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Article

Mechanical Properties Test and Microscopic Mechanism of Lignin Combined with EICP to Improve Silty Clay

by
Cheng Peng
1,
Haiyan Zhou
1,*,
Bo Deng
1,
Dongxing Wang
1,2 and
Jierong Zhu
1
1
School of Civil Engineering, University of South China, Hengyang 421001, China
2
School of Civil Engineering, Wuhan University, Wuhan 430072, China
*
Author to whom correspondence should be addressed.
Sustainability 2025, 17(3), 975; https://doi.org/10.3390/su17030975
Submission received: 25 December 2024 / Revised: 17 January 2025 / Accepted: 21 January 2025 / Published: 25 January 2025
(This article belongs to the Section Sustainable Engineering and Science)
Browse Figures
Versions Notes

Abstract

:
To enhance the improvement effect of Enzyme-Induced Carbonate Precipitation (EICP) technology more effectively, an abundant renewable resource—lignin—was introduced as an additive during the EICP modification process of silty clay. The mechanical properties of the improved soil specimens were analyzed from a macroscopic point of view by using unconsolidated undrained (UU) triaxial tests and unconfined compressive strength (UCS) tests to determine the optimal lignin content and curing time. The micro-mechanism of the improved soil specimens was elucidated from the microscopic point of view by combining scanning electron microscopy (SEM) and X-ray diffraction (XRD) tests. The experimental results showed that lignin synergized with EICP could effectively improve the mechanical properties of the soil, and the mechanical properties of the co-consolidated soil specimens were better than those of the single consolidated and untreated soil specimens as a whole. The single EICP-consolidated soil specimen had undergone brittle damage; lignin could enhance the toughness of the soil and weaken its brittle characteristics. With the increase of lignin content, the mechanical indicators of co-consolidated soil specimens showed the trend of increasing and then decreasing, and reached the optimum at 0.75%. Moreover, the addition of lignin significantly increased the cohesive force, while the friction angle was less affected. With extended curing time, the mechanical indicators of the co-consolidated soil specimens increased overall, and tended to stabilize after 7 days of curing, hence selecting 7 days as the optimal curing time. From the microscopic point of view, lignin provides nucleation sites for the calcium carbonate precipitates generated by EICP, and the joint action of the two can fill the soil pores and cement the soil particles, thereby improving the overall strength of the soil. The results of the study can provide a theoretical basis and practical reference for the construction of foundation projects in silty clay areas.

