Study on Strengthening and Waterproofing Mechanism of Calcium Lignosulfonate in Silty Soil Sites

Abstract

Silty sites are affected by natural and human factors, have a low soil strength and strong water sensitivity, and are prone to cracks, soil peeling, and other failures that urgently need to be repaired. Calcium lignosulfonate (CLS) from paper plant waste fluids is a natural bio-based polymer. In this paper, against the background of the reinforcement and protection project of the Qingtai Site in Xingyang, Henan Province, silty soil was modified by adding CLS, and the material ratio with the best restoration effect was determined by carrying out a series of mechanical and waterproof tests. The mechanism of action of the modified materials was analyzed through microscopic tests such as SEM and XRD. The test results showed that a 1.0% mass fraction of CLS in the silty soil was the optimal ratio of the modified material. The mechanical properties of the modified soil first increase and then decrease with the increase of CLS content, and the waterproof properties increase with the increase of CLS content. The lignosulfonate polymer generated by the displacement reaction between CLS and the soil particles was cemented with the soil particles, meaning that the mechanical and waterproof properties of the modified soil were improved. And the content of the main elements and the mineral composition in the modified soil did not change. The research results provide a reference for the restoration and protection of silty sites.

1. Introduction

Rain erosion and structural instability of earthen ruins have always been important research directions in archaeological engineering. Silty sites are widely distributed in Henan Province, with special grading characteristics, poor stability of the soil particle structure, smooth capillary channels in the soil, and strong water sensitivity [1,2,3]; therefore, there is an urgent need to research related soil site protection materials. The restoration measures for soil sites at home and abroad mainly include physical and chemical agent restoration. The common physical restoration measures include tamping, vibrating, vibration rolling, and fiber reinforcement. The common chemical agent restoration measures include chemical agent surface spraying and stabilizer mixing in the site soil. However, physical restoration measures usually cause irreversible damage to the cultural characteristics of soil sites. Therefore, based on the research idea of “taking local materials and repairing soil with soil”, this study focuses on the restoration and protection of soil sites by mixing stabilizers to explore new methods suitable for the restoration and protection of silty sites.

In addition to traditional soil stabilizers such as cement and lime, many types of non-traditional stabilizers are available in engineering, which are generally divided into ions, enzymes, lignosulfonates, salts, petroleum resins, polymers, and resins [4]. Considering the cultural relics of earthen ruins, the influence of the stabilizer materials used in all aspects of earthen ruins must be fully considered in the restoration. Therefore, scholars have researched various earthen ruin restoration materials and achieved some results. Alkali solution can reduce the swelling capacity of montmorillonite after treating clay [5]; synthetic termite saliva and bamboo particles can improve the compressive strength of adobe and reduce capillary action [6]; composite materials obtained by adding silica nanoparticles and PDMS (polydimethylsiloxane) to TEOS (ethyl silicate) can effectively improve the consolidation effect of adobe materials [7]. After adding glutinous rice pulp and lime to loess, its mechanical properties, water resistance, and salt resistance are improved, but its air permeability is reduced [8]. In addition, glutinous rice pulp has an excellent improvement effect on the wetting performance of silty soil sites [9]. However, the aging and oil resistance of organic materials, such as acrylic resin, silicone resin, and epoxy resin, widely used in the existing research, are generally affected by climates, such as discoloration and debonding. Although the inorganic internal doping materials used have excellent aging resistance, the crystal expansion formed by soluble salts when they meet with water will aggravate the surface weathering of cultural relics in earthen sites [10]. Therefore, it is essential to study more effective mixing of modified materials in the soil to solve the problems of rain erosion and the structural instability of soil sites.

