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Article

Study on Mechanical Properties of Sulfate Saline Soil Improved by CLI-Type Polymer Active Agent

by
Xufen Zhu
,
Zhuoqun Yang
,
Jiaqiang Zheng
,
Jin Liu
*,
Fan Bu
,
Chengjiang Dai
and
Yipin Lu
School of Earth Sciences and Engineering, Hohai University, Nanjing 210098, China
*
Author to whom correspondence should be addressed.
Appl. Sci. 2023, 13(19), 10727; https://doi.org/10.3390/app131910727
Submission received: 7 August 2023 / Revised: 25 September 2023 / Accepted: 25 September 2023 / Published: 27 September 2023
Browse Figures
Versions Notes

Abstract

:
Large amounts of soluble salts in a soil enhance the soil sensitivity to changes in its properties induced by changes in environmental conditions, such as easy dissolution in water and easy occurrences of salt heaving in low-temperature environments, which make the soil volume swell rapidly, leading to a series of engineering disasters. Moreover, the growth and development of surface vegetation will be inhibited due to excessive salinity, resulting in a gradual decline in the ecological functionality of the area. A polymer active agent (CLI) was selected for the ecological improvement of sulfuric acid saline soils. Triaxial compression tests and a test on the soluble salt content of the treated soil were carried out to investigate the effects of polymer active agent content and maintenance time on the mechanical properties and soluble salt content of sulfate saline soils. The results showed that the addition of CLI can improve the soil strength by increasing the cohesion of the specimen, and the improvement increases significantly with the content of CLI and the curing age. Meanwhile, the calcium ions in CLI can react with sulfate ions in sulfate-salted soils to produce calcium sulfate precipitation to alleviate soil salinization. The scanning electron microscopy (SEM) images indicated that an appropriate content of CLI (about 8%) can strengthen the soil structure through an excellent chelating ability, enhancing the strength of the soil.

1. Introduction

Globally, nearly one billion hm2 of land is salinized [1,2], and the soil salinization area is still expanding [3]. Saline soils hurt the geotechnical application and environmental sustainability because of their collapsibility, salt swelling, and corrosiveness. According to the main salt components, saline soils can be divided into carbonate, sulfate, sub-sulfate, chloride, and sub-chloride saline soils [4].
Sulfate saline soils are widely distributed in the world. Sodium sulfate in soils can absorb water and crystallize, resulting in an expansion in soil volume [5]. The distress or failure of infrastructures caused by sulfate-induced swelling has arisen in construction practices, such as subgrades, soil slopes, and tunnels. Soils containing soluble salts that suffer from thermal load may result in changes in dissolution rates, triggering uneven ground settlement and the deformation of building foundations [6]. A large deterioration has been found in the mechanical properties of frozen saline soils [7,8] because sulfates can intensely change the structural characteristics of the soils. Soil salinity weakens the mechanical characteristics of the loess through the alteration of the microstructure (e.g., pore distribution and particle morphology) [9]. Vegetation that grows in saline soils has undergone severe osmotic stress and salinity toxicity [10], resulting in low germination percentage and speed [11,12] and further deterioration of saline soils [13]. Hence, it is necessary to investigate effective amelioration methodologies to reduce the adverse effects of saline soils.
Numerous studies have investigated the remediation of saline soils. Physical (e.g., vibratory compacting) and chemical treatment methods are typically used to improve soil properties. Lime [14], cement [15], fly ash [16], and gypsum [17] have been demonstrated to improve the strength properties, capacity, and durability of soils. However, the presence of sulfates in the treated soils can induce the formation of expansive minerals, resulting in volume swelling, strength loss, and numerous heaves in earthworks [18], further ruining the efficiency of these treatments [19,20]. Moreover, these approaches generally have disadvantages such as high energy consumption and high cost and maintenance, sometimes leading to the secondary pollution of the environment [21]. Therefore, there is an increasing demand for an effective and eco-friendly measure to ameliorate soil salinity.
Polyacrylamide utilization in saline soils can inhibit the accumulation of soil salinity and increase soil fertility [22]. Polyacrylamide addition increased water retention and reduced the cracking degree of saline soils [23,24,25]. A study indicated that polyacrylamide addition improved plant growth in saline soils by restricting the transport of salt ions [26]. Biochar can effectively modify the physical, chemical, and biological characteristics of saline soils [27]. Biochar addition provides carbon [28,29] and other essential soil mineral nutrients, thereby significantly improving vegetation productivity [30] and soil micro-biological properties [31]. Lignin, a kind of byproduct collected from paper mills, performs efficiently in improving unconfined compressive strength [32], shear strength [33], durability [34], and erosion resistance [35]. In addition, the application of lignin-based hydrogels to saline soils increases the water retention capability of the soils [36]. Wei et al. [37] indicated that the strength of a saline soil increased significantly after an SH agent treatment. Previous reviews have indicated that arbuscular mycorrhizal fungi can protect plants from salt stress by regulating the soil macroaggregates [38,39]. Arbuscular mycorrhizal fungi and earthworms can cooperatively promote the remediation of saline soils by changing soil macroaggregates and microbial biomass nitrogen [40].
Most research has focused on the effects of the hydraulic properties, soil quality, and plant productivity of treated saline soils, but the information regarding the effects of amelioration methodologies on the mechanical properties of saline soils is limited. This study aimed to characterize the effect of CLI on the modification of sulfate saline soils. A series of triaxial tests and a test on the soluble salt content of a treated soil were performed on the modified saline soils with various contents of CLI and curing times to explore the effect of CLI on the mechanical properties and salinity variation. The results of this study provide insights for the application of polymer technology to the improvement of sulfate saline soil.

