Open AccessArticle

Study on the Effect of Sodium Silicate Solution Injection Timings on Electrochemical Reinforcement of Dredged Sludge

Jiangdong Lin

¹,

Mi Ai

²,

Guohui Yuan

^3,4,5,*

Long Wang

^3,4,

Ziyang Gao

^3,4,

Xiaobing Li

^3,4,

Hongtao Fu

^3,4 and

Yongfei Fan

⁵

Jiangxi Zhongmei Engineering Group Co., Ltd., Nanchang 330001, China

Zhejiang Geology and Mineral Technology Co., Ltd., Wenzhou 325035, China

College of Civil Engineering and Architecture, Wenzhou University, Wenzhou 325035, China

⁴

Key Laboratory of Engineering and Technology for Soft Soil Foundation and Tideland Reclamation of Zhejiang Province, Wenzhou 325035, China

⁵

College of Civil and Surveying Engineering, Jiangxi University of Science and Technology, Ganzhou 341000, China

Author to whom correspondence should be addressed.

Buildings 2025, 15(1), 70; https://doi.org/10.3390/buildings15010070

Submission received: 10 November 2024 / Revised: 19 December 2024 / Accepted: 26 December 2024 / Published: 28 December 2024

(This article belongs to the Section Building Energy, Physics, Environment, and Systems)

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Browse Figures

Figure 1
Testing model box. (a) Schematic drawing. (b) Detection arrangement point. "> Figure 2
Variation of current during treatment for different injection timing. "> Figure 3
Variation of total drainage volume during treatment for different injection timing. "> Figure 4
Variation of drainage rate during treatment for different injection timing. "> Figure 5
Settlement with time. "> Figure 6
Variation in moisture content with distance from the anode. "> Figure 7
Variation in undrained shear strength with distance from anode. "> Figure 8
Variation in (a) Ca2+, (b) Na+, and (c) K+ ions concentration with time in the discharged water. "> Figure 9
SEM image of the microstructure of soil sample at test S3 after treatment: (a) near the anode; (b) center of soil; (c) near the cathode. "> Figure 10
Cumulative mercury intrusion (a) near the anode, (b) near the middle, and (c) near the cathode after treatment. "> Figure 11
Pore size diameter distribution (a) near the anode, (b) near the middle, (c) near the cathode after treatment. "> Figure 12
Mechanism of injection time of the sodium silicate solution during the electrochemical treatment. ">

Versions Notes

Abstract

To address the issue of uneven shear strength distribution in dredged sediment during electroosmosis treatment, a grouting system was employed to inject CaCl₂ into the anode region and Na₂SiO₃ solution into the central region. An experimental study was conducted to examine the effect of injection timing on the electrochemical treatment of dredged sediment. Five experimental groups, each with different Na₂SiO₃ injection timings, were established. The impact of injection timing on the macroscopic electrochemical reinforcement was assessed based on current, drainage volume, settlement, moisture content, and shear strength. Additionally, the ion concentration of effluent from the cathode was measured, and scanning electron microscopy (SEM) and mercury intrusion porosimetry (MIP) were employed to analyze ion migration and pore characteristics. The results indicated that when CaCl₂ was injected into the anode at the start of the experiment and Na₂SiO₃ was injected into the central region after the current had decayed by 70% from its peak, the drainage volume reached its maximum. Under these conditions, the average shear strength increased from nearly 0 kPa to 48.2 kPa, yielding the optimal reinforcement effect. The strength in both the central and cathode regions also improved, and the strength distribution between the anode and cathode became more uniform, with the strength ratio decreasing from 1.91 to 1.65, thereby enhancing the overall soil strength distribution. The Na⁺ concentration in the cathode effluent was highest, suggesting that Na⁺ migration played a predominant role in electroosmotic drainage. Furthermore, the electrochemical reactions generated cementitious materials that effectively filled the soil pores. SEM imaging and MIP pore size analysis revealed a reduction in porosity and an increase in soil compaction.

Keywords:

dredged sludge; electrochemical reinforcement; injection timing; reinforcement effect; mercury intrusion porosimetry; ion migration

1. Introduction

Coastal regions in China are widely distributed with dredged sludge soft clay foundations, while many countries around the world also have soft soils generated by dredging projects. Soft clays are characterized by high water content, high compressibility, low shear strength, and low permeability [1]. Effective treatment of soft clay is crucial for its use in building construction. Vacuum preloading is a common method for improving soft clay foundations; however, its efficiency is significantly limited by the soil’s hydraulic conductivity. In clay with low hydraulic conductivity, issues such as drainage board clogging and inadequate deep soil improvement frequently occur [2]. Although surcharge preloading can achieve better reinforcement outcomes, it is restricted by an excessively long consolidation period [3]. In contrast, the electro-osmotic method offers distinct advantages for treating soft clay. It is unaffected by hydraulic conductivity or particle size and ensures faster consolidation, particularly for soils with a higher particle content [4,5].

