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Article

Experimental Study on Expansive Soil Improved by Lignin and Its Derivatives

by
Yi Cai
and
Mingxi Ou
*
Faculty of Civil Engineering and Mechanics, Kunming University of Science and Technology, Kunming 650500, China
*
Author to whom correspondence should be addressed.
Sustainability 2023, 15(11), 8764; https://doi.org/10.3390/su15118764
Submission received: 11 April 2023 / Revised: 22 May 2023 / Accepted: 23 May 2023 / Published: 29 May 2023
Browse Figures
Review Reports Versions Notes

Abstract

:
Expansive soil covers the vast area of Mengzi, Yunnan, China, and creates numerous hazards for construction projects. When treating expansive soil, a modifier is usually added to inhibit its expansion and increase its strength. Lignin and its derivatives can better meet the requirements of expansive soil treatment and have become the preferred choice to replace traditional inorganic modifiers. Lignin is a green and environmentally friendly physical improvement material. In this study, lignin was used to improve soil, alone and combined with its derivatives, and the physical and mechanical properties of the improved soil were studied. Combined with an unconfined compressive strength test, a low-stress direct shear test, and a scanning electron microscopy test, the mechanism of lignin and its derivatives for the improvement of expansive soil is discussed. When calcium lignosulfonate alone was added, the improved soil’s expansion rate decreased, the soil’s water-holding capacity decreased, and its strength increased. Furthermore, the inclusion of 3% calcium lignosulfonate was the best. When the expansive soil was improved with the optimal calcium lignosulfonate content (3% CL) and composite lignin fibers, the strength of the soil body was further improved, the toughness was enhanced, and it shows plastic swelling failure and good water stability. 3% calcium lignosulfonate and 1.5% lignin fiber was the best for composite improvement as; it offered the optimal degree of particle aggregation and the development of pores and cracks was better inhibited, even though the fiber distribution was messy. This study shows that lignin and its derivatives can be used instead of inorganic modifiers to treat expansive soils to reduce the number of inorganic modifiers, and provided a sustainable treatment plan for reducing industrial waste.

1. Introduction

Water expansion and dehydration contraction are the primary expansion and contraction characteristics of expansive soil, and they are also significant causes of engineering distress, such as uneven deformation, structural instability, building cracking, road bridge structure damage, etc. [1]. In the early 1970s, during the construction of the Taocha water diversion canal in the Nanyang area, more than ten landslide disasters occurred due to excavations in the expansive soil layer, some of which occurred on gentle slopes of 1:6, and these events attracted the special attention of engineers [2,3]. Numerous studies have been carried out, and it has been determined that the best solution for the problem of sliding on gentle slopes of expansive soils is the employment of the retaining reinforcement method, but this method also dramatically increases engineering costs and causes construction delays [4]. Shallow slope failure is a characteristic problem that occurs during construction in areas of expansive soil [5,6]. For this reason, it is essential to effectively improve this type of soil. At present, many achievements have been made regarding the mechanism, treatment methods, and prediction measures for expansive soil problems. These methods include measuring the free expansion rate of improved soil [7], liquid-plastic limit [8,9], direct shear test, and the unconfined compressive strength [10,11], which will be discussed. Although the traditional methods for improving expansive soil have a remarkable effect, such as adding lime [12], iron tailings [13], fly ash [14], machine-made sand [15], and other substances to improve the strength of the soil, these methods also have many deficiencies, including the resulting brittle failure characteristics of the improved soil, which are significant for the soil’s degree of durability. Furthermore, the resulting environmental pollution problems are also severe, and they do not support the concept of low-carbon emissions and environmental protection proposed by social development. For this reason, it is necessary to move away from non-environmentally friendly improvement means and, at the same time, seek green expansive soil improvement means.
As a recently adopted soil conditioner, lignin and its derivatives have gradually become the first choice in soil engineering improvement [16,17,18,19,20,21]. Lignin has many advantages, such as large reserves, low cost, it is easy to obtain, and it does not cause secondary pollution to the environment during its use [22,23]. China’s paper industry accounts for 26% of the world’s total paper production, it produces vast amounts of lignin and its derivatives, and this creates economic and sustainable conditions for its use as a modifier [24,25]. Alazigha et al. [26] used lignosulfonate to treat expansive soil, and found that the soil’s expansibility was weakened, showing non-brittle failure, and lignosulfonate significantly improved the strength and solidity of the soil samples under freeze-thaw/dry-wet cycle conditions. In conclusion, under the appropriate dosage, the improvement effect of lignosulfonate is better than that of other improvement substances, such as cement and lime. Ijaz et al. [27] studied the plasticity, expansion, shrinkage, strength, and permeability coefficient of expansive soil improved by lignosulfonate composite lime, and through combined XRD and SEM experiments, analyzed the improvement mechanism. The results showed that lignosulfonate could improve expansive soil properties. It can be used instead of lime when appropriate. Hao et al. [14] used sisal fiber composite fly ash to improve the expansive soil and found that the composite-improved soil had better mechanical strength than the single-mixed fly ash-improved soil, and the soil’s characteristic changes from the brittleness of single doping to the plasticity of compound doping. The incorporation of fibers can better inhibit the development of cracks in the soil. Our research group used lignin fiber composite high-calcium fly ash to improve similar expansive soil. The results showed that the engineering properties of the composite-modified soil can better meet engineering needs [28]. In the past, there were inorganic modifiers in the soil conditioners, or lignin sulfonate alone was used for improvement. Currently, there are few studies on the compound improvement of lignin and its derivatives. For this reason, this paper selects lignin and its derivatives (calcium lignosulfonate) as improvement agents. According to the current research results, the advantages of lignosulfonate and fiber in improving expansive soil are complementary, and can be used as a sustainable expansive soil improvement method.
This study involved a series of laboratory tests on the improved soil, such as the liquid plastic limit, free expansion rate, unconfined compressive strength, and water stability tests. In addition, a low-stress direct shear test was carried out according to the characteristics of shallow swelling landslide hazards [29]. The compound improvement mechanism was obtained by comparing and analyzing the changes in the soil properties before and after improvement, and an analysis was carried out on the development of microscopic characteristics and sample failure characteristics.The research content of this paper can provide a theoretical basis for soil engineering treatments in expansive soil areas.