2. Materials and Methods

2.1. Polymer Active Agent

A CLI-type polymer active agent (CLI) was selected for this study (Figure 1). CLI is prepared by purifying and drying paper-making wastewater and is a brownish-yellow powder at room temperature. CLI contains a large number of calcium ions, and the molecular side chain has many sulfonate groups that have strong adhesive properties. Meanwhile, due to the presence of anionic groups (sulfonate and hydroxyl), CLI is water-soluble and ion-exchangeable in solution. In addition, CLI can be used as a binder for poultry and livestock feeds to reduce the content of fine powder and the dust recovery rate. Moreover, CLI is environmentally friendly during soil improvement.

2.2. Tested Soil

The soil used in this study was collected from a construction site of the 33rd Regiment of Nong II Division in Weili County, Bayinguolin Mongolian Autonomous Prefecture, Xinjiang, China. The soil was obtained at a depth of approximately 1 m. The test results show that the undisturbed soil contains a variety of soluble salts, and the content of Na2SO4 was about 3.82%. Therefore, the tested soil is a strong sulfuric saline soil according to the classification criteria of saline soils. The soil was desalinated before the test and then passed through a 2 mm sieve after drying and crushing. The specific gravity (Gs) of the tested soil is 2.72; the maximum dry density (ρmax) is 1.73 g/cm3; the optimum moisture content (wop) is 14.12%; and the plastic limit (wp) and liquid limit (wl) are 14.6% and 31.6%. The contents of sand, silt, and clay in the soil are about 12.81%, 71.77%, and 15.42%, respectively.