The electro-osmotic method was first applied in geotechnical engineering by Casagrande in 1939 [6], and extensive research has since been conducted worldwide. The electroosmotic method has certain limitations. Specifically, the directional flow of pore water from the anode to the cathode results in significantly higher vane shear strength in the anode region compared to the cathode region, leading to uneven reinforcement between the anode and cathode. To address these issues and optimize the effectiveness of electroosmotic reinforcement in soft soil foundations, Gray proposed combining chemical grouting with electroosmosis to enhance the electroosmotic effect [7,8]. Since then, numerous studies have investigated the reinforcement effects and ion migration mechanisms of electrochemical grouting [9,10,11,12]. Chien, Ou, and colleagues conducted laboratory electroosmotic experiments on kaolin and silty clay treated with KCl, NaCl, and CaCl₂ solutions. They found that injecting these salt solutions effectively improved electroosmotic efficiency, with higher-valence and higher-concentration solutions further enhancing the electroosmotic improvement quality [13,14]. Liu [15] tested remolded soil samples with different salinities of KCl, NaCl, and CaCl₂ solutions, concluding that CaCl₂ provided better reinforcement performance than KCl and NaCl, with an optimal Ca²⁺ salinity of approximately 1%. Shang et al. investigated the electrochemical reinforcement effects of various concentrations of CaCl₂ and Al₂(SiO₄)₃·H₂O solutions on marine calcareous soils. Their results showed that a 15% CaCl₂ solution yielded the maximum reinforcement effect [16]. Kong et al. [17] conducted electroosmotic experiments on transparent soil samples treated with calcium chloride solutions of varying concentrations. They demonstrated that increasing the solution concentration reduced electric potential loss, improved drainage rates, and accelerated settlement. Ou et al. [18] performed indoor electrochemical grouting experiments using CaCl₂, varying the voltage, electrode size, and electrode spacing. Their findings revealed that the size of the cemented region was independent of the cathode size and that increasing electric field intensity proportionally expanded the radius of the cathode cemented region. Although calcium chloride injection significantly improved the effectiveness of electrochemical treatment, the cementation zones remained confined to the vicinity of the anode or cathode, and the overall strength improvement was suboptimal.

To further improve the overall strength of the soil between the anode and cathode regions, Asavadorndeja and Glawe [19] injected an alkaline solution into the soil, thereby expanding the alkaline region. As a result, the cemented zone extended toward the anode. Their study concluded that the formation of cementitious substances in the soil by Ca²⁺ is related to the pH level in the soil. To expand the improvement region, a suitable operation procedure was established for the injection of calcium chloride solution, followed by the injection of sodium silicate solution at the anode [20,21]. The results indicate that the improvement region expanded from the anode to the cathode, and the average cone resistance and undrained shear strength increased by 125–130% from the anode to the cathode. An injection time for the calcium chloride solution greater than 36 h might improve the soil near the cathode, forming an impermeable layer around the cathode. Ou et al. [22,23,24] presented a novel electroosmotic chemical treatment method to improve the strength of the clay throughout the entire sample by injecting a 0.75 M CaCl₂ solution for 72 h, followed by a 1.5 M KOH solution for 48 h, a fresh sodium silicate solution for 72 h, and finally injecting deionized water for 168 h. The cone resistance after treatment could achieve a range of 1–5 MPa. Ren et al. found that the optimal reinforcement effect occurred when CaCl₂ solution was first injected at the anode when the current began to decline (after 4.5 h of electroosmosis) and Na₂SiO₃ solution was injected in the middle when the cathode ceased to discharge water. This method effectively improved the uneven reinforcement of the soil. However, at this stage, the injection of Na₂SiO₃ solution did not fully utilize cation migration for drainage [25,26,27]. The aforementioned experimental studies primarily focused on factors such as electrochemical treatment time, grouting types, injection locations, and solution concentrations. Compared to single-agent grouting, the combination of reagents was more effective in improving reinforcement uniformity. However, there is a lack of systematic research on the optimal timing for the injection of sodium silicate solution. The conductivity of soil samples under different conditions varies, and thus, a fixed electrochemical treatment time (e.g., 24 h [20,21] or 4.5 h [26]) cannot be universally applied for the Na₂SiO₃ injection. Since variations in current can comprehensively reflect changes in ion concentration and conductivity in the soil, the current measurements could be used as a criterion to determine the appropriate timings for Na₂SiO₃ injection.

This study investigates the optimal timing for injecting Na₂SiO₃ solution during the grouting process, which involves a combination of CaCl₂ and Na₂SiO₃ solutions. A preliminary experiment was conducted prior to the formal tests. During the initial stage of electroosmosis, CaCl₂ solution was injected at the anode to achieve the peak current in the electrochemical treatment process. Based on this peak current, Na₂SiO₃ solution was injected at various stages when the sample’s current reached 100%, 85%, 70%, 55%, and 40% of the peak current observed in the preliminary experiment. Laboratory observations during electrochemical treatment, strength tests following the completion of the electroosmosis test, and a discussion of the test results are presented herein. The objective of this study was to investigate the optimal sodium silicate injection timings for the treatment. The mechanism underlying the strength improvement through electrochemical treatment was also examined. The results presented in this paper may provide useful guidance for optimizing sodium silicate injection timings to enhance the effectiveness of electrochemical treatment in dredged sludge.

2. Test Materials and Methods

2.1. Experimental Soil Samples

The soil samples used in the experiments were collected from dredged sludge in the Dongtou District of Wenzhou. Prior to the experiment, impurities such as gravel and coarse particles were removed from the undisturbed soil by air-drying, crushing, and sieving the sediments. After adding distilled water, the samples were reconstituted to achieve a water content of 75%. The samples were then allowed to rest for 24 h. The basic parameters of the soil samples are listed in Table 1, following the standard for geotechnical testing methods (GB-T 50123-2019) [28].