1. Introduction

Silty clay exhibits poor engineering properties, including low strength and inadequate stability [1], which pose challenges to direct application. Traditional improvement methods primarily focus on physical [2,3] and chemical [4,5] reinforcement. Although these approaches can enhance soil strength to a certain extent, they each have limitations [6], making it difficult to meet the development of contemporary geotechnical engineering. Moreover, with the increasing attention paid to the ecological environment, energy saving and emission reduction in recent years, as well as the proposal of the dual-carbon target in China, there is an urgent need to find a new type of eco-friendly and sustainable soil improvement method.
Microbial induced carbonate precipitation (MICP) technology used as biological reinforcement has received widespread attention due to its advantages of high efficiency, environmental friendliness and economy [7,8]. However, at present, MICP is mainly used for the reinforcement of coarse-grained soils, such as gravel and sandy soils, and relatively few studies have been conducted on the consolidation of clayey soils. This is because, compared with coarse-grained soils, clayey particles have a smaller particle size, a lower porosity and a poorer permeability, which are not conducive to microbial migration in the soil [9], resulting in the “pore plugging” effect, which leads to an uneven curing effect. The microorganisms serving as the source of urease are not only expensive but also have complicated cultivation processes. In addition, the lack of adequate environmental monitoring methods makes it difficult to assess their practical impact on the environment [10]. In order to solve the above problems, some studies have found that some plants, such as legumes and melons, are rich in urease [11]. The use of small-particle-size free urease induces calcium carbonate precipitation, which can penetrate into finer-grained soils, fill the pores between soil particles and connect the particles together, thereby enhancing the soil strength [12,13]. This method ensures more uniform reinforcement of fine-grained soils, termed the EICP technique. Researchers have conducted in-depth studies on the application of EICP technology in soil improvement. For instance, Gao et al. [14] used EICP technology to improve chalky soil, and the results showed that the soil did not exhibit clogging and its mechanical properties were improved. Zhang et al. [15] investigated the impact of EICP technology on the long-term stability of loess soil, and their research indicated that the surface strength of loess soil was significantly enhanced, thereby improving the stability of the soil mass. Moghal et al. [16] investigated the effect of EICP treatment on the swelling, permeability and adsorption and desorption of heavy metals in two clayey soils from India and found that EICP could improve soil properties and enhance the remediation of pollutants. It can be seen that EICP technology has been widely used in many fields such as pollutant solidification [15], foundation treatment [17], dust and sand consolidation [18] and repair of material cracks [19].
However, most of the ureases extracted from plants are in a free state and lack the nucleation sites required for calcium carbonate precipitation, so the generated CaCO3 crystals are small and disorganized, and the contact points inside the soil are reduced, which makes the cured soil prone to brittle damage and reduces the effect of EICP technology in soil improvement [20]. When addressing the issues of insufficient nucleation sites for calcium carbonate and inadequate soil strength enhancement after modification posed by EICP technology, scholars [21,22] have proposed a solution involving the addition of organic materials or calcium carbonate “seeds” during the EICP process. The aim is to increase the nucleation sites for calcium carbonate, thereby enhancing soil performance. Lignin, which exists in large quantities as a renewable resource and which is inherently non-toxic, environmentally friendly and inexpensive, is considered an amendment capable of optimizing soil properties [23]. Studies have shown that lignin possesses properties such as filling and cementing [24]. Alazigha et al. [25] found that lignin can reduce the hydrophilicity of soil particles, thereby mitigating the swelling degree of expansive soils. Zhang et al. [26] found that incorporating lignin into silt produced lignin-based cemented materials that bond and fill pores between soil particles, forming a stronger soil structure and resulting in an increase in its undrained shear strength. Ji et al. [27] summarized the mechanism of lignin modification in dispersed soils as follows: there is reduction of the thickness of the soil particles’ double electric layer, bridging, cementing, hydrophobicity and lubrication, all of which improve the engineering properties of the soil. Therefore, the integration of lignin with EICP technology emerges as a potential approach to mitigate the shortcomings of the technology [28].
At present, most domestic studies on lignin-modified soils focus on chalky soils [29] and loess [30,31], while there are fewer studies on the improvement of silty clay soils. Therefore, considering the necessity of improving the poor engineering properties of silty clay, the limitations of the existing soil improvement and treatment techniques and the broad prospects of lignin improvement and reinforcement techniques, lignin, as an external admixture was applied to the EICP improvement of silty clay, which was used to cement and fill the pores between soil particles on the one hand, and to improve the attachment of free calcium carbonate as a nucleation site for EICP-generated calcium carbonate on the other hand. Through unconsolidated undrained (UU) triaxial tests and unconfined compressive strength (UCS) tests, mechanical parameters, such as peak shear strength, elastic modulus, cohesion, friction angle and unconfined compressive strength, were analyzed. Based on these analyses, the optimal curing time and lignin content were determined. From a macro perspective, the effect of lignin on the improvement of silty clay using EICP was analyzed. At the same time, scanning electron microscopy (SEM) and X-ray diffraction (XRD) tests were conducted to elucidate the enhancement mechanism of the lignin-combined EICP-improved silty clay from a microscopic perspective. This provides theoretical support and technical reference for reinforcement construction in silty clay areas.

2. Materials and Methods

2.1. Test Materials

2.1.1. Silty Clay

The test soil was taken from the Hengyang area of Hunan Province, China. The fundamental physical indicators of silty clay are shown in Table 1, and the particle grading curve of soil is shown in Figure 1. The plasticity index is 10 < Ip ≤ 17, and according to China’s Design Code for Building Foundations (GB50007-2011), the soil belongs to the category of low-liquid-limit silty clay.