2.3. Test Methods

2.3.1. Triaxial Compression Test

This experiment was mainly carried out through a triaxial shear test to investigate the variation law of soil strength after modification under the conditions of different contents of the polymer active agent ( w c = 0, 4%, 8%, 12%) and different curing times (d = 1, 3, 7, 14). To make the initial salt content of each specimen consistent, equal amounts of anhydrous sodium sulfate crystals were added to the target soil to form an initial salt content of 4%. The prepared samples (the sample is a cylinder with a height of 80 mm and a base diameter of 39.1 mm) were placed in a chamber with a temperature of 25 ± 2 °C and a relative humidity of approximately 50%. The calculation methods of the initial salt and CLI contents are expressed as Formulas (1) and (2).
The confining pressures in the triaxial shear tests were 100 kPa, 200 kPa, and 300 kPa, respectively. The experimental process used the Chinese standard (GB/T 50123-2019) [41], and the strain rate of the triaxial shear equipment was controlled at 0.8 mm/min. The test procedure was as follows: (1) After the latex film passes through the tube, the two ends of the film are turned out and fastened to the mouth of the tube, and the gas is sucked out through the air holes with the use of an aurilave, so that the latex film is tightly attached to the inner wall of the tube to ensure that the sample can be placed in the tube. (2) Release the aurilave after placing the sample in the film-bearing cylinder so that the latex film can tightly adhere to the sample. (3) Transfer the sample to the pressure chamber, fill the pressure chamber with water and then close the air vent, set the appropriate confining pressure, and start the test. According to the test, if the axial force reading does not have a significant reduction, the deviatoric stress from shearing to the time when the axial strain reaches 15% is defined as the peak deviatoric stress. The instrument used in the experiment was a TSZ-1 full-automatic triaxial shear equipment (Figure 2), which was made by Nanjing soil Instrument Factory.
w s = m s m
where w s is the initial salt content (%), m s is the mass of anhydrous sodium sulfate crystal (g), and m is the mass of dry soil used (g).
w c = m c m
where w c is the content of CLI (%), m c is the mass of the polymer active agent (g), and m is the mass of dry soil (g).

2.3.2. Soluble Salt Content Test

Barium ions and sulfate ions react to form an insoluble barium sulfate precipitate under acidic conditions (chemical Equation (3)). Therefore, the soil leaching solution of the treated sample could be prepared, which could react to form barium sulfate precipitation within a certain time by adding barium chloride solution. Furthermore, the change in the sulfate ion content in the soil before and after the improvement was revealed according to the weight of barium sulfate precipitation (Figure 3).
SO 4 2 − + Ba 2 + = BaSO 4 ↓
The test process was as follows. First, the cured sample was sifted with a 0.075 mm sieve after it was dried and ground with a mortar. Second, a soil sample with a weight of 50.00 g was poured into a 500 mL dry shake flask, and 250 mL of distilled water was added. The shake flask was placed on a reciprocating electric oscillator for continuous shock for 5 min. After standing for 24 h, the soil leaching solution of the sample was obtained by extracting the supernatant liquid. Then, the soil leaching solution was poured into a 400 mL beaker, to which was added diluted hydrochloric acid until the solution turned exactly red after dropping the methyl red indicator. After that, the solution was heated to near-boiling in the sand bath, and then 40 mL 0.02 mol/L barium chloride solution was quickly added, after which the solution was stirred intensely for 2 min. When the solution was cooled to room temperature, a little barium chloride solution was dropped into it to test whether the precipitation was complete. Next, the solution containing the precipitation of barium sulfate was pumped through the filtration device. After the suction filtration was completed, the precipitation was placed in the crucible (the mass of which was known) and put in the oven at 110 °C for drying. During the period, the mass of the crucible was weighed every 1 h until the difference between the two masses was less than 0.0001 g. Finally, the mass of the barium sulfate precipitation was calculated according to Equation (3).