2.2. Experimental Apparatus

The experiment employed an electro-osmotic consolidation model box made of organic glass, featuring a central testing soil sample chamber and an anode chamber and cathode chamber on each side. A schematic of the experimental setup is shown in Figure 1. The internal dimensions of the testing chamber were 500 mm × 200 mm × 200 mm, and each side collection tank measured 50 mm × 200 mm × 200 mm internally. A drainage hole with a 5 mm radius was located at the center of the drainage channel, with a graduated cylinder positioned beneath the cathode drainage hole to collect discharged water. The anode consisted of an iron plate measuring 220 mm × 200 mm × 2 mm, while the cathode was an iron mesh of identical dimensions, facilitating drainage throughout the experiment. Permeable geotextile was placed on the outer sides of the electrodes, serving as a filter layer. The chemical injection grouting pipe was constructed from a perforated PPR pipe with a 10 mm diameter, sealed at the bottom to ensure uniform diffusion of chemical reagents at various soil depths. A GWSPD-3606 regulated direct current (DC) power supply was used as the power source.

2.3. Experimental Method

In this study, CaCl₂ solution was injected into the anode zone through a grouting pipe at the start of electro-osmosis. Na₂SiO₃ solution was subsequently injected into the middle of the soil when the current decayed to 100%, 85%, 70%, 55%, and 40% of its peak value. To determine the peak current (I_max), two preliminary tests were conducted prior to the formal experiment. In these tests, the prepared soil samples were placed in a model box, and 100 mL of 0.5 mol/L CaCl₂ solution was injected into the anode region. During the experiment, the output voltage was controlled at 25 V (volt), resulting in an electrical potential gradient of 0.5 V/cm across the soil. Electroosmotic drainage was then initiated, and the current variation over time was recorded. Initially, the current increased gradually, reaching a peak before decaying. The average peak current from the two preliminary tests was 2.3 A (Ampere), which was defined as I_max. In the formal experiment, 100 mL of 0.5 mol/L Na₂SiO₃ solution was injected into the center of the soil, while the other experimental conditions remained consistent with those of the preliminary tests. A total of five experimental groups were established to investigate the effect of different timings of chemical solution injection on electrochemical soil stabilization (as shown in Table 2). To monitor energy consumption variations within the soil, potential probes were placed 10 mm from the electrodes.

2.3.1. Experimental Procedure

The detailed experimental procedure is as follows: (1) A thin layer of petroleum jelly was evenly applied to the inner walls of the model box to minimize frictional resistance between the walls and the soil samples, as well as to facilitate easy separation and cleaning of the box afterward. The electrode plates and grouting pipes, pre-moistened with deionized water, were then placed in the model box as illustrated in the schematic diagram. (2) The soil samples were added in layers, with each layer compacted and leveled to eliminate air bubbles. The power supply was connected to the soil sample, with the positive electrode attached to the anode and the negative electrode to the cathode. A plastic measuring cup was placed beneath the cathode drainage port to collect discharged water. (3) The CaCl₂ solution was injected into the grouting pipe at the anode. Once the solution had fully diffused into the soil, the output voltage was set to 25 V, and the power supply was activated to initiate electro-osmosis. (4) The current was monitored from the DC power supply. Based on the preliminary test results, Na₂SiO₃ solution was injected into the center of experiment S1 when the current reached 2.3 A. Similarly, Na₂SiO₃ injection was initiated at the center of experiments S2 through S5 when the current decayed to 1.96 A, 1.61 A, 1.27 A, and 0.92 A, respectively.

2.3.2. Measurement of Experimental Parameters

During the initial phase of the experiment, when the changes in values were rapid, the weight of the water collected in the measuring cup was recorded every hour to calculate the drainage rate. The discharged water from each time interval was collected separately in bottles for inductively coupled plasma mass spectrometry (ICP-MS) analysis. Current values within the soil were also recorded from the power supply. After 12 h, as the rate of change slowed, measurements were taken every 2 h, and the collected water was also stored separately. Following the injection of Na₂SiO₃ solution, the ion concentration in the effluent could vary significantly. Therefore, effluent samples were collected every hour for the first four hours, with a total of four collections. Afterward, the effluent was collected every four hours. After 20 h of testing, measurements and sample collections continued at four-hour intervals. When the drainage rate of all experimental groups dropped below 5 mL/h, electroosmotic drainage was terminated, and the DC power supply was disconnected to conclude the experiment [26].

Upon completion of the experiment, settlement, shear strength, and moisture content of the soil were measured at designated points, as illustrated in the schematic diagram (Figure 1b). The electrodes were removed, and their mass was measured. Soil samples were extracted from the measurement points for porosity analysis using MIP and SEM. The tests were conducted using the AutoPore 9500 high-performance fully automated mercury porosimeter (Test standard ASTM).

3. Results and Discussion

The effect of electrochemical treatment on dredged sludge was studied through measurements of current, drainage rate, total drainage volume, settlement, moisture content, vane shear strength, changes in ion concentration of discharged water, and analysis using scanning electron microscopy (SEM) and mercury intrusion porosimetry (MIP). The mechanism underlying the increase in dredged sludge strength is discussed in the following section.

3.1. Current

The variations in soil current for each experimental group are shown in Figure 2. In all five groups, 100 mL of 0.5 mol/L CaCl₂ solution was injected at the start of electro-osmosis. During the initial phase, the current gradually increased, reaching a maximum value of 2.3 A at the 7th hour for all groups, after which it began to decline. Na₂SiO₃ solution was injected into the central pipe at different current values. During electro-osmosis, fine cracks developed in the soil, and the Na₂SiO₃ solution partially filled these cracks, reducing interface resistance and introducing a substantial number of cations, which caused a noticeable surge in current. Comparing the amplitude of the current surges across experimental groups, it was observed that delaying the Na₂SiO₃ injection appropriately resulted in a higher surge amplitude. However, injecting Na₂SiO₃ too late led to a diminished current surge. This was because, in the later stages of the experiment, large cracks had already formed in the soil. While Na₂SiO₃ injection could mitigate some potential losses, the overall resistance remained high, leading to a less pronounced current surge.