As a derivative of lignin, calcium lignosulfonate (CLS) contains a variety of active functional groups. It is an anionic surfactant of multicomponent polymers with good adhesiveness, chelation, and corrosion resistance and has the advantages of being environmentally friendly, innocuous, non-toxic, and non-corrosive [11]. Industrial lignin mainly comes from the waste liquid of paper mills and by-products of biofuel. Suppose industrial lignin is used as a soil stabilizer. In that case, it can improve the soil’s engineering properties, reuse industrial by-products, and reduce the environmental problems caused by improper disposal [12]. In recent years, the research on applying lignosulfonate in geotechnical engineering has been gradually developed, especially in subgrade engineering. Lignosulfonate is mainly used in cohesive soil to improve its strength and natural expansion rate [13,14,15,16]. The improvement of loess’s mechanical properties, durability, and collapsibility has also been studied [17,18]. Scholars have also conducted some research work on the improvement of silt using lignosulfonate. For example, Zhang Tao [19,20] used a series of indoor tests, theoretical analyses, and field tests to verify that lignin effectively improves the basic engineering characteristics of silt. Zhang Miaoxin [21] found that 0.75% calcium lignosulfonate and 1% fly ash improved silty clay in a certain area of Harbin and, at the same time, improved its frost resistance and compressive strength. Li Yizhou [22] measured the influence of the lignin sulfur salt content on the strength and durability improvement effect of silt through laboratory tests of mechanics and durability, and the results showed that a low content of lignin sulfur salt was effective in improving the strength and durability of the modified soil. The research results are consistent in the microscopic mechanism of performance improvement. It is generally believed that lignin improves soil by binding soil particles together, and the improvement of soil anti-erosion ability is due to a decrease in the surface charge of clay particles, thus reducing the thickness of the electric double layer and forming a stable flocculent structure [4,23].

It is generally believed that adding a certain amount of lignosulfonate as a stabilizer (curing agent) can effectively improve the mechanical strength, durability, and engineering characteristics of various original soils to a certain extent. However, there are few studies on applying lignin materials in the protection and restoration of earthen sites. Because of the cultural relics of earthen sites, the requirements for repair materials in terms of mechanical and waterproof properties are more stringent. Thus more targeted experimental research should be carried out. The purpose of this study was to use CLS as the modified material for the original site silt. Through a direct shear test, an unconfined compressive test, a capillary water rise test, and a water-resistant disintegration test, the change rules of the mechanical properties and waterproof properties of the site silt before and after modification were obtained, and the optimal content of CLS was determined. At the same time, the changes in the microstructure, element composition, and mineral composition before and after CLS modification were analyzed via SEM, EDS, XRD, and the internal mechanism of CLS in improving silt mechanics and waterproof properties was discussed. We combine our experimental results with the existing research to clarify the potential of lignin as a material for repairing earthen sites. The research results provide some references for the protection of silty soil sites.

2. Materials and Methods

2.1. Materials and Sample Preparation

• Materials

Soil samples were taken from the soil around the Qingtai Site in Zhengzhou City, China. The soil was crushed, air-dried, and passed through a 2 mm sieve. The basic physical properties of the soil samples were determined according to ASTM D4318 [24], as shown in

Table 1. Basic parameters of soil samples.

Property	Value
Sand (0.074~2 mm)	17.69%
Silt (0.002~0.074 mm)	82.21%
Clay (<0.002 mm)	0.10%
Liquid limit	25.2%
Plastic limit	16.4%
Plasticity index	8.8%
Maximum dry density	1.709 g·cm−3
Optimum moisture content	12.42%

.

Calcium lignosulfonate (CLS) was produced by Hefei Qiansheng Biotechnology Co., LTD (Hefei City, China). Its basic physicochemical properties are shown in

Table 2. Basic parameters of CLS.

Appearance	Lignin Content	Carbon Content	Sulfur Content	Moisture Content	Water Insoluble Substance	PH
Brown powder	98%	40%	5%	≥5%	<1.5%	≈7.0

. The sample and molecular structure of CLS are shown in Figure 1.

2.