3. Results

3.1. Triaxial Shear Test

The stress–strain curves of the modified sulfate saline soil samples after 1 day and 14 days of curing for different confining stresses are shown in Figure 4 and Figure 5. Figure 4 and Figure 5 show that the samples displayed strain-softening behavior. The deviatoric stress of all the samples increased rapidly at a lower axial strain before reaching the peak stress. Then, the deviatoric stress decreased to the residue deviatoric stress. The slope of the stress–strain curves declined gradually with axial strain and finally became zero when the peak deviatoric stress was reached. As the axial strain increased, the slope of the stress–strain curves turned negative. As shown in Figure 4a–c, both the content of CLI and the confining pressure had a significant influence on the stress–strain curves. As the confining pressure increased from 100 kPa to 300 kPa, the samples exhibited a higher deviatoric stress. Compared with the non-modified samples (the content of CLI = 0%), the shear strength of modified samples was effectively improved. As the content of CLI increased from 0% to 12%, the deviatoric stress increased and then decreased. When the content of CLI was 8%, the deviatoric stress reached the maximum. Take the confining pressure of 100 kPa as an example: the peak deviatoric stress of the sample with 8% CLI content was enhanced by about 122.22%, 89.38%, and 70.56 compared to the samples with CLI content of 0%, 4%, and 12%, respectively. Moreover, the slope of the stress–strain curves of the 8% CLI-treated soil before reaching zero was steeper than the others. The stress–strain curves of samples with 4% and 12% CLI showed a similar tendency.
Figure 4d shows the effect of CLI content and confining pressure on the secant modulus, in which the secant modulus was defined as the ratio of the peak deviatoric stress to the corresponding axial strain. In general, the secant modulus increased with confining pressure. The secant modulus increased as the content of CLI increased from 0 to 8%, which showed improvements of about 2–3 folds. As the content of CLI increased to 12%, the secant modulus decreased but was still higher than that of 4% CLI.
In Figure 4a–c, the deviatoric stress response of non-modified samples (the content of CLI = 0%) displays slight post-peak softening, while other samples behave in an obvious strain-softening manner. The deviatoric stress shows an obvious increase with confining pressure for the same content of CLI. Similarly, with samples after 1 day of curing, the deviatoric stress of samples after 14 days of curing increased first and then decreased as the content of CLI increased, with the turning point in the content of 8% CLI. Moreover, the slope of the rising phase of the stress–strain curves was the steepest at the 8% content of CLI, indicating that the use of 8% CLI effectively improves the modified soil’s ability to resist shearing at the same axial strain. CLI is a natural soil stabilizer, and it needs to take a period of curing time to form bonding with soil particles. Figure 4 and Figure 5 illustrate that the samples cured for a longer time showed higher deviatoric stresses, which was more phenomenal for a higher content of CLI. With the increase in the curing time from 1 day to 14 days, the stress–strain curves of the samples moved upward and toward the left. The axial strain generated to reach the peak deviatoric stress decreased as the curing time increased. As for the samples after 14 days of curing, the peak deviatoric stress was reached with the axial strain < 5%, which is less than that of the samples after 1 day of curing.
The effects of CLI content and confining pressure on the secant modulus are presented in Figure 5d. The secant modulus increased with confining pressure. This could be attributed to the increasing ductile behavior of the modified soil under higher confining pressures. At lower contents of CLI, on increasing the content of CLI from 0 to 8%, the secant modulus of samples showed a notable increase from 2.68 to 23.85 MPa, 3.36 to 27.31 MPa, and 3.50 to 33.13 MPa under different confining pressures, respectively. With the increase in the content of CLI from 8% to 12%, a reduction in the secant modulus could be observed. Figure 4d and Figure 5d show the variation in the secant modulus with the curing time. When the CLI content was 8%, the secant modulus of the modified soils increased from 5.80 MPa to 23.85 MPa, 7.13 MPa to 27.31 MPa, and 3.50 MPa to 33.13 MPa under different confining pressures as the curing time increased from 1 day to 14 days, respectively. It indicated that adequate curing was essential for CLI to improve the shear strength of the modified soil.
Table 1 presents the peak deviatoric stress of samples for various curing periods and confining pressures. The results show that on increasing the confining pressure from 100 kPa to 300 kPa, the peak deviatoric stress showed a phenomenal increase for samples of 0–8% content of CLI. As for samples of 12% content of CLI, it can be observed that by increasing the confining pressure from 100 kPa to 300 kPa, the peak deviatoric stress increased first and then showed a reduction. In addition, with the increase in the content of CLI, the peak deviatoric stress increased to 8% content of CLI. This phenomenon became more remarkable as the curing period increased. But with the increase in content to 12%, it showed a reduction irrespective of the confining pressure and curing period. Moreover, the increase in the curing period played an active role in the enhancement of the peak deviatoric stress. It showed a more significant increase in increasing the curing period from 1 day to 7 days than from 7 days to 14 days. When the CLI content in the sample was 8%, the peak deviatoric stress of the samples after curing for 14 days was elevated compared to that of the samples cured for 1, 3, and 7 days by about 36.29%, 13.93%, and 2.11%, respectively, under the confining pressure of 100 kPa. This could be attributed as follows: after stabilizing the soil with CLI, it must take time to coat and bond soil particles to form a stable soil structure for the improvement of shearing resistance.