After 50 h, the current stabilized across all groups, showing minimal further variation. The stabilized current values followed the order: S3 > S4 > S2 > S1 > S5. Injecting Na₂SiO₃ when the current decayed to 70% of its peak value resulted in the highest stabilized and average current throughout the experiment. This outcome is attributed to the alkaline environment that developed near the cathode during the middle and later stages. The presence of Ca²⁺ ions facilitated the formation of cementitious materials from the soil center to the cathode, delaying crack propagation and reducing overall soil resistance. In contrast, injecting Na₂SiO₃ too early—before an alkaline environment formed near the cathode—consumed free Ca²⁺ ions, resulting in the formation of insoluble CaSiO₃ precipitates in the soil center. This reduced the availability of Ca²⁺ for subsequent cementation reactions, thereby increasing overall soil resistance. Conversely, injecting Na₂SiO₃ too late, when soil resistance was already high, led to a lower increase in current and a reduced average current throughout the experiment.

3.2. Drainage Rate and Total Drainage Volume

The variations in total drainage volume for each group are shown in Figure 3. Prior to the injection of Na₂SiO₃ solution, minimal changes in total drainage volume were observed across the groups. However, a significant increase was observed after the Na₂SiO₃ solution was injected. By the end of the experiment, the total drainage volumes for the S1 to S5 groups were 2705 mL, 2876 mL, 3186 mL, 2989 mL, and 2745 mL, respectively. These results indicate that adjusting the timings of sodium silicate injection can effectively increase the total drainage volume. The highest drainage volume was achieved when Na₂SiO₃ solution was injected when the current had decreased to 70% of its peak value. This is attributed to the fact that injecting Na₂SiO₃ too early leads to the formation of insoluble CaSiO₃ precipitates in the middle of the soil, as SiO₃²⁻ reacts with Ca²⁺, consuming free-moving Ca²⁺ ions and blocking drainage channels, thereby hindering drainage. Conversely, if the injection is delayed too long, large cracks will have already formed in the soil, reducing both the current and drainage rate and resulting in a lower overall drainage volume.

Figure 4 illustrates the changes in drainage rate over time. The trend of the drainage rate closely mirrors that of the current: it initially increases, then decreases before the injection of Na₂SiO₃ solution. Following the injection, the drainage rate exhibits a surge, followed by a rapid decline. This behavior is attributed to electro-osmotic drainage, which primarily depends on the movement of free cations that transport water molecules toward the cathode. The injection of Na₂SiO₃ solution increases the number of free-moving cations, causing a surge in the drainage rate and enhancing the drainage process. After 40 h, the drainage rate stabilizes. Comparing the current and drainage rate curves, it is evident that the S3 group maintains the highest stable drainage rate, consistent with the current trend. These observations underscore the importance of injecting CaCl₂ solution at the anode at the start of electro-osmosis, followed by the timely injection of Na₂SiO₃ solution. Injecting Na₂SiO₃ too early reduces the availability of Ca²⁺ ions for ion exchange, leading to the formation of CaSiO₃ precipitates, which block drainage channels and reduce the total drainage volume. Conversely, injecting Na₂SiO₃ too late, when the soil current is low and cracks are already pronounced, has minimal impact on the drainage rate and results in lower overall drainage. Injecting Na₂SiO₃ at the optimal timing significantly enhances the total drainage volume.

3.3. Settlement

As the electro-osmosis process progresses, cations, influenced by the electric field, drag water molecules from the anode to the cathode, resulting in water discharge. This movement creates numerous pores in the soil, and under the combined effects of soil self-weight and interparticle forces, varying degrees of surface settlement occurs. Measurements taken at different points in the experimental groups using a steel ruler are shown in Figure 5. Settlement on both sides of the soil sample is consistently lower than that in the middle, with the maximum settlement observed at the center. This phenomenon is attributed to the boundary effect, as the sides of the soil are in close proximity to the electrode plates. Settlement near the anode is primarily caused by the expulsion of pore water, while the cathode area, which retains higher moisture content, becomes alkaline in the later stages of the experiment. This alkalinity promotes the formation of calcium silicate hydrate (CSH) and calcium aluminate hydrate (CAH) gels that fill the pores, resulting in minimal settlement near the cathode. In contrast, cracks develop in the middle of the soil during the later stages, causing the drainage paths to shift downward, which leads to the greatest settlement at the center. The settlement patterns across the experimental groups correspond to the drainage volume trends: groups with higher drainage volumes also exhibit higher settlement. Injecting Na₂SiO₃ solution too early causes significant CaSiO₃ precipitate formation in the soil center. These precipitates block drainage channels, causing water to accumulate in the central region and reducing settlement. Additionally, the excessive consumption of Ca²⁺ ions limits the formation of cementitious materials at the cathode, further decreasing settlement. Conversely, injecting Na₂SiO₃ too late, when the current is already low, results in a lower overall drainage volume compared to the S3 group, leading to less settlement.

3.4. Moisture Content and Vane Shear Strength

Soil samples were collected from each experimental group at designated measurement points, including the surface, middle, and bottom layers. The average of these measurements was used to determine the mean moisture content at each point, as illustrated in Figure 6. The moisture content across all five experimental groups ranged from 30% to 50%, representing a reduction of 31.4% to 45.1% compared to the initial moisture content of the remolded soil. The mean moisture content for each group followed the order S3 > S4 > S2 > S5 > S1, consistent with the trends observed in drainage volume. Regions I and III exhibited the lowest moisture content, followed by region II, while region IV had the highest moisture content. This distribution is attributed to the initial injection of CaCl₂ solution at the anode, which introduced a significant amount of Ca²⁺ ions. The increased concentration of Ca²⁺ facilitated ion exchange reactions, accelerating drainage and reducing moisture content near the anode. Following the injection of Na₂SiO₃ solution in the middle of the soil, Na⁺ ions dragged water molecules toward the cathode, enhancing drainage in region III. By the end of the experiment, a considerable amount of pore water remained near the cathode, resulting in higher moisture content in that area.

