Carbonate Produced by EICP

Figure 7 shows the influence of the concentration ratio of the cementing solution on the formation of calcium carbonate under different urea concentrations (1 mol/L and 2 mol/L). The influence law was nearly identical for the two urea concentrations: At low cementing solution ratios (Ca²⁺–urea), CaCO₃ production rose with increasing calcium ion proportion. CaCO₃ production peaked at a 1:1 cementing solution ratio, indicating the most sufficient EICP reaction. When the concentration of calcium ions was too high, the amount of calcium carbonate produced showed a downward trend. This phenomenon is more obvious when the concentration of urea is 2 mol/L. Excessive Ca²⁺ binds specifically to urease active sites and competitively occupies them, thereby inhibiting urea hydrolysis. Notably, at Ca²⁺ concentrations < 1 mol/L, CaCO₃ yields were nearly identical under the two urea concentrations tested. However, at a 1:1 cementing solution ratio, the low-urea group produced more CaCO₃ than the high-urea group, indicating excess urea (higher than Ca²⁺ concentration) does not enhance EICP reaction efficiency.

From the perspective of reaction mechanism, this phenomenon can be explained as follows: Firstly, the urease catalytic process follows the ‘enzyme–substrate complex’ formation mechanism. When the urease concentration is constant, excessive urea does not increase the concentration of intermediate products, resulting in the reaction rate reaching a maximum. Secondly, the hydrolysis of high-concentration urea will lead to a change in the pH value of the solution. Meanwhile, an excessive calcium ion concentration in the cementing solution also inhibits urease activity; these factors together restrict the further increase in calcium carbonate production.

3.1.5. The Effect of Cementing Solution Concentration on the Formation of EICP Calcium Carbonate

Figure 8 shows the relationship between the concentration of cementing fluid and the amount of calcium carbonate produced. It can be seen from the figure that with the increase in the concentration of cementing solution, the development law of urease activity and the calcium carbonate content curve is basically the same, showing a trend of increasing first and then decreasing. Urease activity and CaCO₃ content rose sharply below 1.0 mol/L cementing solution concentration, peaked at 1.0 mol/L, and then gradually declined. The main reason is that when the concentration of the cementing solution increases, more substrate urea and reactant calcium chloride will be provided, which not only increases the formation probability of calcium carbonate but also promotes an increase in soybean urease activity and calcium carbonate production. With a further increase in the concentration of the cementing solution, the phenomenon of gel formation or particle aggregation may occur, which will lead to the limitation of the mass transfer step, thereby reducing the effective contact between the soybean urease and the substrate, and thus limiting the reaction rate. A high concentration of the cementing fluid will increase the viscosity and viscosity of the cementing fluid, which limits the diffusion capacity of the reactants and makes the effective contact between the soybean urease and the substrate difficult. Therefore, the effect of cementing solution concentration on soybean urease activity and calcium carbonate production is multifaceted, including both an increase in substrate supply and a change in mass transfer and reaction condition control.

3.1.6. Effect of Reaction Time on the Amount of Calcium Carbonate Produced

Figure 9 shows the effect of reaction time on the production of calcium carbonate. When the reaction time was 1–4 days, the soybean urease activity and calcium carbonate content increased significantly. At this time, the substrate concentration gradually increased, and the urease molecular activity gradually reached a higher level. When the reaction time reached 4 days, the growth rate of urease activity and calcium carbonate content decreased. With the further extension of the reaction time, there may be a combination of multiple key factors, including the reaction equilibrium, the change in the enzyme–substrate ratio, and the inhibition effect. These factors limit the possibility of a significant increase in urease activity and calcium carbonate production again, leading them to a relatively stable state. When the reaction time exceeds 4 days, the growth rate of the curve continues to decline and gradually tends to be gentle.

3.1.7. Effect of Aluminum Ion Flocculant on Calcium Carbonate Production

In order to more intuitively reflect the effect of the aluminum ion flocculant, three kinds of aqueous solution systems based on 50 mL and 1 mol/L cementing solution were placed in the colorimetric tube: soybean urease–cementation liquid system, soybean urease–cementation liquid–aluminum ion flocculant system, and cementation liquid–aluminum ion flocculant system. Following the standing reaction, we compared the appearance changes and precipitation characteristics of each group. Figure 10a shows the state after standing for 10 min, and Figure 10b shows the state after standing for 6 h.