3.2. Shear Strength Parameters

Cohesion and internal friction angle are two essential shear strength parameters. Figure 6 plots the effects of the content of CLI and curing time on cohesion and internal friction angle. Both parameters were obtained by using the least squares fitting method to fit the major and minor principal stresses. As shown in Figure 6a, for the samples after the same curing time, the cohesion generally increased with the increase in the content of CLI from 0 to 8%, while the cohesion showed a strong decreasing trend for the content of CLI increasing from 8% to 12%. This indicates that soil particles were cemented with CLI, and the pores in the soil were clogged, forming a more stable structure. When the content of CLI increased beyond the optimum value, the excessive aggregates formed with CLI and soil particles induced some large pores forming in the soil. The curing time contributed marginally to the cohesion of non-modified samples: the cohesion increased by 1.11 kPa as the curing time increased from 1 day to 14 days. The CLI-treated soils increased by 6.48 kPa, 5.55 kPa, and 5.88 kPa with contents of 4%, 8%, and 12% with an increase in the curing time from 1 day to 14 days. The results show that the improvement in cohesion was more significant before 7 days, and the enhancement of cohesion was marginal after 7 days.
Figure 6b reveals that the trend of the internal friction angle with the content of CLI and curing time was similar to that of cohesion. As shown in Figure 6b, the internal friction angle increased first and decreased with the content of CLI. When the content of CLI was 8%, the internal friction angle reached the peak value, which was about 2 times that of the non-modified samples after different curing times (1, 3, 7, 14 days). For the non-modified samples, the effect of curing time on the internal friction angle was marginal. It increased by 0.36–1.11° as the curing time increased from 1 day to 14 days. Moreover, with the increase in the curing time, the effect of different contents of CLI on the internal friction angle was more significant. It can be seen that the internal friction angle of the samples after curing for 14 days was enhanced compared to that of the samples cured for 1 day by about 49.16% (content of CLI = 4%), 24.74% (content of CLI = 8%), and 31.24% (content of CLI = 12%), respectively.