2. Materials and Methods

2.1. Testing Material

The expansive soil used in the test was collected from a construction site in Mengzi City, Yunnan Province. The sampling depth was 2–3 m, and the color of the soil was grayish yellow, with a smooth surface and a complex texture. First, the impurities were removed, then the soil was air-dried and ground. The basic physical properties of the soil samples are shown in Table 1 and Figure 1. The free expansion rate of the soil sample was determined to be 50%. According to the technical code for buildings in expansive soil regions (GB50112-2013) [30], this soil sample is weakly expansive. Figure 2 presents the XRD pattern of the untreated Mengzi expansive soil.
Lignin fiber is produced by Langfang Tianya Energy Saving Technology Co., Ltd. (Langfang, China). Lignin is fibrous, gray-white in color, pH neutral, insoluble in water, and non-polluting to the environment, and it has good hydrophilicity. It is prepared through the chemical treatment of natural wood. The treatment temperature reaches as high as 260 °C. It has strong chemical stability and corrosion resistance, as shown in Figure 3a. The calcium lignosulfonate used in the experiment was from Guangzhou Yicheng Chemical Co., Ltd. (Guangzhou, China). Calcium lignosulfonate is a brownish-yellow powder that is easily soluble in water and has good stability. It has strong dispersibility, cohesiveness, and chelation. It can be used as a water reducer in engineering applications, as shown in Figure 3b.

2.2. Test Scheme and Test Method

2.2.1. Test Scheme

Once the sample soil was brought to the lab, it was dried and the impurities were removed. Then, the sample was placed in a mortar and ground into powder. Once ground, the soil was passed through 0.5 mm and 2 mm sieves, respectively. Calcium lignosulphonate was sieved through a 0.5 mm sieve and mixed with 1%, 2%, 3%, and 4% water in a beaker until fully dissolved. Then, it was added to the dried soil, the mixture was passed through the 0.5 mm sieve, and then stirred to determine the Atterberg limit before the free expansion rate tests were carried out. The sample preparation was repeated and passed through the 2 mm sieve. Then, the sample was thoroughly mixed with the optimal water content and placed in a closed container and left to soak for 24 h. Then, the improved soil was weighed according to the maximum dry density standard and placed in a ring cutter and an unconfined compressive specimen mold. The resulting specimen was formed using several soil layers and the interface of each layer was roughened with the soil cutter so that no fault occurred after the specimen was made. The height of the ring knife sample was 20 mm, and the diameter was 61.8 mm. The direct shear test and water stability test were then carried out. The unconfined compressive strength test was carried out on a sample that measured 80 mm in height and a 39.1 mm in diameter. The production process of the sample is shown in Figure 4.
According to Fan, more than 2% of lignin will agglomerate in expansive soil, which will affect its integrity and reduce the strength of the soil [31]. Therefore, the 0–2% range was selected for our study. The calcium lignosulfonate content with good physical and mechanical properties was combined with 0.5%, 1%, 1.5%, and 2% lignin fibers for the composite improvement. The material ratio of soil samples, as shown in Table 2, and the sample was prepared using a single improvement method. Each test was completed according to the standards set for geotechnical test methods [32] or technical specifications for building in expansive soil areas (GB50112-2013).