It can be seen that after pouring the cementation solution into the urease solution, it quickly becomes turbid, but it does not precipitate rapidly. After the addition of the aluminum ion flocculant, a large amount of colloidal substances rapidly forms. After standing for a long time, calcium carbonate added with the aluminum ion flocculant is gradually precipitated. In the EICP group added with the aluminum ion flocculant, calcium carbonate is adsorbed by aluminum hydroxide to form a floating aluminum ion cementing group. In the control group, it was observed that when soybean urease was not added, the aluminum ion flocculant did not react with the cementing liquid to form flocs.

The amount of calcium carbonate produced in the aqueous solution test at different AlCl₃·6H₂O concentrations is shown in Figure 11. There are some differences in the amount of calcium carbonate produced in each group. According to the difference in the amount of calcium carbonate produced, each group can be divided into three categories: a control group without AlCl₃·6H₂O, a low-concentration group (concentrations of 0.002 mol/L and 0.004 mol/L), and a high-concentration group (concentrations of 0.006 mol/L, 0.008 mol/L, and 0.1 mol/L). The addition of a small amount of AlCl₃·6H₂O increases the precipitate. However, with the increase in the concentration of AlCl₃·6H₂O, the amount of calcium carbonate produced gradually decreases. Among them, the amount of reactants produced by adding a small amount of AlCl₃·6H₂O is higher than that without adding AlCl₃·6H₂O. This is mainly because the formation of Al(OH)₃ precipitates in the system will form flocs and lead to ion aggregation, thus promoting the formation of calcium carbonate. However, excessive AlCl₃·6H₂O will reduce the pH during the ionization of aluminum ions and interfere with the normal progress of the EICP reaction. Therefore, in general, a small amount of aluminum ion flocculant plays an increasing role in the EICP process.

This phenomenon can be explained by combining relevant theories: Wei et al. proposed that after an aluminum salt is dissolved in an aqueous solution, aluminum ions are prone to hydrolysis reactions due to their high charge density and small ion radius, forming a variety of mononuclear complexes. In the process of urea decomposition catalyzed by urease, the reaction system presents an alkaline environment, which promotes the conversion of aluminum ions in the solution into Al(OH)₃ precipitates. From the perspective of mineral sedimentology, a change in the pH value directly affects the equilibrium concentration of various ions in the pore solution and then regulates the crystallization kinetics and final yield of calcium carbonate.

The effect of reaction time on calcium carbonate formation in the presence of AlCl₃·6H₂O is shown in Figure 12. From the time dimension, there are significant differences in the short-term and long-term effects between the group with aluminum ions (A-1 to A-5) and the control group without the flocculant (W-1). In the early stage of the experiment (1–2 days), the production of calcium carbonate in the low-concentration aluminum ion group (such as A-1 and A-2) was slightly higher than that in the control group, indicating that the low concentration of aluminum ions could accelerate the precipitation process, and the acceleration effect was very obvious. From the second day, the production of the low-concentration aluminum ion group was still slightly ahead, and the production of other aluminum ion groups (A-3 to A-5) was gradually lower than that of the control group. This indicates that the inhibitory effect of aluminum ions on the EICP reaction is sustained. However, at this time, the calcium carbonate production of the low-concentration aluminum ion group had initially reached equilibrium, and its speed of reaching equilibrium was significantly better than that of the control group without the flocculant.

With the prolongation of the reaction time (3–5 days), the production of calcium carbonate in all AlCl₃·6H₂O groups was significantly inhibited. The final calcium carbonate production of the aluminum ion group decreased with the increase in concentration, and the production of the high-concentration group (A-5) decreased by about 8% compared with the control group. This indicates that although the introduction of aluminum ions may briefly increase precipitation efficiency through flocculation in the early stage, in the long run, their competitive binding with calcium carbonate precipitation or interference with urease activity gradually dominates and finally inhibits the continuous generation of calcium carbonate.