3.3. Effect of CLI Content and Curing Time on Sodium Sulfate Content in the Soil

CLI addition exhibited a satisfactory performance in improving shearing strength. It also showed great potential to enhance soil water-holding capacity. Figure 7a shows the effect of CLI content and curing time on the moisture content of soil samples. For samples after the same curing time, the reduction in moisture content showed the tendency of a V-shaped decrease and increase as the content of CLI increased, with the turning point at the content of 8%. This indicated that the CLI addition could increase water storage capacity, which was the most effective when the CLI content reached 8%. In comparison to the non-modified soils, the moisture content of the samples of 8% CLI was higher by 0.25%, 0.93%, 2.27%, and 2.75% after different curing times, respectively. Moreover, the results reveal that the curing time significantly influences the water-holding capacity of CLI-treated soil. For samples after a day of curing, the moisture content was approximately 12%. As the curing time increased, the difference between the moisture content of samples with different CLI contents became larger. As the curing time increased from 7 days to 14 days, the effect of curing time on the modified soil became negligible.
To describe the variation of salt content in the sulfate saline soil, the mass of barium sulfate was calculated. The content of CLI added and the curing time had significant effects on the concentration of sulfate ions in the sulfate saline soil (Figure 7b). CLI treatments caused a notable decrease in the concentration of sulfate ions in the soils (by 1.49–2.31 g after the curing time from 1 day to 14 days), with the greatest decrease found in the samples of 12% CLI. With the increase in the curing time, the largest decrease was found between 1 and 3 days. When the curing time increased from 7 days to 14 days, the reduction in the mass of barium sulfate was slight, indicating that CLI-induced decreases in the content of sulfate ions were more significant during the first 7 days than those of the following 7 days. In addition, Figure 8 shows that the amount of precipitation sodium sulfate crystals gradually decreased as the CLI content increased. Further evidence is that CLI is effective in reducing soluble salt content in sulfate saline soil.

4. Discussion

Under the action of surface runoff and underground seepage, the crystalline salts inside a saline soil are dissolved to form a salt solution and become lost in water. This process weakens the cementation ability between soil particles and increases the porosity of the soil, thus leading to a decrease in the stability of the overall soil structure. On this basis, when the saline soil is subjected to self-weight and external load, the soil particles break the original stable structure and reorganize, which easily induces a series of geological disasters such as soil subsidence and collapse. Therefore, the changes in the mechanical properties and the soluble salt content of the study samples are discussed to further elucidate the action mechanism of polymer active agents on saline soil.
The effect and mechanism for improving a sulfate saline soil are shown in Figure 9. Dry soil was mixed with the appropriate amount of water and CLI to obtain the desired specimens. For the unmodified plain soil (0% CLI content), a thick hydration film existed on the surface of the soil particles with a large number of connected pores between the particles. The molecular structural formula of CLI is shown in Figure 9a, which is weakly acidic due to the hydroxyl and sulfonic groups and chelates with calcium ions, providing strong stability. In water, soil, and ion (sodium and calcium ions are provided by anhydrous sodium sulfate and polymer active agent, respectively) environments, the weakly acidic CLI makes the pH lower, i.e., hydroxyl ion decreases, leading to a decrease in the net negative charge number on the particle surface and a thinner double electric layer. Meanwhile, the high-valence calcium ions provided by CLI experience an ion-exchange reaction with sodium on the surface of the soil particles (Figure 9a). It is known that when the surface charge of clay particles is constant, ion valence increases with the ion concentration and decreases with the thickness of the double layer of soil particles according to the theory of a double layer of soil. Therefore, the double layer thickness of the modified soil particles decreases, leading to an increase in the attraction between the particles, which in turn produces an agglomeration effect to achieve the effect of compacting the soil. Additionally, as an anionic polymer, the sulfonic group of CLI is negatively charged in an aqueous environment, which can be adsorbed onto the surface of soil particles under the effect of cation bridging and binding the cations in situ. Due to the characteristics of macromolecular groups, CLI forms a spatial network structure between neighboring soil particles, which plays a strong bonding effect. As shown in Figure 9b, at the optimal doping content, CLI filled the intergranular pores, and it wrapped and bonded the soil particles. It also induced the formation of the agglomerated flocculated structure of soil particles, which decreased the specific surface area of the soil body, and the mechanical properties of the modified soil were optimal.
However, CLI preferentially binds to itself to form macromolecules as the content continues to increase since the chelating property is good. Some of the sulfonic groups are bonded to the neighboring sulfonic groups of CLI by hydrogen bonds (Figure 6a). This is not as significant as cation bridging, resulting in a decrease in the friction force between the soil particles. As shown in Figure 9b, an excessive CLI matrix increases the distance between soil particles, making compaction of the soil difficult and thus exposing more voids. As a result, the skeleton effect of soil particles is weakened, and the mechanical strength is reduced. Meanwhile, it can be found that when the polymer active agent content is 8%, the moisture content of the specimens is slightly higher than that of the specimens in which the polymer active agent content is 12% under the same maintenance time, as shown in Figure 5a. This further indicates that the addition of excess polymer active agents increased the soil porosity and thus elevated the moisture evaporation.
Furthermore, after CLI was added to the sulfate saline soil, free calcium ions reacted with sulfate ions in the hygroscopic water to form a slightly water-soluble calcium sulfate precipitate (Figure 9a). This indicates that CLI can reduce the soluble salt content of saline soils by reducing the concentration of sulfate ions. The precipitated salt crystals were bound in the CLI matrix, thus preventing the salt migration. In addition, throughout the whole test, it was found that the improvement of the saline soil by CLI was influenced by the curing time. The mechanical properties, evaporation characteristics, and soluble salt content of the improved soil all changed rapidly and gradually slowed down with the increase in the curing time. This means that CLI reacts violently with the soil at the early stage of improvement but gradually reacts completely and tends to stabilize as time goes on. It follows that there is a better benefit at a curing time of 7 days.