Sample Preparation

CLS was mixed into the dried silt according to different mass ratios, and the particle size of the silt was controlled within 2 mm after screening with a screen. The proportions of calcium lignosulfonate (the mass ratio of repair materials in the dry soil) were 0%, 0.5%, 1.0%, 1.5%, and 2.0%, which were marked as 1 (plain soil), 2, 3, 4, and 5, respectively, in the test. According to ASTM D698 [25], 61.8 mm × 20 mm standard circular cutter samples were produced, with the water content controlled at 12.42% and the pressure stabilization lasting for about 1 min. cylindrical samples with a diameter of 39.1 mm and a height of 80 mm were produced using the layered compaction method, and the water content was the same as the above. The demolded samples were placed into a constant temperature and humidity box for curing for 28 days. Some samples are shown in Figure 2.

2.2. Direct Shear Testing

The instrument used in this experiment was a ZFY-1A strain-controlled direct shear apparatus for unsaturated soil. The shear rate was 1 mm/min, and vertical pressures of 100 kPa, 200 kPa, 300 kPa, and 400 kPa were applied. The readings were observed every 20 s, the shear displacement was calculated according to the turntable speed, and the shear strength was calculated from the readings. In the experiment, five groups of samples were tested, and standard circular cutter samples were selected. The proportions of calcium lignosulfonate in the samples were 0%, 0.5%, 1.0%, 1.5%, and 2.0%, which were marked as 1, 2, 3, 4, and 5, respectively.

2.3. Unconfined Compressive Strength Testing

The instrument used in the unconfined compressive test was a universal material tester, which can be controlled by a computer and automatically record the load and displacement of the sample. In the experiment, five groups of samples were tested, and cylindrical samples were selected. The proportions of calcium lignosulfonate in the samples were 0%, 0.5%, 1.0%, 1.5%, and 2.0%, which were marked as 1, 2, 3, 4, and 5, respectively.

2.4. Capillary Water Rise Testing

The occurrence environment of the site was simulated to study the capillary water rise phenomenon of the modified soil. Five groups of samples were tested in the test, and the types of samples used were the same as those used in the unconfined compression test. The specific test steps are as follows:

• Drying the sample

The sample needs to be cured for 28 days. One piece of each proportion is taken, and it is dried in an electric heating constant temperature drying oven at 50 °C for 8 h.

2.

Capillary water rise test process.

A permeable stone is placed in a plexiglass tank, and water is added. The water level should exceed the bottom of the permeable stone by 2~3 mm and not exceed the top. To reduce the test error, the water level should be checked frequently during the test. When the water level is low, water should be added to the tank to avoid the test tank being too low due to the water absorption of the sample.

3.

Experimental observation and recording

The change in the rise height of the capillary water in the sample is recorded over time, being marked every 5 mm until the capillary water reaches the top of the sample, and the test time is 24 h.

2.5. Water-Resistant Disintegration Testing

Soil disintegration is one of the essential reference indexes to evaluate the degree of soil erosion by water. To analyze the improvement effect of CLS on the disintegration resistance of silt, the water-resistant disintegration test was carried out on the modified soil samples. Five groups of samples were tested in the test, and the types of samples used were the same as those in the direct shear test. The specific test steps of the test are as follows:

• Drying the sample

This step is consistent with the first step in the capillary water rise testing.

2.

Water-resistant disintegration test process

Water is added to a water tank with a water level of 75 mm to ensure that the samples are completely submerged.

3.

Experimental observation and record

The disintegration of the samples in the water is recorded over time, and the immersion time for this test is three days.

2.6. Scanning Electron Microscopy (SEM) Analysis

Scanning electron microscopy (SEM, FEI, Hillsboro, OR, USA) is a common microscopic test method to study the engineering properties of soil, which can observe the size, shape, and connection of soil particles and pores and helps explore the reinforcement mechanism of the modified soil from the perspective of microstructure. The test instrument used in this section was the environmental scanning electron microscope produced by the FEI company in the United States, and the model was Quanta 650. The scanning electron microscope was used to observe the cut pieces of plain soil and 1.0%CLS-modified soil, and the magnification of the samples was 200 times, 500 times, 1000 times, and 2000 times. Before observation, spraying a layer of heavy metal film on the sample block with a high-vacuum sputter coating instrument is necessary to make the image clearer during observation.