3.2. Analysis of Unconfined Compressive Strength Test

The preparation process of the sample is as follows: Firstly, the sand sample was dried at a constant temperature of 108 °C for standby. The inner wall of the mold was coated with Vaseline and covered with PVE film. The bottom was covered with filter paper and a rubber mat with holes. The coral sand with different proportions was weighed and mixed evenly. After being loaded into the mold in four layers, it was compacted, and the top was blocked according to the bottom mode. The mold was wrapped with a water stop, splice peristaltic pumps, silicone tubes, and other equipment (the specific process is shown in Figure 13); finally, CO₂ was injected into the full mold, ultrapure water was injected from the bottom to the top at a very slow speed to discharge the bubbles, and the soil was saturated by standing.

Figure 14a shows the relationship between the unconfined compressive strength of the sand column and the amount of calcium carbonate generated under different cementing fluid concentrations of conventional EICP. The change trend of strength and calcium carbonate production of sand column samples without AlCl₃·6H₂O in this experiment is basically the same as that of the previous aqueous solution test; that is, the change trend of unconfined compressive strength and calcium carbonate production is the same. It can be seen from the diagram that the amount of calcium carbonate generated is positively correlated with the unconfined compressive strength. With the increase in the calcium carbonate content, the unconfined compressive strength shows an exponential upward trend.

The calcium carbonate production and unconfined compressive strength of the sample in the 0.5 mol/L cementing solution group are the smallest, which is 366.7 kPa. The calcium carbonate production and unconfined compressive strength of the 1.0 mol/L cementing solution sample are the largest, up to 761.6 kPa, which is 107% higher than that of the 0.5 mol/L concentration cementing solution. The calcium carbonate precipitation generated by the sample can reduce the porosity of the sand sample, and the sand particles can produce a strong cementation ability to improve the strength of the sand. When the sand column is subjected to an external load, relative sliding will occur between the sand particles. At this time, the calcium carbonate precipitation connecting the sand particles prevents the relative movement between the sand particles and constrains the deformation of the sand particles, which improves the overall strength of the sand column. Therefore, the amount of calcium carbonate produced will directly affect the strength of the sample.

Figure 14b presents the analysis of unconfined compressive strength obtained by changing the concentration of aluminum ions after adding the AlCl₃·6H₂O group. The growth trend of unconfined compressive strength between sand columns with different concentrations of AlCl₃·6H₂O is different from the growth trend of calcium carbonate production. After adding the aluminum ion flocculant, the unconfined compressive strength can reach 2389.1 kPa, which is significantly higher than that of the conventional group, and the strength is 3.14 times that of the conventional group. This shows that the mechanism of AlCl₃·6H₂O underlying the improvement in the unconfined compressive strength is not increasing the amount of calcium carbonate but providing more calcium carbonate attachment points. The aluminum ion electric power produces gelation, so that more calcium carbonate is accompanied by Al(OH)₃ precipitation, ultimately increasing the compressive strength.

3.3. Axial Stress–Strain Curve

Figure 15a,b show the axial stress–strain curves of the coral sand specimens under different reinforcement conditions. Through the analysis of the test data, the deformation process can be divided into four typical development stages: The first stage is the initial compaction stage; the curve presents a slightly convex feature, reflecting the gradual compaction process of the pores inside the coral sand column. The second stage is the linear elastic stage, where the stress and strain show a good linear relationship, and the sample shows stable elastic deformation characteristics. The third stage is the plastic development stage, where the slope of the curve gradually decreases, and the micro-cracks in the sample begin to expand and gradually penetrate. The fourth stage is the failure stage, where the sudden strength loss occurs after the specimen reaches the peak stress, showing a typical brittle failure mode.

A concentration change in the aluminum ion flocculant and cementing solution also affects the curve, which usually shows that as the curing strength increases, the peak first moves up and then down, and the curve moves to the right as a whole. After adding AlCl₃·6H₂O, the degree of mutation at the peak of the curve becomes lower, indicating that AlCl₃·6H₂O can limit the brittle failure of the soil to a certain extent and improve its ductility, which can be changed from (2–2.5%) in the conventional group to (2.5–4%). The reinforcement effect of different AlCl₃·6H₂O concentrations on the samples is also very different. The peak value of AlCl₃·6H₂O at high concentration decreases from (2.5–4%) to (2.5–3.5%), and the ductility and compressive strength are greatly reduced.