2.1.2. Lignin

Calcium lignosulfonate, purchased from Shandong Yousuo Chemical Technology Co., Ltd., Linyi, China, with the molecular formula C20H24CaO10S2, is a yellow-brown, slightly aromatic, non-toxic and harmless powder that is easily soluble in water. It has good stability and belongs to the anionic surfactants. The basic properties of lignin are shown in Table 2.

2.1.3. Cementing Liquid

The cementing solution used in this test consisted of urea and calcium chloride anhydrous, both at a concentration of 2.1 mol/L, mixed in a volume ratio of 1:1. These chemicals were purchased from Tianjin Zhiyuan Chemical Reagent Co., Tianjin, China.

2.1.4. Soybean Urease

The steps of soybean urease extraction were as follows. Firstly, the commercially available soybean powder was put into a beaker after passing through a steel sieve of 80 mesh. It was then mixed with deionized water in a ratio of 1:10 to obtain a soybean liquid of 100 g/L. The soybean liquid was stirred by a magnetic stirrer for 30 min and subsequently placed in a low-temperature environment at 4 °C for 24 h. Afterwards, the suspension was filtered through a filter cloth into a centrifuge tube and centrifuged at 4500 r/min for 15 min. Finally, after centrifugation, the supernatant was filtered through a filter cloth again to obtain the crude soybean urease extract (urease solution).

2.2. Specimen Preparation

The specimen preparation steps strictly followed the provisions of China’s Standard for geotechnical testing method (GB/T 50123-2019) [32]. The specific process was as follows. Firstly, the soil was dried in an oven at 105 °C, crushed and passed through a 2 mm sieve, and impurities were removed. Then, the mixing treatment was carried out according to the test program (Table 3). The reaction liquid was a 1:1 mixture of soybean urease and cementing liquid by volume, the lignin mixing ratio was based on the lignin mass to dry soil mass and the water content of the manufactured specimens was set to the optimal water content corresponding to the mixing ratio. Then, the processed mixed soil was filled into a cylindrical mold, with a diameter of 39.1 mm and a height of 80 mm in five layers, with a scraping treatment applied between each layer. After the specimen preparation was completed, it was necessary to ensure that the surfaces at both ends were even, and subsequently, the mold was stripped. Finally, the specimens were sealed and placed into a constant temperature and humidity curing box set at 30 °C and 95% humidity for curing until the specified time.

2.3. Test Method

Triaxial Compression Test: To evaluate the improvement effects on mechanical parameters, such as peak shear strength, elastic modulus, cohesion and friction angle, an unconsolidated undrained (UU) triaxial compression test was conducted using the TSZ fully automatic triaxial apparatus. The test was set with a loading rate of 0.8 mm/min and was conducted at confining pressures of 100, 150 and 200 kPa, respectively [33]. The test was terminated once the axial strain reached 20%.
Unconfined Compressive Strength Test: Similarly, the TSZ fully automatic triaxial apparatus was utilized to conduct a compression test at a constant loading rate of 0.8 mm/min for the purpose of determining the UCS of soil specimens. During the test, loading ceased when the stress–strain curve exhibited a distinct peak and commenced to decline, at which point the peak value was recorded. In cases where no clear peak was observed, loading was terminated when the axial strain reached 20%.
Microscopic Test: After the UCS test, the crushed soil specimens were cut into 5 mm × 5 mm × 5 mm specimens. Vacuum metal sputtering technology was applied to the surfaces of these specimens, and then a scanning electron microscope (Sigma 300, Zeiss, German) was used to analyze the microstructural morphology and distribution of both untreated and improved soil specimens. Furthermore, the crushed test specimens were ground into powders, and their elemental compositions were analyzed using an X-ray diffractometer (D8-Advance, Bruker, German) with a scanning angle range of 10° to 80° and a step-scan rate of 10°/min.
The flow chart of the test is shown in Figure 2.