2.7. Energy-Dispersive Spectrometer(EDS) Analysis

An EDS is a testing instrument used to analyze the types and contents of component elements in local areas of materials, which is used in conjunction with a scanning electron microscope. Because the EDS test is used with SEM, the samples used in the test are the same as those used in SEM, so there is no need to produce another sample. The instrument captures and analyzes the intensity of X-rays with different energies, develops the X-rays of the detected substance into an energy spectrum analysis diagram, and observes each peak of the energy spectrum diagram to deduce the elemental composition and content of the sample. There are three analysis modes of an energy spectrometer: point analysis, line scanning, and area analysis. This experiment adopted the area analysis mode, which has the characteristics of high accuracy and wide range.

2.8. X-ray Diffraction (XRD) Analysis

The mineral composition of the soil is an essential factor affecting the material structure characteristics of the soil and a necessary basis for determining the physical characteristics of the soil. XRD tests show that different minerals will produce different diffraction patterns upon X-ray diffraction of materials, and the analysis of diffraction patterns can identify the minerals contained in the sample, which can be used to judge the interaction between additive materials and silt minerals effectively. The test instrument used in this study was a Brooke D8 X-ray diffractometer made in Karlsruhe, Germany, and the scanning angle was 0~80, which reflects the mineral composition in the excavation. Diffraction tests were carried out on plain soil and 1.0%CLS-modified soil samples using the instrument, and the changes in the mineral composition of the soil samples before and after modification were analyzed.

3. Results and Discussion

3.1. Direct Shear Test Results

From sorting out the direct shear test data, the shear strength of each group is shown in

Table 3. Direct shear test record sheet (Unit: kPa).

Vertical Stress	Sample 1	Sample 2	Sample 3	Sample 4	Sample 5
100	64.6	71.7	81.6	76.1	70.7
200	102.7	124.4	145.4	131.0	119.7
300	130.0	158.9	175.6	168.0	155.9
400	172.3	195.5	222.5	207.9	194.0

, and the changing trend is shown in Figure 3. According to the fitting curve of shear strength, the cohesion and angle of internal friction of the sample can be calculated, as shown in

Table 4. Results of shear strength index.

Calcium Lignosulphonate Content (%)	Cohesion c (KPa)	The angle of Internal Friction φ (°)
0	29.76	19.29
0.5	36.08	22.10
1	43.01	24.37
1.5	37.69	23.37
2	33.62	22.09

, and the relationship curve between them and the content of the modified materials is shown in Figure 4.

The direct shear test showed that the best improvement effect was obtained when the content of 1.0% CLS was added. With the increase in the CLS content, the shear strength, cohesion, and angle of internal friction of the sample increased first and then decreased. The increase in shear strength was mainly due to the cementation between the lignosulfonate polymer and soil particles, which enhanced the cementation between the soil particles. At the same time, calcium lignosulfonate can increase the molecular force between soil particles, the roughness of soil particles, and the bite force between particles, thus increasing the cohesion and angle of internal friction of the soil.

3.2. Unconfined Compressive Strength Test Results

From sorting out the unconfined compressive test data, the compressive strength of each group is shown in

Table 5. Unconfined Compressive Strength Test record sheet.

Sample	Calcium Lignosulphonate Content (%)	Average Unconfined Compressive Strength (kPa)
Sample 1	0	294.7
Sample 2	0.5	466.5
Sample 3	1.0	449.1
Sample 4	1.5	408.7
Sample 5	2.0	378.7

, and the changing trend is shown in Figure 5.

The unconfined compressive test showed that 0.5% CLS had the best improvement effect. With the increase in the CLS content, the compressive strength of the sample gradually decreased. The main reason for the increase in compressive strength is that the lignosulfonate polymer produced by CLS causes the soil particles to bond and fill the pores between the soil particles. However, excessive addition of CLS causes the compressive strength to gradually decrease because the unreacted CLS fills the pores of the soil. Because of the low strength of CLS itself, soil particles slide against each other after compression, which is consistent with the reason for the decrease in shear strength in the direct shear test.