2.2.2. Test Method

The unconfined compressive strength test was carried out using a YYW-2 strain-controlled unconfined compressive instrument produced by Nanjing Soil Instrument Factory. The loading rate was 2.4 mm/min.
The stress condition for the direct shear test was set to low stress (quick direct shear test), and this was achieved using the sandbag equivalent substitution method. The downward vertical normal pressure was set to 15, 25, 35, and 50 kPa, for a total of four levels. The test instrument was a ZJ type of strain-controlled direct shear apparatus produced by the Nanjing Soil Instrument Factory, and the hand wheel speed was set to 4 r/min.
Scanning electron microscopy (SEM) was used to observe the microstructure characteristics of the soil. The test instrument was a TESCAN MIRALMS, produced by TESCAN Trading Co., Ltd(Shanghai, China). When the sample was prepared, a cross-section of the model was selected first for observation, and the complete section was cut and freeze-dried. Once frozen, the vacuum and gold spraying operations were carried out. Then, the sample was examined under an electron microscope. The magnification of the scanning electron microscope was 1000 times.

3. Test Result Analysis

3.1. Changes in the Physical Properties

3.1.1. Atterberg Limit

As shown in Figure 5, the expansive soil’s liquid limit and plasticity index decreased significantly after adding calcium lignosulfonate, and it reached its lowest values of 28.72 and 5.18 when the content was 1%, with a decrease of 46.82% and 83.52%. When the mixing amount was greater than 1%, it showed an increasing trend, and the boundary water content in the improved soil increased, but the increase rate was slow. For the plastic limit of the improved soil, the maximum value was 23.54 when mixed with 1% calcium lignosulfonate, which was 4.25% higher than the prime expansive soil. Upon adding calcium lignosulfonate, the moisture content of the improved soil was significantly lower than that of the plain expansive soil, and this was mainly related to the properties of calcium lignosulfonate itself, which is often used as a water-reducing agent in actual engineering applications [18].

3.1.2. Free Expansion Rate Test

Figure 6 is a histogram of the change in the free expansion rate of the sample at various dosages. Following the addition of calcium lignosulfonate, the lowest free expansion rate of the improved soil was observed in the sample with 3% calcium lignosulfonate content, and this dropped to 38.4%. As this was lower than 40%, the sample was identified as non-expandable soil because the content exceeded 3%. Then, the free expansion rate of the sample rebounded slightly and increased to 39.2%. This was mainly caused by the addition of calcium lignosulfonate, which is known to form a floc structure that adsorbs on the clay’s surface and coats it, and then agglomerates. The ion exchange between the admixture and the expansive soil increases the interlayer distance of the mineral components. Both significantly inhibit the water penetration to the interior and reduce the free expansion rate of the improved soil; if too much calcium lignosulfonate is added, the flocs formed in the soil increase, which increases the repulsion effect between the same-sex ions, causing the expansion rate to increase [27].