Figure 7 illustrates the distribution of shear strength across the soil samples. Shear strength is influenced by both moisture content and the cementitious substances formed through chemical solution injection. The moisture content curve indicates that areas with higher moisture content exhibit lower shear strength, and vice versa. After the injection of Na₂SiO₃ solution in the middle of the soil, the solution diffused throughout the soil matrix, forming CaSiO₃ precipitates and SiO₂, which filled the voids created by drainage in region II, thereby increasing shear strength. Although the moisture content near the cathode remained the highest, pozzolanic reactions in the acidic environment of the soil adjacent to the cathode during the later stages of the experiment produced calcium silicate hydrate (CSH) and calcium aluminate hydrate (CAH) gels, which partially increased shear strength in this region.

The average vane shear strength values for the sampling points of all experimental groups were calculated. The average strengths for tests S1 to S5 were 42.11 kPa, 45.94 kPa, 48.2 kPa, 44.84 kPa, and 42.78 kPa, respectively. The ratio of the strength in the anode region to that in the cathode region was defined as the unevenness coefficient. The unevenness coefficients for tests S1 to S5, under different Na₂SiO₃ solution injection timings, were 1.91, 1.74, 1.65, 1.69, and 1.76, respectively. Comparing the shear strength and moisture content curves, it can be concluded that under the experimental conditions of test S3, the moisture content was minimized, the average shear strength reached its maximum value of 48.2 kPa, and the unevenness coefficient was the lowest, thereby achieving the optimal reinforcement effect.

3.5. Changes in Ion Concentration of Discharged Water

In the early stages of the experiment, the concentration of Ca²⁺ in the discharged water was similar across all groups, exhibiting a rapid increase, as shown in Figure 8. This trend occurred because the initial injection of CaCl₂ solution at the anode raised the concentration of Ca²⁺ in the soil, which was subsequently expelled with the water. In the S1 and S2 experimental groups, the Ca²⁺ concentration in the discharged water began to decrease after the injection of Na₂SiO₃ solution. Additionally, the peak Ca²⁺ concentrations in these groups were lower than those in the latter groups (S3–S5). This difference is attributed to the premature injection of Na₂SiO₃ solution, where the introduced SiO₃²⁻ reacted with Ca²⁺ in the soil center, forming insoluble CaSiO₃ precipitates. This reaction depleted the free-moving Ca²⁺, leading to a continuous decline in Ca²⁺ concentration in the discharged water. In contrast, the trends in the S3 to S5 groups were similar, with a gradual decline of Ca²⁺ concentration after reaching a peak. The peak Ca²⁺ concentrations in these groups were also comparable. This is because, after the current dropped to 70% of its peak value, an alkaline environment gradually developed near the cathode. This environment facilitated the formation of calcium aluminate hydrate (CAH), calcium silicate hydrate (CSH), and Ca(OH)₂ cementitious materials from the free Ca²⁺ moving from the soil center to the cathode, thereby reducing the Ca²⁺ concentration in the discharged water.

Figure 8b illustrates the changes in Na⁺ concentration in the discharged water for each experimental group. The Na⁺ concentration was significantly higher than that of other ions, primarily because ions with a smaller atomic mass and lower valence state, such as Na⁺, migrate more rapidly under an electric field compared to higher valence cations [29]. In all five groups, the Na⁺ concentration increased to a peak and then gradually declined. Following the injection of CaCl₂ solution at the start of electro-osmosis, the Na⁺ concentration in the discharged water rose rapidly due to the introduction of large amounts of Ca²⁺. The increased Ca²⁺ concentration facilitated ion exchange, displacing Na⁺ and K⁺ from soil particles and promoting Na⁺ discharge in the early stages. After the injection of Na₂SiO₃ solution, Na⁺ discharge increased in all groups, with similar peak concentrations observed across the experiments. The injection introduced a substantial amount of Na⁺ into the soil center, enhancing drainage from the center to the cathode. Among all groups, the S3 group exhibited the highest Na⁺ discharge. In the S1 and S2 groups, excessive consumption of Ca²⁺ reduced the amount of Ca²⁺ available for ion exchange, resulting in lower Na⁺ displacement. In the S4 and S5 groups, the delayed injection of Na₂SiO₃ occurred when the soil current was already low, reducing Na⁺ migration speed and quantity. Injecting Na₂SiO₃ solution when the current was at 70% of its peak value ensured that Ca²⁺ available for ion exchange was not prematurely depleted, while the soil current remained high, optimizing Na⁺ migration.

Figure 8c illustrates the changes in K⁺ concentration in the discharged water. The K⁺ concentration increased gradually and remained relatively consistent across all groups, indicating that the timings of Na₂SiO₃ solution injection did not significantly affect the involvement of K⁺ in the drainage process. During the early stages, when the drainage rate was high, the primary contributors to drainage were the introduced Ca²⁺ and Na⁺ ions. As the drainage rate decreased and the concentrations of Ca²⁺ and Na⁺ diminished, the K⁺ concentration in the discharged water gradually increased.