3.3. Capillary Water Rise Test Results

The phenomenon of capillary water rise in the samples with different proportions of lignosulphonate is shown in Figure 6a, and samples 1, 2, 3, 4, and 5, from left to right, respectively, and the data obtained from the test are plotted in Figure 6b.

It can be seen in Figure 6b that with the increase in the lignosulphonate content, the capillary water rise rate of the lignosulphonate-modified soil showed a decreasing trend. The speed of sample 1 was the fastest, and that of sample 5 was the slowest. The time it took the capillary water of sample 5 to rise to the top of the sample was 8.14 times that of sample 1. In the capillary water rise experiment, the rise height of the capillary water of the samples showed little difference in the first 15 s. In the subsequent experiments, the difference in the rising rate became more and more obvious. The higher the lignosulphonate content, the slower the rise rate of the capillary water. The time it took the capillary water to rise to the top of sample 2 was only 1.84 times that of sample 1, which was quite different from that of the capillary water increase with a higher content of lignosulphonate. At the same time, in the compressive and shear tests, lignosulphonate with a 0.5% content showed good mechanical properties, indicating that lignosulphonate with a 0.5% content mainly plays the role of bonding soil particles and, less so, filling the pores between the soil particles. In the test, the sample with a 1.0% lignosulphonate content began to show strong resistance to capillary water invasion. The higher lignosulphonate content filled the pores between the silt particles, which reduced the porosity of the soil, weakened the connectivity between the pores, and improved the compactness of the soil. The higher the content of calcium lignosulfonate modifying the soil, the better the effect in restraining the capillary action of silt.

3.4. Water-Resistant Disintegration Test Results

The phenomenon of water-resistant disintegration in the samples with different CLS contents is shown in Figure 7. During the experiment, it was found that the disintegration process of the modified soil had three stages: water absorption (bubble generation), crack generation, and disintegration.

As seen in Figure 7, the disintegration speed and amount of modified soil decreased with the increase in the CLS content.

Sample 1 reacted strongly in water, and many bubbles began to appear after the sample was soaked in water, with cracks starting to appear at the edge of the sample in 15 s. In the 25 s, the cracks at the edge of the sample became larger, and the granular soil fell off from the outside to the inside, rapidly disintegrating from the inside. In the 70s, sample 1 completely disintegrated and lost shape, and the whole sample was powdery after disintegration.

At the beginning of the experiment, sample 2 produced small bubbles, and the rate of bubble generation was slow. In the 30 s, the soil particles at the edge of sample 2 fell off slightly in granular form, and in the 90 s, cracks developed at the edge and then gradually extended inward. In the 450 s, the central part of sample 2 lost its shape and disintegrated, and after disintegration, the whole of sample 2 showed massive soil accumulation.

The bubble generation rate of sample 3 was very slow after the start of the test. In the 800 s, the soil particles at the edge of sample 3 fell off slightly, and in the 1400 s, tiny cracks appeared on the surface of sample 3. With the continuous entry of water, each crack continued to extend. At 1.9 h, the sample finally disintegrated in blocks, but the whole sample maintained a certain shape, and there was no large-scale soil collapse. The performance of samples 4 and 5 was consistent with that of sample 3. However, fewer bubbles were generated, the speed was lower, and cracks were generated in the 4.3 h and 12.6 h, respectively; no obvious disintegration occurred.

It can be seen that CLS can effectively improve the water-resistance of the samples. Although CLS fills the pores between the soil particles, which reduces the porosity of the soil and improves its compactness, the sample with a low content of CLS cannot fill the pores between the soil particles. After water gradually enters, the interaction between particles is destroyed by the water, and the sample peels off and disintegrates layer by layer from the outside to the inside. The lignosulfonate polymer produced by adding lignosulfonate to silt increases the bonding force between the soil particles. At the same time, the bonding bond is stable and difficult to break. This polymer has a more obvious effect of promoting the agglomeration of the soil particles and their bonding to form a more stable reinforced skeleton. The hydrophobic group (phenylpropane group) of exposed lignosulphonate macromolecules plays a role in draining water, forming a “water-blocking layer” around the wrapped silt particles. Higher content of lignosulphonate can provide an effective water-blocking layer for silt; therefore, higher contents of lignosulphonate have better water resistance.