3. Results and Discussion

3.1. Triaxial Compression Test

3.1.1. Stress–Strain Curves

The stress–strain curves for the UU tests of EICP-amended soil, with varying lignin contents under different curing times and confining pressures, are depicted in Figure 3. Due to space limitations, the stress–strain change rules for the improved specimens are only illustrated under the same confining pressure for different curing times and under the same curing time for different confining pressures. Specifically, only the stress–strain curves for the improved specimens under a confining pressure of 200 kPa and a curing time of 14 days are presented.
From the stress–strain curves depicted in Figure 3, it was observed that under low confining pressures (100 kPa and 150 kPa), the soil specimens exhibited weak strain-softening characteristics, where the deviatoric stress gradually decreased as the axial strain increased after having reached its maximum value. However, under higher confining pressures (200 kPa), the soil specimens demonstrated strain-hardening characteristics, where the deviatoric stress continued to increase or remained constant as the axial strain increased after reaching its maximum value. The underlying reason is that when the confining pressure is low, the soil specimens exhibit consolidated behavior, with dilation occurring during shearing, resulting in a softening stress–strain curve. Conversely, when the confining pressure is high, the soil specimens display normal consolidation characteristics, with shear compression occurring during shearing, leading to a hardening stress–strain curve [34]. Compared to untreated soil, adding lignin and reaction liquid gradually increased the shear stress of silty clay, shifted the entire stress–strain curves upwards, and significantly enhanced the soil’s resistance to shear deformation. This indicated that both lignin and reaction liquid can effectively improve the soil’s performance.
To further analyze in depth the effects of curing time and confining pressure on the stress–strain change characteristics, the EICP-modified specimen with 0.75% lignin is taken as an example, as shown in Figure 4. These specimens all rapidly reached the linear elasticity stage when the internal structure of the soil was tight and within the elastic range. Figure 4a demonstrated that, under the same curing time, the stress–strain curves shifted upwards as a whole as the confining pressure increased. The peak deviatoric stress increased, and the curve morphology transitioned from a weakly softening type to a weakly hardening type. At that point, the shear strength also increased, and the damage mode of the soil changed from brittle damage to plastic damage. Meanwhile, Figure 4b indicated that, under the same confining pressure, extending the curing duration would also cause the stress–strain curve to shift upward, suggesting that a longer curing duration can enhance the shear strength of the soil.

3.1.2. Peak Shear Strength

To further investigate the strength evolution pattern of soil under varying confining pressures and curing times, the deviatoric stress at the peak was adopted as the peak shear strength for strain-softening curves, while for strain-hardening curves, the deviatoric stress corresponding to 15% axial strain was considered as the peak shear strength. An analysis was conducted using an example with a curing time of 3 days and a confining pressure of 100 kPa, as illustrated in Figure 5.
Overall, the modified soil enhanced the peak shear strength of the soil. Compared to single consolidated soil specimens (TM, TE), the co-consolidated soil specimens (TME) exhibited a higher peak shear strength. The peak shear strength first increased and then decreased as the lignin content increased, reaching its maximum for modified samples with a lignin content of 0.75%. According to Figure 5a, there was a positive correlation between the peak shear strength and confining pressure. This is because an increase in confining pressure led to closer contact between soil particles and more substantial lateral restraint, requiring a greater shear strength to cause the failure of the sample, thereby resulting in an increase in the peak shear strength. Figure 5b shows that, under the same confining pressure, when the curing duration was relatively short, the reinforcement effect of lignin and the reactive solution was not fully exerted. However, when the curing time had been 7 days, the increase in peak strength of the amended soil specimens had far exceeded that of the untreated soil specimens, further validating the enhancing effect of lignin and EICP on soil strength.