3.2. Changes of in the Mechanical Properties of Expansive Soil Modified by Single Addition of Calcium Lignosulfonate

3.2.1. Direct Shear Test

Slope collapse in expansive soil usually occurs in the shallow surface layer, that is, under low everyday stress [29]. In this regard, it is necessary to conduct shear strength tests of improved soil under low-stress conditions to explore the variation of c, φ, and shear strength with the addition of calcium lignosulfonate, which is very necessary for evaluating the shear effect of calcium lignosulfonate improved expansive soil. A slope collapse in expansive soil usually occurs in the shallow surface layer, that is, under low everyday stress [29]. In this regard, it was necessary to conduct shear strength tests on the improved soil under low-stress conditions to explore the variations of c and φ. This test was necessary for evaluating the shear effect of calcium lignosulfonate-improved expansive soil.
As shown in Figure 7, after adding calcium lignosulfonate, the shear strength of the improved soil was significantly improved compared with that of the prime expansive soil. The change rule is that with increasing calcium lignosulfonate content, the shear strength first increases and then decreases. Therefore, the shear strength of the improved soil with 3% calcium lignosulfonate content was the highest. For the changing law of c and φ values of soil with different calcium lignosulfonate content, Table 3 shows the fitting parameters of the cohesion and internal friction angle of the soil improved with different amounts of calcium lignosulfonate. With increased calcium lignosulfonate content, the cohesion force maintained an overall upward trend, up to 72.897 kPa, especially when the calcium lignosulfonate was just added; the increase rate was pronounced, from 37.702 kPa to 68.158 kPa, which was an increase of 80.7%. In comparison, the change of the internal friction angle was slightly smaller, it fluctuated up and down in the range of 24.2~46.6% compared with the plain soil. This is mainly because, after adding calcium lignosulfonate, cementing substances are generated in the soil to fill the pores. Compared with the prime expansive soil, the specific surface area of the particles in the improved soil increases and has a more substantial bonding effect, which is reflected in the improved soil macroscopically. In our study, the cohesion and internal friction angle increased, and the increase in cohesion was more pronounced. Our test results show that under low-stress conditions, the improved soil with calcium lignosulfonate alone has better shear strength and a significant improvement effect, and can be used appropriately in the soil improvement in slope engineering with expansive soil distributed in the shallow surface layer.

3.2.2. Unconfined Compressive Strength Test

Figure 8 shows the stress–strain curve of the modified expansive soil mixed only with calcium lignosulfonate. It can be seen from the figure that the plain soil exhibits strain hardening and its stress–strain turn is “short and fat” as a whole, with an obvious yield process; after adding calcium lignosulfonate, the strength of the sample begins to increase, and the stress–strain curve of the improved soil is 1% content, which is consistent with the plain soil performance. With the gradual content increase, the sample begins to show strain softening, and the stress–strain curve is “thin and tall”. Then, it begins to decrease after reaching its peak strength, and the residual strength of the sample is also reduced with the increase in calcium lignosulfonate content. It can be seen that 3% calcium lignosulfonate is optimal when only calcium lignosulfonate is added.
Figure 9 shows the variations in the unconfined compressive strength of soil improved with different amounts of calcium lignosulfonate. The curing period of the sample was seven days. It can be seen from the figure that with the addition of calcium lignosulfonate, the unconfined compressive strength of the improved soil increases first and then decreases, which is 1.14–1.41 times higher than that of the plain soil sample. When the calcium lignosulfonate content is 3%, the unconfined compressive strength reaches its maximum value, which is 238.96 kPa, and the power of the sample begins to decline after exceeding this value.

3.3. Changes in the Mechanical Properties of Expansive Soil Modified by Calcium Lignosulfonate-Lignin Fiber Composite

3.3.1. Direct Shear Test

Based on the unconfined compressive strength test and the direct shear test results, 3% calcium lignosulfonate alone mixed with 0.5%, 1%, 1.5%, and 2% lignin fiber, was used for the unconfined compressive strength test of the composite-improved soil.
Figure 10 is the direct shear fitting curve of the 3% calcium lignosulfonate samples with different amounts of lignin fiber. It can be seen from the figure that when 3% of calcium lignosulfonate amended soil is mixed with lignin fiber, the shear strength is improved, and it increases with the increase in lignin fiber content. Combined with the data in Table 4, the cohesion of the improved soil mixed with lignin fiber is higher than that of the enhanced soil mixed with 3% calcium lignosulfonate alone, and the increase ranges from 5.2% to 21.1%. The internal friction angle varies with the addition of lignin fiber, and the fluctuation range is 1.43~20%. This is because the addition of lignin fibers makes the calcium lignosulfonate particles more connected to each other. Once the sample is sheared, the fibers in the soil share part of the tensile stress. The more fibers are added, the more the shearing effect is inhibited. Hu Xuhui [6] and Xiao Jie et al.’s [29] studies on the change law of the shear strength index of expansive soil under low-stress conditions have determined that the cohesion of the soil deteriorates significantly when the shallow layer collapses, but the internal friction angle changes little. The sharp drop in cohesion at the sliding surface is an essential factor that can induce the external collapse of the expansive soil slope. The improvement method proposed in this paper contributes to improvements in soil cohesion. The increase in cohesion is more significant, and the effect is remarkable. This can provide a theoretical basis for shallow slope control.