It is worth noting that the spatial distribution of the EICP reaction in the PVC tube is uncontrollable, resulting in a significant inhomogeneity in the distribution of calcium carbonate in the sample, which has an important influence on the mechanical properties of the sample. The specific performance is as follows: in the 1.0 mol/L cementing fluid group, the stress–strain curve shifts to the left as a whole, which may be caused by insufficient cementation in local areas; in the 0.5 mol/L cementing solution group, the curve showed obvious stress fluctuation in the 1.0–1.5% strain interval (first decreased, then gentle, and then decreased). This may be due to the blockage of the grouting port during the grouting process, resulting in the formation of a weak cementation layer in the middle of the sample. These phenomena fully illustrate the important influence of cementation uniformity on the reinforcement effect.

3.4. Permeability

The change in permeability with the concentration of cementing fluid is shown in Figure 16a. For the same grouting round, the permeability change in the sand column still has the same trend. Under different cementing solution concentration environments, the permeability coefficient is consistent with the sand column strength and calcium carbonate production trend in the previous article; that is, it gradually decreases with the increase in the cementing solution concentration. In the initial state of the sand column without cementation treatment, the permeability coefficient is 7.56 × 10⁻² cm/s; when the concentration of the cementing solution is 1.0 mol/L, the permeability coefficient reaches the minimum value of 0.77 × 10⁻² cm/s, which is about one order of magnitude lower than the initial state.

Increasing the concentration of the cementing solution significantly alters the pore structure of coral sand. As the amount of calcium carbonate precipitation generated in the cementing solution increases, these newly formed cementitious substances not only effectively enhance the bond strength between sand particles but also significantly reduce pore connectivity through selective filling of interconnected pores. As the cementing solution concentration further increases, the cementitious substances extend from the sand particle contact points toward the middle of the pores; calcium carbonate crystals are preferentially deposited at the particle contact points, forming cementation between sand particles via calcium carbonate deposition. This leads to a decrease in effective porosity, selective blockage of critical seepage paths, and a substantial reduction in permeability. This indicates that the formation of calcium carbonate is a key factor contributing to the significant reduction in permeability.

The permeability coefficient after adding AlCl₃·6H₂O is shown in Figure 16b. The permeability coefficient of the standard EICP after curing is 0.77 × 10⁻² cm/s, while the permeability coefficient of the EICP coral sand column optimized by AlCl₃·6H₂O is significantly reduced. When the addition of AlCl₃·6H₂O is 0.004 mol/L, the permeability coefficient is 0.57 × 10⁻² cm/s, which is 26% lower than that of the standard EICP test.

The decrease in permeability can be attributed to two aspects. On the one hand, the addition of AlCl₃·6H₂O generated a small amount of Al(OH)₃ precipitation, which slightly reduced permeability. However, more importantly, AlCl₃·6H₂O flocculated a large amount of free calcium carbonate during the curing process, which filled the gaps in the coral sand column and provided more nucleation sites for soybean urease. As a result, the EICP reaction was further promoted to generate more calcium carbonate precipitates to strengthen the solidification effect, further fill the pores and block the seepage path, and finally significantly reduce the permeability of the coral sand column.

3.5. Micro-Analysis

3.5.1. XRD Results

Figure 17 shows the X-ray diffraction results of the EICP aqueous solution precipitate and the EICP-AlCl₃·6H₂O aqueous solution precipitate. Figure 17a,c,e,g,i are the XRD diffraction patterns of the precipitate from the EICP aqueous solution after 6 h, 24 h, 48 h, 72 h, and 96 h, and the proportion of EICP crystal content in Figure 18 is obtained by quantitative analysis. It can be seen that in the first six hours, the EICP reaction generated a large amount of calcite and vaterite, while the amount of aragonite was very small. As the reaction progressed, vaterite, as a metastable calcium carbonate, gradually turned to calcite, accompanied by the formation of a small part of aragonite. From 6 h to 72 h of the curing reaction, the content of calcite increased from 65% to 73%, while the content of vaterite decreased from 42% to 24%, and the content of aragonite increased to 5% in the first day and then decreased gradually. This indicates that at the beginning of the reaction, a certain amount of vaterite and aragonite will be generated, and as the reaction progresses, vaterite and aragonite will gradually convert into more stable calcite, and the conversion of vaterite will be close to 40%. The reaction will gradually slow down after 72 h.