To sum up, silt has extremely poor water resistance and can easily collapse and dissolve in water. Overall, the water-resistant disintegration ability of silt can be improved by adding lignosulfonate polymer generated by lignosulfonate in silt, and the hydrophobic groups of lignosulfonate macromolecules play a role in improving the collapsibility and deformation of silt in the water.

3.5. SEM Test Results

The SEM images of the plain soil (sample 1) and the modified soil mixed with 1.0% calcium lignosulfonate (sample 3) are shown in Figure 8 and Figure 9, respectively.

Figure 8 shows the microscopic morphology diagram of the plain soil under the scanning electron microscope. The diagram indicates that the composition structure type of silt is mainly a stacked mosaic structure with irregular particle size, blocky shape, angular surface, and random particle distribution. Although compacted, the overall structure of the soil is still loose, and there are many pores with different shapes among the soil particles. Large pores easily form between large particles, and the pores are mainly formed by staggered stacking of soil particles with different shapes. The overlapping particles are point-to-point and point-to-face contacts, with apparent boundaries between the soil particles. Under the scanning electron microscope at 2000 times magnification, it is found that the surface of the soil particles is smooth, and there is no attachment on the surface of the particles.

Figure 9 shows the microscopic morphology of the 1.0%CLS-modified soil at various times under the scanning electron microscope. Compared with Figure 8, it is found that the graininess of the surface in Figure 9 is reduced, the porosity is greatly reduced, and the compactness of the soil is significantly improved. Hardly any pores can be seen in the 1.0%CLS-modified soil, and the connectivity between the soil particles is excellent, forming a relatively complete structural unit.

It is considered that when lignosulphonate is added to soil, OH⁻ and high-valence Ca²⁺ cations are separated in the pore water of the soil, which reacts with low-valence cations on the surface of the silt particles, changing the structure of the electric layer on the surface of the silt particles, resulting in a decrease in the thickness of the electric double layer and the surface charge density, thus increasing the attraction between particles. The lignin polymer with a positive charge is adsorbed on the surface of negatively charged soil particles, and the lignosulfonate polymer is produced as an ionic bond. The increase in this polymer will increase the bonding force between the soil particles, and the bonding bond is stable and difficult to break. In the microstructure, the cemented polymer will fill the pores between the soil particles, which will reduce the pores and increase the compactness of the silt. From a macroscopic view, the mechanical properties of the silt will be improved. In addition, the reduction in pores will make it more difficult for water to enter the soil, which will strengthen the ability of the silt to resist water erosion and achieve the effect of improving soil reinforcement and waterproofing. The mechanism of improving silt using CLS is shown in Figure 10, and the chemical reactions are shown in Formulas (1) and (2).

H₂O ⇄ OH⁻ + H⁺

(1)

nC₂₀H₂₄CaO₁₀S₂ + nH⁺ → nC₂₀H₂₃CaO₉S₂⁺ + nH₂O

(2)

3.6. EDS Test Results

Figure 11, Figure 12 and Figure 13 show the EDS detection area and the energy spectrum of plain soil and 1.0% CLS modified soil. The comparative analysis of the determination results of sample elements before and after 1%CLS modification in

Table 6. Elemental content determination results.