5. Conclusions

Triaxial compression tests and a test on the soluble salt content of a treated soil were carried out to investigate the effects of polymer active agent content and maintenance time on the mechanical properties and soluble salt content of sulfate saline soils. The following conclusions were drawn:
(1) The calcium ions in the CLI-type polymer active agent react with the sodium ions in the sulfate-type saline soil, which reduces the thickness of the double electric layer of the soil particles. This reaction further enhances the attraction between the particles and finally compacts the soil through the agglomeration effect.
(2) The optimal improvement effect can be obtained when the content of the CLI-type polymer active agent is around 8%, and the reaction between the polymer active agent and the soil is completed when the maintenance time reaches 7 days.
(3) The calcium ions in the CLI-type polymer active agent can react with sulfate ions in sulfate-impregnated soils under a suitable moisture condition to produce calcium sulfate precipitation, which further alleviates soil salinization.

Author Contributions

Conceptualization, methodology, and data curation, X.Z., J.Z. and Z.Y.; writing—original draft preparation, X.Z.; project administration, Z.Y.; validation, J.L., F.B. and Z.Y.; formal analysis, C.D.; investigation, Y.L. and J.L.; resources, J.L., Y.L., X.Z. and Z.Y.; writing—review, editing, and visualization, X.Z., F.B., C.D. and Z.Y.; supervision, J.Z. and Z.Y.; funding acquisition, J.L. All authors have read and agreed to the published version of the manuscript.

Funding

This research was financially supported by Open Project of Technology Innovation Center for Ecological Monitoring & Restoration Project on Land (Arable), Ministry of Natural Resources (Grant No. GTST2021-006).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data presented in this study are available on request from the corresponding author.