3.6. Scanning Electron Microscopy (SEM) Analysis

After the completion of the electro-osmosis experiments, various sizes of pores and cracks were observed in the soil. This phenomenon occurs because electro-osmotic reinforcement primarily relies on cations dragging water molecules toward the cathode, expelling water without compressing the soil skeleton. As water is expelled, pores form in the soil, and the soil gradually compresses under gravity and interparticle forces. Variations in pore structures are attributed to variations in soil moisture content and the degree of chemical cementation within the soil. To compare pore structures in different regions, SEM analysis was conducted on samples from the anode, middle, and cathode regions. The samples were magnified 3000 times for observation and analysis, with a focus on the S3 experimental group. SEM images from the S3 group show that the anode region contains numerous small and relatively uniform pores (Figure 9). These small pores result from the movement of water molecules; as water migrates toward the cathode and is expelled, micro-pores form under the combined effects of soil self-weight and interparticle forces. In contrast, SEM images from the middle region reveal a denser structure, with most of the small pores disappearing. Soil particles are tightly bonded, significantly reducing pore volume, which explains why the middle region exhibits the highest shear strength. The cathode region, however, contains many large pores, primarily due to the higher moisture content. Even after freeze-drying the soil samples from the cathode, large pores remain, highlighting the effect of water retention in this region.

3.7. MIP Pore Analysis

Mercury intrusion porosimetry (MIP) tests were conducted on soil samples from the anode, middle, and cathode regions to assess pore characteristics under different injection timings. To preserve the pore structure and avoid sample disturbance, the soil samples were immediately immersed in liquid nitrogen after electrochemical reinforcement, then freeze-dried in a vacuum dryer. Figure 10 presents the cumulative mercury intrusion curves. The cumulative mercury intrusion volumes for the soil samples from the anode region in experiments S1 to S5 were 0.28 mL/g, 0.304 mL/g, 0.307 mL/g, 0.296 mL/g, and 0.304 mL/g, respectively. For the soil samples from the central region, the cumulative mercury intrusion volumes were 0.316 mL/g, 0.291 mL/g, 0.267 mL/g, 0.283 mL/g, and 0.299 mL/g, respectively. The cumulative mercury intrusion volumes for the soil samples from the cathode region were 0.391 mL/g, 0.375 mL/g, 0.33 mL/g, 0.348 mL/g, and 0.363 mL/g, respectively. In all experimental groups, the cumulative mercury intrusion volume was lowest in the middle region, followed by the anode, and highest at the cathode. This trend aligns with the distribution of shear strength, indicating that the middle region had the smallest pore volume, while the outer regions had larger pores. The differences in pore volume at the anode were minimal among the experimental groups, likely due to similar soil moisture content across these regions. The S3 group exhibited the lowest cumulative mercury intrusion volume, representing the smallest total pore volume. This outcome is attributed to the optimal timing of Na₂SiO₃ injection. Injecting Na₂SiO₃ too early would lead to significant consumption amounts of Ca²⁺ when the current is still high, reducing drainage and limiting the formation of cementitious materials in the middle to cathode regions during the later stages. This premature injection also exacerbates soil cracking and increases overall pore volume. Conversely, injecting Na₂SiO₃ too late allows larger cracks to develop in the soil. Although more cementitious materials form from Ca²⁺ in the middle to cathode regions, the overall current and drainage volume remain lower than in the S3 group, resulting in a larger total pore volume. In the S3 group, Na₂SiO₃ was injected when the current dropped to 70% of its peak, maintaining the lowest moisture content. This optimal timing prevented excessive Ca²⁺ consumption in the formation of CaSiO₃ precipitates in the middle region, sustaining a higher current level. Consequently, cementitious materials such as Ca(OH)₂, calcium aluminate hydrate (CAH), and calcium silicate hydrate (CSH) formed during the later stages, filling pores and cracks, reducing pore volume, and inhibiting further cracking.

Figure 11 shows the differential pore size distribution curves for the different regions of the soil. The curves exhibit a single-peak distribution pattern, with the peak corresponding to the dominant pore size. Analysis of the anode region pore size distribution curve (Figure 11a) reveals that the timing of grouting has little effect on the pore structure in the anode region, with the dominant pore size for all experimental groups ranging between 500 and 600 nm. By comparing Figure 11b,c, it is evident that for experiment S3, where Na₂SiO₃ is injected when the current has decayed 70% of its peak value, the “peak” is the sharpest, indicating the largest peak value. This suggests that injecting Na₂SiO₃ solution at this stage results in a greater concentration around the dominant pore size, with the peak of the pore size curve shifting to the left. As a result, the dominant pore size decreases, leading to a reduction in the overall pore diameter of the soil. In the middle region, significant differences are observed in the dominant pore size, which are 850 nm, 750 nm, 550 nm, 600 nm, and 700 nm, respectively. These variations in pore size distribution correlate with the shear strength patterns, with the S3 group exhibiting the smallest dominant pore size at 550 nm, indicating a more compact and optimized soil structure. At the cathode region, the dominant pore size distribution in the T1 and T2 groups is more irregular, with a predominance of large pores and a weaker structure. This is attributed to the high moisture content in the cathode region and the limited diffusion of Na₂SiO₃ when injected too early, resulting in fewer cementitious materials and inadequate reinforcement. The dominant pore sizes for the remaining three groups are 600 nm, 800 nm, and 900 nm, showing a pattern consistent with that observed in the middle region.