3.3.2. Unconfined Compressive Strength Test

It can be seen from Figure 11 that the strength of the composite-improved soil mixed with lignin fiber increased first and then decreased with increasing lignin content at d 1 and d 7 of maintenance, and the highest intensity was 293.72 kPa for the composite-improved soil with 3% calcium lignosulfonate (CL) + 1.5% lignin fiber (LF) at d 7 of care. Compared with the 3% calcium lignosulfonate content and curing for seven days, and with the exception of 3% calcium lignosulfonate + 0.5% lignin fiber, the other samples saw increases in the lignin fiber content, which was 1.08~1.23 times of the single-mixed 3% calcium lignosulfonate-improved soil. When the lignin fiber content was 0.5%, the strength of the composite-improved soil was weaker than that of the single-mixed 3% calcium lignosulfonate-improved soil. The main reason for this is that it is difficult to improve the soil’s strength with the incorporation of a tiny amount of lignin fiber, but this small quantity can destroy the cemented structure of the sample mixed with only calcium lignosulfonate; therefore, the power of the sample is lost. The improved composite soil mixed with lignin had a better toughness in the small strain range. The strain corresponding to the peak strength of the sample was more significant, and the strength of the sample began to decrease when it reached its peak value. With the addition of lignin, the stress–strain curve of the sample gradually changed to a “thin and tall” type, but there was a higher residual strength after the sample was destroyed. Among these samples, the 3% calcium lignosulfonate + 1.5% lignin fiber composite improved the soil and had the best performance.
Figure 12 shows the effect of the lignin fiber content and curing age on the unconfined compressive strength of the improved soil. The unconfined compressive strength of the composite-improved samples increased with the increase of the curing age, and when the curing age increased from 1d to 7d, the strength of the sample increased by 1.05~1.18 times. The test shows that when more fibers are added, the curing time can be appropriately increased so that the composite-improved soil has greater strength. Combined with the research results of Wang Huan, on the use of lignin to improve expansive soil, calcium lignosulfonate-improved expansive soil mainly forms cementing substances and cohesive particles to fill pores, strengthen the compactness of the soil, and then improves the mechanical strength of the improved soil. In addition, after the soil is compacted, water can be prevented from entering the soil, the expansibility of the improved soil is weakened, and the stability of water immersion is enhanced. Since no new mineral components are formed before or after improvement, the increase in the strength of the sample and curing age does not significantly increase the strength of the soil [33,34]. In this study, based on single-doped calcium lignosulfonate, lignin fiber was added for the composite improvement, which can interconnect the particles formed in the improved soil and enhance the failure toughness of the soil. Since no new mineral components are included before or after improvement, the increase in the strength of the sample and the curing age will not increase the strength of the soil.

3.4. Failure Characteristics of the Improved Soil Samples

Figure 13 shows the failure patterns of various improved soil samples in the unconfined compressive strength test. It shows the prime soil, 3% calcium lignosulfonate-improved soil, and 3% calcium lignosulfonate + 1.5% lignin fiber composite-improved soil (7d). It can be seen from the figure that when the loading process of the bare soil reaches failure, cracks first appear in the middle and extend outwards from the center until it has lost its strength. The overall shape of the sample can be maintained during the test, the strain-hardening characteristics are apparent, and the plasticity is better. For the 3% calcium lignosulfonate-modified soil, oblique cracks appeared during the loading process, and the gaps were wide and gradually extended to both ends, transversely penetrating through the outer surface of the sample. The failure surface was more prominent, forming a 45° angle with the horizontal plane and the loading process. There were peeling fragments in the middle skin, the strength of the sample rapidly deteriorated after reaching its peak value, the characteristics of strain softening became apparent, and this was identified as “brittle failure.” The crack development of the 3% calcium lignosulfonate + 1.5% lignin fiber composite-improved soil was slightly weaker than in the former two samples, and the “plastic swelling” type of damage was more prominent as there were many cracks in the bulging section. However, it maintained its overall shape, and the development of pores was not apparent. During the test, the soil was mainly compressed in the axial direction, with no outward cracking.

3.5. Water Stability Test

We selected the prime expansive soil and 3% calcium lignosulfonate + 1.5% lignin fiber composite samples for the water stability test, and observed the changes in the water stability of the samples before and after improvement. Then, we analyzed the difference in the water stability before and after modification, as shown in Table 5. Following the addition of the improved substance, the water stability of the sample was enhanced, no cracks developed during the soil sample’s long-term immersion state, and the degree of disintegration was significantly reduced. This is because incorporating calcium lignosulfonate strengthens the bonding performance of the soil and fully exerts its function as a water-reducing agent. At the same time, it is combined with lignin fibers to form a tighter overall structure, which significantly inhibits the disintegration of soil samples in water, and improves the water stability of the soil. It can be used for slope engineering construction in expansive soil areas with rainy conditions [35,36].