Acknowledgments

The authors would like to thank Shefeng Hao, Yongxiang Yu, and Jinghua Ren from the Geological Survey Institute of Jiangsu Province for their contributions to this research.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. A CLI-type polymer active agent.
Figure 1. A CLI-type polymer active agent.
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Figure 2. TSZ-1 full-automatic triaxial shear equipment.
Figure 2. TSZ-1 full-automatic triaxial shear equipment.
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Figure 3. Schematic illustration of the CLI polymer reacting with sulfate saline soil to produce calcium sulfate precipitation.
Figure 3. Schematic illustration of the CLI polymer reacting with sulfate saline soil to produce calcium sulfate precipitation.
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Figure 4. The deviatoric stress–strain curves and secant modulus of samples after a day of curing: (a) 100 kPa confining pressure; (b) 200 kPa confining pressure; (c) 300 kPa confining pressure; (d) Secant modulus.
Figure 4. The deviatoric stress–strain curves and secant modulus of samples after a day of curing: (a) 100 kPa confining pressure; (b) 200 kPa confining pressure; (c) 300 kPa confining pressure; (d) Secant modulus.
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Figure 5. The deviatoric stress–strain curves and secant modulus of samples after 14 days of curing: (a) 100 kPa confining pressure; (b) 200 kPa confining pressure; (c) 300 kPa confining pressure; (d) Secant modulus.
Figure 5. The deviatoric stress–strain curves and secant modulus of samples after 14 days of curing: (a) 100 kPa confining pressure; (b) 200 kPa confining pressure; (c) 300 kPa confining pressure; (d) Secant modulus.
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Figure 6. Shear strength parameters of samples: (a) Relationship between cohesion and curing time; (b) Relationship between internal friction angle and curing time.
Figure 6. Shear strength parameters of samples: (a) Relationship between cohesion and curing time; (b) Relationship between internal friction angle and curing time.
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Figure 7. Changes in the moisture content of samples and mass of barium sulfate in samples with different CLI contents and curing times: (a) moisture content of samples; (b) mass of barium sulfate in samples.
Figure 7. Changes in the moisture content of samples and mass of barium sulfate in samples with different CLI contents and curing times: (a) moisture content of samples; (b) mass of barium sulfate in samples.
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Figure 8. Precipitation of sodium sulfate crystals from the surface of treated sulfate saline soil after curing for 14 days: (a) Content of CLI is 0%; (b) Content of CLI is 4%; (c) Content of CLI is 8%; (d) Content of CLI is 12%.
Figure 8. Precipitation of sodium sulfate crystals from the surface of treated sulfate saline soil after curing for 14 days: (a) Content of CLI is 0%; (b) Content of CLI is 4%; (c) Content of CLI is 8%; (d) Content of CLI is 12%.
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Figure 9. Schematic diagram of CLI modification mechanism: (a) Interaction mechanism between sulfuric saline soil and CLI; (b) Improvement effect of different CLI contents.
Figure 9. Schematic diagram of CLI modification mechanism: (a) Interaction mechanism between sulfuric saline soil and CLI; (b) Improvement effect of different CLI contents.
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Table 1. The peak deviatoric stress of specimens for various curing periods and confining pressures.
Table 1. The peak deviatoric stress of specimens for various curing periods and confining pressures.
Confining Pressure
(kPa)		Curing Period (Day)Peak Deviatoric Stress (kPa)
0%4%8%12%
1001436.25511.89969.43568.37
3462.22588.371159.71905.82
7532.38658.661294.081111.63
14551.66708.781321.261128.24
2001625.73704.651292.261126.90
3621.36759.061402.021172.51
7678.27836.291578.681378.64
14703.571048.521770.431460.54
3001682.43821.991463.42887.69
3781.92912.321622.211021.21
7812.431018.811824.411251.76
14824.731165.351956.541324.65
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MDPI and ACS Style

Zhu, X.; Yang, Z.; Zheng, J.; Liu, J.; Bu, F.; Dai, C.; Lu, Y. Study on Mechanical Properties of Sulfate Saline Soil Improved by CLI-Type Polymer Active Agent. Appl. Sci. 2023, 13, 10727. https://doi.org/10.3390/app131910727

AMA Style

Zhu X, Yang Z, Zheng J, Liu J, Bu F, Dai C, Lu Y. Study on Mechanical Properties of Sulfate Saline Soil Improved by CLI-Type Polymer Active Agent. Applied Sciences. 2023; 13(19):10727. https://doi.org/10.3390/app131910727

Chicago/Turabian Style

Zhu, Xufen, Zhuoqun Yang, Jiaqiang Zheng, Jin Liu, Fan Bu, Chengjiang Dai, and Yipin Lu. 2023. "Study on Mechanical Properties of Sulfate Saline Soil Improved by CLI-Type Polymer Active Agent" Applied Sciences 13, no. 19: 10727. https://doi.org/10.3390/app131910727

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