The quantitative analysis of the X-ray diffraction test results of the EICP-AlCl₃·6H₂O aqueous solution precipitate is shown in Figure 18b. Similarly, compared with EICP, in the first six hours, the curing reaction generated more calcite and aragonite, while the amount of vaterite generated was less than that of the conventional group. As the reaction progressed, vaterite transformed to calcite more. From 6 h to 72 h of the curing reaction, the content of calcite increased from 67% to 81%, while the content of vaterite decreased from 30% to 15%; the aragonite content reached 7% in the first day and then gradually decreased. This shows that in the early stage of the reaction, AlCl₃·6H₂O can produce calcite more effectively, accompanied by the formation of a small amount of aragonite, which has a certain inhibitory effect on the formation of vaterite. With the progress of the reaction, the amount of calcite also increases, and the conversion rate of vaterite can be increased from 40% to 50%.

3.5.2. Nuclear Magnetic Resonance Test

The NMR test results of the optimized EICP-reinforced samples with different AlCl₃·6H₂O contents are shown in Figure 19. The unoptimized EICP samples show bimodal pore characteristics. With the increase in the AlCl₃·6H₂O content, the peak amplitudes decreased by 6.41%, 13.12%, and 11.20%. In comparison, when the content of AlCl₃·6H₂O was 0.004 mol/L, the development of macropores in the sample could be effectively improved. Compared with the amplitude peaks corresponding to the medium pores, they reduced by 49.47%, 45.17%, and 35.78%. In comparison, when the AlCl₃·6H₂O content was 0.004 mol/L, the medium signal amplitude was the smallest. It shows that the incorporation of AlCl₃·6H₂O can reduce the development of medium and large pores in the sample and increase its compactness.

Based on the change in the total peak area, the total peak area of the blank group without AlCl₃·6H₂O was the largest. With the increase in the AlCl₃·6H₂O content, the total peak area of each group decreased by 30.09%, 39.42%, and 26.40%, respectively, indicating that AlCl₃·6H₂O can effectively reduce the number of pores in the sample. For macropores, with the increase in the AlCl₃·6H₂O content, the maximum peak area of each group decreased by 27.39%, 32.06%, and 22.17%. This shows that AlCl₃·6H₂O also has an excellent inhibitory effect on the development of macropores in the sample.

3.5.3. SEM Test Analysis

According to the relationship between sand particles and calcium carbonate, the effect of calcium carbonate cementation can be divided into three main types: coating, bonding, and bridging [30]. Figure 20a shows the microstructure of a sand sample treated with a cementation solution without added Al³⁺, magnified 2000, 10,000, and 20,000 times. In the EICP process, the calcium carbonate produced functions primarily in a surface-coating mode: its crystalline particles form on the surface of coral sand grains and gradually cover them, thus achieving the encapsulation of the sand particles. However, constrained by the amount of calcium carbonate generated, the coral sand particles achieved only partial surface coverage, failing to form a continuous and uniform protective layer, with some areas remaining exposed. In terms of crystal morphology, the calcium carbonate produced under these conditions was dominated by calcite and vaterite, with aragonite as a minor component. The crystals were sparsely distributed, resulting in relatively weak inter-particle bonding.

Figure 20b shows the microstructure of the sand samples treated with an Al³⁺ concentration of 0.004 mol/L. The calcium carbonate produced mainly exerts bonding and bridging effects: the calcium carbonate crystals deposit, accumulate, and grow in the gaps between adjacent sand particles, successfully cementing the dispersed sand particles into a coherent whole. At this stage, most of the generated calcium carbonate crystals are of the calcite type. Calcite crystals and sand particles together form a dense cementation system, while the aluminum hydroxide produced simultaneously adheres to the surface of calcium carbonate particles and between sand particles.