4. Composite Improved Micro-Mechanism Analysis

Figure 14a–c shows the microstructure of the sample of plain soil, 3% calcium lignosulfonate, and 3% calcium lignosulfonate + 1.5% lignin fiber content (curing age 7d) magnified 1000 times. Observing the change law of the microstructure inside the soil before and after improvement and under the dosage of various improvement substances, we thus obtain the improvement mechanism.
It can be seen from Figure 14a that there are many pores distributed in the prime expansive soil, and these are accompanied by the development of cracks. The cracks are wide and connected, providing favorable conditions for the penetration of water. The hydrophilic mineral component montmorillonite forms micro soil body particles that have a flat and curved structure, are loosely arranged, and are prone to expansion and contraction deformation when exposed to water. It can be seen from Figure 14b that after adding 3% calcium lignosulfonate, the pores and cracks in the sample are greatly reduced, and the sheet structure is transformed into a micro-particle structure. The condensed micro-particles are scattered and filled, and the soil’s compactness is improved, effectively improving the soil strength. From Figure 14c, it can be seen that after adding 1.5% lignin fiber following the addition of 3% calcium lignosulfonate, the micro-cracks in the soil are further reduced, the micro-particles are close to each other to form a larger particle structure, and they are distributed in the micro-cracks. Lignin fibers are also found in some places, and the fibers’ bonding effect improves the soil’s strength. The addition of fibers inhibits the further development of pores and cracks, forming an intricate network structure that increases the toughness of the sample when it fails.
Prime expansive soil contains many hydrophilic minerals, and water molecules enter the mineral crystal layer, increasing the interlayer spacing of crystals and causing the clay surface to thicken the water layer, and then the soil expands. The mineral components present in the plain expansive soil form a flat and curved structure, and a large number of surface-surface structures weaken the sample’s strength when they are destroyed. In calcium lignosulfonate-modified soil, calcium lignosulfonate is randomly embedded between expansive soil particles, flocculation occurs in the diffuse double layer of the expansive mineral components, calcium lignosulfonate agglomerates to form aggregates, and externally wraps adsorbed non-expandable mineral components to jointly change the soil structure of the plain expansive soil and promote the transformation of a flat, curved shape into a particle aggregate structure [16]. Calcium lignosulfonate dissolves in water to form positively charged aggregates, and when it is combined with negatively charged expansion substances, neutralization occurs, reducing the surface potential of minerals and reducing the distance between soil particles. Calcium lignosulfonate agglomerates have cementing properties, and when cemented with soil particles, they jointly fill the pore structure [37]. Calcium lignosulfonate improved the soil before and after new substances were formed, and the improvement effect was more stable, which was considered a physical improvement [33]. Upon adding lignin fibers, the material structure in the improved soil has become more complex; the fibers further connect the cement and soil particles, and the fiber connection structure contributes to the sample’s ability to resist external forces. As the fibers are randomly distributed in the soil, when the surface of the soil is sheared and damaged, the fibers protrude through the surface at different angles, which inhibits the shear movement between the soil particles, and plastic swelling occurs when it is destroyed, thereby achieving the compound improvement effect [38,39,40].