Analyzing Figure 10 and Figure 11 together reveals that the timings of Na₂SiO₃ injection has minimal impact on the cementation and precipitation effects at the anode, where reinforcement primarily relies on drainage. The middle region, characterized by lower moisture content and significant precipitation of cementitious materials, exhibits the smallest pore volume. In contrast, the cathode region, with higher moisture content, depends on the formation of calcium aluminate hydrate (CAH) and calcium silicate hydrate (CSH) gels under alkaline conditions to reinforce the soil, which is consistent with SEM observations. Early injection of Na₂SiO₃ depletes free-moving Ca²⁺, reducing the formation of cementitious material near the cathode, leading to weaker reinforcement and a larger total pore volume. Additionally, premature injection results in higher soil moisture content and an increased overall pore volume.

3.8. Discussion

The mechanism of electrochemical reinforcement for enhancing sludge strength, shown in Figure 12, can be analyzed from two perspectives. On the one hand, electroosmotic treatment not only drains free water from the pores but also removes a portion of the weakly bound water, thereby enhancing the soil strength. On the other hand, the electrochemical reactions at the anode and cathode, combined with the chemical cementation reactions, significantly contribute to strength enhancement. Initially, the injection of CaCl₂ solution increases the Ca²⁺ concentration in the soil, enhancing its conductivity. Subsequently, the Na₂SiO₃ solution raises the Na⁺ concentration. The cations carry water molecules and migrate toward the cathode, expelling more pore water and thus consolidating the sediment. Upon injection of Na₂SiO₃, the solution promotes the alkaline environment, and some SiO₃²⁻ ions react with Ca²⁺ to form CaSiO₃ precipitates, which help fill the pores and strengthen the central region of the soil. Meanwhile, some SiO₃²⁻ ions migrate toward the anode. At the anode, water electrolysis creates an acidic environment, causing SiO₃²⁻ to precipitate as SiO₂, which promotes strength enhancement in the anode region. In the cathode region, electrolysis generates an alkaline environment, with OH⁻ ions migrating toward the central region, where they react with Ca²⁺ to form calcium silicate hydrate (CSH), calcium aluminate hydrate (CAH), and Ca(OH)₂ colloids, thereby improving the strength throughout the anode–cathode span [21]. The main chemical reactions formulas for field testing are as follows. The unevenness coefficient of strength decreased from 1.91 to 1.65, addressing the limitations of previous studies that focused solely on strengthening the anode or cathode regions.

Near anode : 2 H_{2} O - 4 e^{-} \to O_{2} + 4 H^{+}; H^{+} + S i O_{3}^{2 -} \to Si O_{2};

Middle region : {C a}^{2 +} + S i O_{3}^{2 -} \to {C a S i O}_{3};

From middle to cathode : 2 H_{2} O + 2 e^{-} \to 2 {O H}^{-} + H_{2}; {C a}^{2 +} + 2 {O H}^{-} \to {C a (O H)}_{2};

{C a}^{2 +} + 2 {O H}^{-} + S i O_{2} \to CSH; {C a}^{2 +} + 2 {O H}^{-} + {A l}_{2} O_{3} \to CAH;

Figure 12. Mechanism of injection time of the sodium silicate solution during the electrochemical treatment.

It is important to note that the timing of Na₂SiO₃ solution injection is critical, with the optimal moment occurring when the current in the soil decays to 70% of its peak value. At this point, the drainage volume is maximized and the vane shear strength is at its highest. This optimal timing is due to the fact that the injection of Na₂SiO₃ relative to the current influences both drainage and cementation reactions in the soil. If injection occurs before the current reaches 70%, excessive Ca²⁺ ions in the central region are consumed by SiO₃²⁻ ions, which impedes electroosmotic drainage. Additionally, an adequate alkaline environment has not yet developed in the cathode region. If the Na₂SiO₃ solution is injected too late, the current will be lower, and a significant portion of Ca²⁺ ions will have already formed impermeable colloids in the alkaline environment of the cathode region. At this stage, Na⁺ ions from the injection will have a weak effect on promoting drainage, thereby limiting the advantages of solution in the later stage of experiment.

4. Conclusions

This study, through experiments with five different Na₂SiO₃ injection timings, demonstrated that the timings of Na₂SiO₃ injection directly impact the macroscopic reinforcement effects of electro-osmotic consolidation and explored the underlying mechanisms of micro-ion migration. The key conclusions are as follows:

(1): The timings of Na₂SiO₃ solution injection significantly influence the electroosmotic reinforcement of soft soil, with an optimal injection time identified. When Na₂SiO₃ solution is injected into the middle of the soil at the point when the current decays to 70% of its peak value, the electroosmotic reinforcement effect is maximized. This results in the highest drainage volume and the greatest average shear strength, which reaches 48.2 kPa.
(2): The use of CaCl₂ and Na₂SiO₃ solutions for electrochemical grouting in silts enhances the shear strength between the anode and cathode, reducing the unevenness coefficient to 1.65. This effectively mitigates the issue of uneven reinforcement typically observed between the anode and cathode in traditional electroosmotic drainage methods.
(3): In the effluent from the cathode, the discharge of Na⁺ ions is significantly higher than that of K⁺ and Ca²⁺. Furthermore, after the injection of Na₂SiO₃ solution, the discharge of Ca²⁺ begins to decrease. Premature injection of Na₂SiO₃ solution reduces the discharge of Ca²⁺. By controlling the timings of Na₂SiO₃ injection, the discharge of Na⁺ can be maximized, leading to the highest cation migration, a reduction in the overall pore volume of the soil, and optimal filling of soil pores by the cementing material.
(4): Based on the experimental conditions, it is recommended in engineering practice to use the current as the criterion for determining the injection timings of Na₂SiO₃ solution. According to preliminary electrochemical experiments in silts, CaCl₂ solution should be injected at the initial stage of electroosmosis in the anode region, with Na₂SiO₃ solution injection beginning when the current decays 70% of its peak value.