3.5.4. Mechanism Analysis of EICP and Aluminum Ion Flocculant-Reinforced Sand Soil

EICP technology catalyzes the hydrolysis of urea by soybean urease to form free carbonate, which is combined with calcium ions in calcium chloride to form calcium carbonate with mostly calcite crystal morphology. Due to the rough surface of sand particles, calcite can be well attached to the surface of sand particles, filling the pores between sand particles and forming an excellent ‘calcium carbonate bridge’ by growth to bond adjacent sand particles together. However, since the urease solution does not have ‘biological control ability’, the diffusion effect in the sand column depends on the seepage velocity. An inappropriate seepage velocity will lead to uneven diffusion of the urease solution in the sand particles; this may lead to local supersaturation and the formation of large calcium carbonate crystals, which will affect the subsequent EICP curing effect. It may also lead to a situation in which the newly produced calcium carbonate crystals are not completely attached to the sand particles and are washed away by the cementing solution. Therefore, the formation of EICP calcium carbonate is relatively random, which is one of the reasons why the strength of EICP is generally lower than that of MICP.

EICP-reinforced calcareous sand differs from quartz sand and other siliceous sand in terms of hydrogen bonds or van der Waals forces. Coral sand, which is calcareous sand, has a natural advantage. The generated calcium carbonate can be highly matched with the lattice structure of calcareous sand (such as calcite–calcite), which may lead to epitaxial growth; that is, the newly generated calcite can directly continue the lattice arrangement on the surface of the substrate to form a coherent or semi-coherent interface, thereby enhancing the bonding strength. Therefore, in principle, EICP has a better reinforcement effect on calcareous sand.

The strengthening mechanism of AlCl₃·6H₂O can be analyzed from two aspects. On the one hand, aluminum ions can inhibit the growth of vaterite and generate calcite to a greater extent, providing a stable crystal state for the curing of EICP. On the other hand, AlCl₃·6H₂O, which can be used as a flocculant, can be adsorbed on the surface of sand particles to provide more heterogeneous nucleation sites, gather calcium carbonate free in water, and fix it in the pores of the sand column; as a result, the CaCO₃ crystal is finer and more evenly deposited and the bridging effect is enhanced. At this time, the induced calcium carbonate formation reaction of EICP is complementary to the cementation regulation of aluminum ions. The flocculation effect of aluminum ions constructs the cement skeleton, and the fine and stable calcite crystals fill the fine pores. The multi-dimensional effect can increase the compressive strength of EICP by three times. At the same time, it can improve the impermeability to a certain extent. In order to clarify the mechanism of AlCl₃·6H₂O in the curing process, the chemical reaction formula and the microscopic mechanism diagram were used to explain the mechanism. First of all, the reaction of urease hydrolysis and calcium carbonate formation can be expressed as [29]:

$$\mathrm{CO}{\left({\mathrm{NH}}_2\right)}_2+2{\mathrm{H}}_2\mathrm{O}\to 2{\mathrm{NH}}_4^{+}+{\mathrm{CO}}_3^{2-},$$

(3)

$${\mathrm{Ca}}^{2+}+{\mathrm{CO}}_3^{2-}\to {\mathrm{CaCO}}_3\downarrow ,$$

(4)

The hydrolysis of aluminum ions is as follows [26]:

$$A{l}^{3+}+3{(OH)}^{-}\to Al{(OH)}_3\downarrow$$

(5)

Combined with the above reaction, the reaction process of AlCl₃·6H₂O can be visually presented by the following microscopic mechanism diagram (Figure 21).

In short, AlCl₃·6H₂O enhances the stability of calcium carbonate crystals by inhibiting the growth of aragonite and vaterite and increasing the content of calcite. At the same time, AlCl₃·6H₂O adsorbs free calcium carbonate by means of aluminum ion flocculation, constructs the cement skeleton and fills small pores, and finally optimizes the curing effect of EICP.