Element		O	Na	Mg	Al	Si	K	Ca	Fe
Mass ratio%	(a)	46.31	1.8	1.41	9.04	29.37	2.92	1.75	4.16
	(b)	55.05	1.46	1.8	9.1	25.64	1.57	2.31	3.07
Atomic ratio%	(a)	62.56	1.69	1.25	7.24	22.6	1.61	0.95	1.61
	(b)	69.07	1.27	1.48	6.77	18.33	0.81	1.16	1.11

found that the oxygen content in the modified soil was higher than that in the plain soil. The content of silicon was lower because the large amount of oxygen contained in CLS led to a change in the proportion of the two elements after modification. At the same time, it was found that the calcium content increased slightly because the modified calcium lignosulfonate polymer was glued to the surface of the soil particles, leaving calcium in the soil. Overall, the addition of 1.0%CLS had no significant effect on the content of each element in the soil, which shows that the main components of the original soil did not change, and the modified soil had good compatibility (as shown in Figure 11, Figure 12 and Figure 13).

3.7. XRD Test Results

Figure 14 shows the XRD diffraction patterns of the plain and 1.0%CLS-modified soil.

The following can be seen in the peak curve in Figure 14: (1) The main components of the two samples did not change before and after doping the modified materials, and they all contained quartz, illite, calcite, albite, potash feldspar, and clinochlore. (2) Comparing the XRD diffraction patterns before and after modification, it was found that the XRD diffraction patterns of the two samples coincided, quartz with the highest diffraction peak was contained in the soil itself, and no new diffraction peaks or diffraction characteristic values appeared in the two patterns. Comparing the diffraction peaks of other minerals in the two samples, each peak had no obvious change before or after modification. Therefore, a small amount of lignosulphonate does not affect the main components of soil.

The results of the microscopic mechanism analysis are consistent with those of Bai [18], who improved loess with CLS, and Li [22], who improved silt with ammonium-based lignosulfonate. They both suggest that the enhancement of the properties of the modified soil is due to the coaxing, wrapping, and filling of soil particles with the CLS polymer, and no new compounds are generated.

4. Conclusions

In this paper, the optimum ratio of calcium lignosulfonate (CLS) to silt of earthen ruins was studied through mechanical and waterproof tests, and the reinforcement mechanism of CLS in silt was further analyzed through microscopic tests. The specific conclusions are summarized as follows:

(1)

In the direct shear test, under the same vertical stress condition, the shear strength of the samples mixed with CLS first increased and then decreased with the increase in the content. The modified soil samples’ cohesion and internal friction angle with a 1.0%CLS content reached the optimal values. In the unconfined compressive strength test, the variation law of compressive strength with the content of the sample was consistent with the shear strength, and the compressive strength of the modified soil sample with a 0.5%CLS content reached the optimal value. A low dose of CLS helps improve the mechanical properties of modified soil, while excessive addition of CLS will cause unreacted CLS to gather in the pores of the soil particles: on the one hand, it acts as a lubricant; on the other hand, its strength is small, which will affect the improvement of the samples’ mechanical properties.

(2)

In the capillary water rise test, the capillary water rise time of the modified soil samples increased with the increase in the CLS content. In the water-resistant disintegration test, the disintegration speed and amount of modified soil samples decreased with increased CLS content. From the results of the waterproof test, when the content of CLS reached 1.0%, the water resistance of the modified samples was significantly improved compared with that of the plain soil. Considering the factors of economic benefit, mechanical properties, and waterproof properties, 1.0%CLS was determined to be the best content for modified soil in this study.

(3)

The comparative analysis of the SEM, EDS, and XRD microscopic tests between the plain soil and 1.0%CLS-modified soil showed that the soil particles and the lignosulfonate polymer generated by CLS bonded with each other after adding 1.0%CLS, which made the type of connection between the particles mainly face-to-face and improved the compactness of the soil, thus improving the mechanical properties and waterproof properties of the silt to a certain extent. The EDS and XRD tests found that the content of the main elements in the modified soil did not change, nor did the mineral composition.

Previous studies have shown that adding a proper amount of CLS has the potential to improve the mechanical and waterproof properties of silty soil, and its characteristics of being non-toxic, harmless, and non-corrosive are more in line with the principles of cultural relic protection compared with traditional soil modification additives. This study provides a feasible scheme for the restoration and reinforcement of silty soil sites. However, further chemical analysis is needed to investigate the enhancement mechanism.