5. Conclusions

The test results show that the physical properties, macro-mechanical indicators, and micro-structural characteristics of the improved soil with different dosages can be obtained. The feasibility and effect of compounding lignin and its derivatives to improve expansive soil were explored. The specific research results are as follows:
  • Upon adding calcium lignosulfonate, the improved soil’s liquid limit and plasticity index decreased, the lowest results were 28.72 and 5.18, with a decrease of 46.82% and 83.52%;
  • Following the addition of calcium lignosulfonate, the combination of hydrophilic minerals and water in the soil decreased, achieving the effect of weakening the expansion; thus, the expansion rate of the improved soil was reduced, and the free expansion rate dropped to 38.4%, which can be identified as non-expansive soil. When carrying out the free expansion rate test, the expansion rate increased slightly with the continuous addition of calcium lignosulfonate;
  • The shear strength of the modified soil mixed only with calcium lignosulfonate increased significantly under low-stress conditions, especially in terms of cohesion, by up to 72.897 kPa. The change of the internal friction angle was slightly smaller than that of the prime expansive soil, which fluctuated internally in the range of 24.2~46.6%. The 3% dosage of calcium lignosulfonate was determined to be the best, but its strength would have been reduced if too much had been added. When the composite lignin fiber was mixed with the 3% dosage of calcium lignosulfonate, the shear strength of the improved soil was further improved, the cohesion increased by 5.2–21.1%, and the internal friction angle increased by a maximum of 20%. This result could be used as the theoretical basis for shallow landslide prevention;
  • The unconfined compressive strength of the improved soil mixed with calcium lignosulfonate increased first and then decreased. It was 1.14–1.41 times higher than that of the prime expansive soil sample. The unconfined compressive strength of the improved soil reached its maximum when the content of calcium lignosulfonate was 3%, which is 238.96 kPa. The unconfined compressive strength of composite lignin fiber-improved soil increased by 1.08–1.23 times based on single mixing, and the strength of 3% calcium lignosulfonate + 1.5% lignin fiber composite-improved soil was the highest, which was 293.72 kPa. This improvement method is a physical improvement, no hydration product was formed, and the effect of increasing the curing time did not improve the soil’s strength;
  • Upon adding lignin and its derivatives, the internal connection effect of the improved soil was better, and the degree of particle aggregation was increased. The mechanical properties of the composite-improved sample were further enhanced based on the improvement of calcium lignosulfonate, manifested as a plastic bulging failure. Incorporating lignin and its derivatives inhibited the development and extension of tiny pores and cracks, reduced water penetration, and enhanced the water stability of the sample. Three percent calcium lignosulfonate + 1.5% lignin fiber has been determined to be the best ratio for compound improvement;
  • Lignin and its derivatives are non-polluting to the environment, and the improvement process does not produce new chemical substances. Improper materials are cheap and easy to obtain, which is not in line with the concept of sustainable development. Combined with the low-stress direct shear test set up in this paper, the shallow surface condition makes the application of the test results more feasible in terms of treatment depth and can be applied to expansive soil shallow slopes or in projects with low-overlying loads.

Author Contributions

Conceptualization, Y.C. and M.O.; methodology, Y.C.; formal analysis, Y.C.; investigation, M.O.; resources, M.O.; data curation, M.O.; writing—original draft preparation, M.O.; writing—review and visualization, Y.C.; supervision, Y.C.; project administration, M.O.; funding acquisition, M.O. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by National Natural Science Foundation of China, grant number 51968036.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data used to support the findings of this study are included in the article.

Conflicts of Interest

The author declares no conflict of interest.