4. Discussion

4.1. The Feasibility and Potential Challenges of Field Application of EICP Technology

This study confirmed the excellent reinforcement effect of EICP technology and the EICP-AlCl₃·6H₂O optimization scheme on coral sand at the laboratory scale. However, the following key issues still need to be considered when applying it to field engineering.

First of all, grouting uniformity and flow control are the core factors affecting the field application effect. Under laboratory conditions, we used a constant flow pump to accurately grout at a low speed of 3 mL/min to ensure the uniform distribution of the reaction liquid in the sand column. However, in large-scale construction on site, in order to achieve the same precision of infiltration and distribution in large volume coral sand, the layered and segmented grouting process should be adopted, and the grouting parameters should be adjusted in real time in combination with the pore water pressure monitoring of soil to prevent the uneven deposition of calcium carbonate caused by the local enrichment of slurry. In view of the fact that excessive Al³⁺ will inhibit urease activity and the risk of environmental residues, strict dose control and monitoring mechanisms must be established for aluminum ion flocculants in future field applications.

Secondly, the technology’s long-term viability hinges on its performance under complex environmental hydrodynamics, such as tidal and wave action. While laboratory curing occurs in a static setting, coral sand in situ is subjected to dynamic forces, seawater chemical interaction, and cyclic wetting–drying. Therefore, the stability of the aluminum-enhanced cement—its resistance to leaching, ion exchange, and physical degradation—must be validated through long-term field monitoring and further study.

4.2. Ecological Security and Sustainability Assessment

The optimal AlCl₃·6H₂O concentration in this study was 0.004 mol/L, a trace addition. In the alkaline EICP environment, Al³⁺ rapidly hydrolyzes to form a low-solubility Al(OH)₃ precipitate. XRD analysis confirmed that these products are fixed within the carbonate matrix, significantly reducing aluminum mobility. Therefore, the associated leaching risk is well below levels threatening to marine ecology. Of course, future field applications should still include standardized tests for verification.

5. Conclusions

In this paper, the effects of soybean urease concentration, cementing liquid ratio, and aluminum ion flocculant on EICP-reinforced coral sand were systematically studied. Combined with the aqueous solution test, the sand column test, and microscopic analysis, the following main conclusions were drawn:

• The curing effect of EICP and AlCl₃·6H₂O was greatly affected by the environment. The urease activity increased with the increase in temperature, but the activity decreased significantly when the temperature exceeded 50 °C. The optimum pH was 8. When the concentration of cementation solution was 1 mol/L, the reinforcement effect of EICP was the best. The addition of AlCl₃·6H₂O at a concentration of 0.004 mol/L had the best synergistic effect on EICP.
• EICP can significantly improve the unconfined compressive strength and impermeability of coral sand. When the concentration of the cementing solution is 1 mol/L, the unconfined compressive strength can reach 761.6 kPa, and the permeability coefficient can be reduced by approximately one order of magnitude compared with the uncured sand sample. On this basis, AlCl₃·6H₂O at a concentration of 0.004 mol/L was added. The unconfined compressive strength can be increased to 2389.1 kPa, which is 3.14 times that of the conventional group, and the permeability is further reduced by 26%.
• The X-ray diffraction test and nuclear magnetic resonance test reveal the mechanism of EICP technology and EICP-AlCl₃·6H₂O optimization. The mechanism of EICP is to generate calcium carbonate, with the main component of calcite and a small amount of aragonite and vaterite, gather and fill the pores of the sand column, and improve the strength and impermeability of the sand column. It can be seen from the microscopic analysis that the optimization mechanism of AlCl₃·6H₂O is to improve the overall stability of calcium carbonate by limiting the formation of aragonite and vaterite. At the same time, Al(OH)₃ and other insoluble substances are formed to produce flocculation. The flocculation precipitates are adsorbed on the surface of sand particles and can adsorb the free calcium carbonate precipitation in the pores, so that the free calcium carbonate in the pores can be converted into ‘effective’ calcium carbonate with cementation. Therefore, the unconfined compressive strength of the sand column is significantly improved when the calcium carbonate content is slightly lower. This also explains why the amount of calcium carbonate produced after the addition of AlCl₃·6H₂O in the sand column experiment does not match the trend of compressive strength.