Author Contributions

Conceptualization, J.L. and X.L.; methodology, H.F. and G.Y.; software, G.Y., X.L. and Y.F.; validation, Y.F.; formal analysis, J.L.; investigation, L.W. and Z.G.; resources, H.F.; data curation, Y.F.; writing—original draft preparation, M.A., J.L. and Y.F.; writing—review and editing, J.L. and G.Y.; visualization, H.F.; supervision, G.Y. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Natural Science Foundation of China (Grant No. 52108338, No. 52178350) and the Programs of Science and Technology of Wenzhou (No. S20220008, No. S2023005).

Data Availability Statement

The original contributions presented in the study are included in the article, further inquiries can be directed to the corresponding author.

Acknowledgments

The findings presented in this article were made possible through the diligent guidance of the authors’ supervisor, as well as the care and help of the teachers in the research group. With their collective help, the authors were able to obtain the lake test data. The unwavering support of the family and girlfriend of one of the authors was also instrumental. The authors extend sincere gratitude to all of them.

Conflicts of Interest

Author Jiangdong Lin was employed by the company Jiangxi Zhongmei Engineering Group Co., Ltd. Author Mi Ai was employed by the company Zhejiang Geology and Mineral Technology Co., Ltd. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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Figure 1. Testing model box. (a) Schematic drawing. (b) Detection arrangement point.

Figure 2. Variation of current during treatment for different injection timing.

Figure 3. Variation of total drainage volume during treatment for different injection timing.

Figure 4. Variation of drainage rate during treatment for different injection timing.

Figure 5. Settlement with time.

Figure 6. Variation in moisture content with distance from the anode.

Figure 7. Variation in undrained shear strength with distance from anode.

Figure 8. Variation in (a) Ca²⁺, (b) Na⁺, and (c) K⁺ ions concentration with time in the discharged water.

Figure 9. SEM image of the microstructure of soil sample at test S3 after treatment: (a) near the anode; (b) center of soil; (c) near the cathode.

Figure 10. Cumulative mercury intrusion (a) near the anode, (b) near the middle, and (c) near the cathode after treatment.

Figure 11. Pore size diameter distribution (a) near the anode, (b) near the middle, (c) near the cathode after treatment.

Table 1. Basic physical and mechanical indicators of soil.

Physical Properties	Value
$Specific gravity, G_{s}$	2.75
Water content, w (%)	75
$Liquid limit, w_{L}$ (%)	52.1
$Plastic limit, w_{P}$ (%)	25.4
Acid-base properties,	7.4
$Void ratio, e_{0}$	1.68
Undrained shear strength, kPa	~0
Hydraulic conductivity k (10⁻⁸ cm/s)	3.56
Particle size distribution, <0.002 mm (%)	28.37
Particle size distribution, 0.002~05 mm (%)	56.86
Particle size distribution, 0.05~2 mm (%)	14.77

Table 2. Summary of electrochemical reinforcement tests.

Test	Type of Anode Injection	Injection Timings	Middle of the Soil Injection	Injection Timings	Potential Gradient
S1	CaCl₂	0 h	Na₂SiO₃	I_max	0.5 V/cm
S2	CaCl₂		Na₂SiO₃	85% I_max
S3	CaCl₂		Na₂SiO₃	70% I_max
S4	CaCl₂		Na₂SiO₃	55% I_max
S5	CaCl₂		Na₂SiO₃	40% I_max

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Lin, J.; Ai, M.; Yuan, G.; Wang, L.; Gao, Z.; Li, X.; Fu, H.; Fan, Y. Study on the Effect of Sodium Silicate Solution Injection Timings on Electrochemical Reinforcement of Dredged Sludge. Buildings 2025, 15, 70. https://doi.org/10.3390/buildings15010070

AMA Style

Lin J, Ai M, Yuan G, Wang L, Gao Z, Li X, Fu H, Fan Y. Study on the Effect of Sodium Silicate Solution Injection Timings on Electrochemical Reinforcement of Dredged Sludge. Buildings. 2025; 15(1):70. https://doi.org/10.3390/buildings15010070

Chicago/Turabian Style

Lin, Jiangdong, Mi Ai, Guohui Yuan, Long Wang, Ziyang Gao, Xiaobing Li, Hongtao Fu, and Yongfei Fan. 2025. "Study on the Effect of Sodium Silicate Solution Injection Timings on Electrochemical Reinforcement of Dredged Sludge" Buildings 15, no. 1: 70. https://doi.org/10.3390/buildings15010070

APA Style

Lin, J., Ai, M., Yuan, G., Wang, L., Gao, Z., Li, X., Fu, H., & Fan, Y. (2025). Study on the Effect of Sodium Silicate Solution Injection Timings on Electrochemical Reinforcement of Dredged Sludge. Buildings, 15(1), 70. https://doi.org/10.3390/buildings15010070

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Study on the Effect of Sodium Silicate Solution Injection Timings on Electrochemical Reinforcement of Dredged Sludge

Abstract

1. Introduction

2. Test Materials and Methods

2.1. Experimental Soil Samples

2.2. Experimental Apparatus

2.3. Experimental Method

2.3.1. Experimental Procedure

2.3.2. Measurement of Experimental Parameters

3. Results and Discussion

3.1. Current

3.2. Drainage Rate and Total Drainage Volume

3.3. Settlement

3.4. Moisture Content and Vane Shear Strength

3.5. Changes in Ion Concentration of Discharged Water

3.6. Scanning Electron Microscopy (SEM) Analysis

3.7. MIP Pore Analysis

3.8. Discussion

4. Conclusions

Author Contributions

Funding

Data Availability Statement

Acknowledgments

Conflicts of Interest

References

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