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Figure 1. Particle grading curve of the Mengzi expansive soil.
Figure 1. Particle grading curve of the Mengzi expansive soil.
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Figure 2. XRD pattern of the un-treated Mengzi expansive soil.
Figure 2. XRD pattern of the un-treated Mengzi expansive soil.
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Figure 3. Lignin fiber and calcium lignosulfonate powder: (a) lignin fiber and (b) calcium lignosulfonate powder.
Figure 3. Lignin fiber and calcium lignosulfonate powder: (a) lignin fiber and (b) calcium lignosulfonate powder.
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Figure 4. Sample production process.
Figure 4. Sample production process.
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Figure 5. Influence curve of the different amounts of calcium lignosulfonate on the Atterberg limits.
Figure 5. Influence curve of the different amounts of calcium lignosulfonate on the Atterberg limits.
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Figure 6. Free expansion rate change histogram sample.
Figure 6. Free expansion rate change histogram sample.
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Figure 7. The fitting curves of the direct shear test for the improved soil with different amounts of calcium lignosulfonate.
Figure 7. The fitting curves of the direct shear test for the improved soil with different amounts of calcium lignosulfonate.
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Figure 8. Stress–strain curves of the modified expansive soil mixed only with calcium lignosulfonate (7d).
Figure 8. Stress–strain curves of the modified expansive soil mixed only with calcium lignosulfonate (7d).
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Figure 9. Variations in the unconfined compressive strength of the improved soil with different amounts of calcium lignosulfonate (7d).
Figure 9. Variations in the unconfined compressive strength of the improved soil with different amounts of calcium lignosulfonate (7d).
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Figure 10. Direct shear fitting curves of the 3% calcium lignosulfonate samples with different amounts of lignin fiber.
Figure 10. Direct shear fitting curves of the 3% calcium lignosulfonate samples with different amounts of lignin fiber.
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Figure 11. Stress–strain curves of the composite-improved expansive soil at different curing ages: (a) stress–strain curve of the compound-improved expansive soil (d 1); (b) stress–strain curve of the compound improved expansive soil (d 7).
Figure 11. Stress–strain curves of the composite-improved expansive soil at different curing ages: (a) stress–strain curve of the compound-improved expansive soil (d 1); (b) stress–strain curve of the compound improved expansive soil (d 7).
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Figure 12. Effects of lignin fiber content and curing age on unconfined compressive strength of improved soil.
Figure 12. Effects of lignin fiber content and curing age on unconfined compressive strength of improved soil.
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Figure 13. Failure mode of the samples: (a) prime expansive soil; (b) 3% calcium lignosulfonate-improved soil; and (c) 3% calcium lignosulfonate + 1.5% lignin fiber composite-improved soil.
Figure 13. Failure mode of the samples: (a) prime expansive soil; (b) 3% calcium lignosulfonate-improved soil; and (c) 3% calcium lignosulfonate + 1.5% lignin fiber composite-improved soil.
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Figure 14. Microstructure of the samples: (a) prime expansive soil; (b) 3% calcium lignosulfonate-improved soil; and (c) 3% calcium lignosulfonate + 1.5% lignin fiber composite-improved soil.
Figure 14. Microstructure of the samples: (a) prime expansive soil; (b) 3% calcium lignosulfonate-improved soil; and (c) 3% calcium lignosulfonate + 1.5% lignin fiber composite-improved soil.
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Table
Table 1. Physical parameters of the Mengzi expansive soil.
Table 1. Physical parameters of the Mengzi expansive soil.
Liquid Limit (%)Plastic Limit (%)Plasticity IndexMaximum Dry Density (g/cm3)Optimal Moisture Content (%)Free Swelling Ratio (%)
54.0122.5831.431.6519.0350.0
Table
Table 2. The material ratio of soil samples (mass ratio).
Table 2. The material ratio of soil samples (mass ratio).
Sample NameCalcium Lignosulfonate Content/%Lignin Fiber/%
Prime expansive soil//
Calcium lignosulfonate-improved soil1.0/2.0/3.0/4.0/
Composite-improved soilOptimal Calcium lignosulfonate content0.5
1.0
1.5
2.0
Table
Table 3. Fitting parameters of the cohesion and internal friction angle of soil improved with different amounts of calcium lignosulfonate.
Table 3. Fitting parameters of the cohesion and internal friction angle of soil improved with different amounts of calcium lignosulfonate.
Calcium Lignosulfonate Content %C/kPaφ/(°)R2
037.70230.6890.9803
168.15830.5750.9407
256.79942.3230.9644
365.92444.9910.9016
472.89723.2580.9138
Table
Table 4. Fitting parameters of the cohesion and internal friction angle of the 3% calcium lignosulfonate-improved soil with different amounts of fiber.
Table 4. Fitting parameters of the cohesion and internal friction angle of the 3% calcium lignosulfonate-improved soil with different amounts of fiber.
Lignin Fiber Content %C/kPaφ/(°)R2
Prime expansive soil37.70230.6890.9803
065.92444.9910.9016
0.576.18135.9900.9367
1.069.35145.2100.9379
1.575.04145.6350.9887
2.079.83741.3240.9460
Table
Table 5. Changes in water stability of samples before and after improvement.
Table 5. Changes in water stability of samples before and after improvement.
Sample NameBeginning of the Soaking in Water Process (within 30 min)Soaking Process (48 h)Final Result (96 h)
Prime expansive soilThere are many tiny gaps on the sample’s surface, accompanied by bubbles.The cracks in the sample begin to develop, gradually extending to both ends of the sample, and there are small cracks at the ends.The cracks in the sample gradually increase, running through the entire sample, the end cracks are fully developed, and the crack width increases.
Three percent calcium lignosulfonate+1.5% lignin fiber composite samplesThere is no noticeable change in the overall sample, a small amount of air bubbles are generated on the surface, and no cracks are formed.Some “foaming” particles appear in the sample, and the overall sample increases slightly.Cracks appear on the edge of the sample and start to peel off, but no shots are formed in the middle, and the integrity can still be maintained.
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Cai, Y.; Ou, M. Experimental Study on Expansive Soil Improved by Lignin and Its Derivatives. Sustainability 2023, 15, 8764. https://doi.org/10.3390/su15118764

AMA Style

Cai Y, Ou M. Experimental Study on Expansive Soil Improved by Lignin and Its Derivatives. Sustainability. 2023; 15(11):8764. https://doi.org/10.3390/su15118764

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Cai, Yi, and Mingxi Ou. 2023. "Experimental Study on Expansive Soil Improved by Lignin and Its Derivatives" Sustainability 15, no. 11: 8764. https://doi.org/10.3390/su15118764

APA Style

Cai, Y., & Ou, M. (2023). Experimental Study on Expansive Soil Improved by Lignin and Its Derivatives. Sustainability, 15(11), 8764. https://doi.org/10.3390/su15118764

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