3.1.3. Elastic Modulus

Based on the characteristics of the soil stress–strain curves, the axial strain reaching 1% was selected as the reference point for the elastic deformation stage of the soil, and the ratio of axial stress increment to axial strain increment was defined as the elastic modulus, as shown in Figure 6. Overall, the addition of lignin and EICP increased the elastic modulus of the soil, enhancing its ability to resist deformation, which was consistent with the characteristics of the stress–strain curves. Meanwhile, the lignin content affected the elastic modulus of the co-consolidated soil specimens. As the lignin content increased, the elastic modulus first increased and then decreased, indicating that an appropriate amount of lignin is beneficial for improving the elastic modulus of silty clay. At the same time, excessive incorporation will reduce the elastic modulus of the soil. When the curing time remained constant, the elastic modulus of the samples increased with the increase in confining pressure. When the confining pressure was the same, the elastic modulus of the modified soil increased with the extension of the curing time, which was consistent with the aforementioned characteristics of peak shear strength changes.

3.1.4. Shear Strength Indicators

The shear strength indicators of lignin-EICP-improved soil under different curing times are shown in Figure 7. Overall, the cohesion and friction angle of the improved soil specimens were enhanced compared to the untreated soil. The cohesion of the co-consolidated soil specimens was superior to that of the soil improved by a single method. The cohesion trend first rose and then decreased with the increase in lignin content, reaching a maximum of 0.75% of the doping level. This indicated that, while the combined improvement effect is significant, it requires careful control of the lignin content. With the prolongation of the curing duration, the cohesion of EICP-amended soil increased significantly. Then, it slowed down, whereas the cohesion of lignin-only amended soil specimen gradually increased, suggesting that improving soil by lignin required time accumulation [24]. Based on the conclusions drawn by Shu et al. [35], the EICP process mainly was completed within 7 days. However, in the diagram, the soil cohesion improved by EICP only continued to increase on the 14th day. This is attributed to the transition of some calcium carbonate crystals changing from a metastable state to a stable state, which subsequently enhanced the soil strength. Taking the example of soil modified with 0.75% lignin combined with EICP, after a 7-day curing period, its cohesion reached 94.24 kPa, which represents a significant increase compared to the 68.77 kPa of untreated soil and the 84.69 kPa of soil that underwent a 3-day curing period. When the curing period was extended to 14 days, the cohesion of the modified soil further increased to 99.09 kPa. Although there was an increase in soil strength between the 7-day and 14-day curing periods, the difference was insignificant.
The increased friction angle of the improved soil specimens is attributed to the cementing effect of calcium carbonate crystals and lignin on the soil matrix, which facilitates closer contact between soil particles, enhances interlocking friction and reduces sliding friction between soil particles. Over time, the friction angle of soil improved solely with lignin continued to increase but with a modest magnitude, whereas the friction angle of soil improved through EICP fluctuated less. Overall, the changes in the friction angle of the improved soil specimens were relatively stable, and the influence of the curing duration on it could be neglected. Therefore, the improved soil enhanced its shear strength primarily by increasing soil cohesion.

3.2. Unconfined Compressive Strength Test

The effect of varying lignin contents in combination with EICP technology on the strength of silty clay under different curing times was investigated using the UCS test. Figure 8 demonstrates that the UCS of the improved soil specimens was higher than that of the untreated soil. Overall, the UCS of the co-consolidated soil specimens increased substantially and then slowly as the curing duration increased. Moreover, it first increased and then decreased as the lignin content increased, reaching a peak at a lignin content of 0.75%.
The reasons for the variation in strength concerning curing durations were as follows. In the initial stage of the reaction, the high concentration of cementing solution inhibited the activity of urease, preventing it from fully hydrolyzing urea, at which point the soil strength increased by approximately 74%. As time progressed, the reaction gradually completed, and the calcium carbonate cementation formed with lignin as nucleation sites increased continuously, resulting in a rapid strength increase of about 1.65 times. After 7 days of curing, the generation of new calcium carbonate crystals ceased forming, and at this stage, the calcium carbonate crystal gradually transitioned from a metastable state to stable calcite. The cementation effect of lignin with the soil continued to function, allowing the strength to continue to slowly increase after 7 days of curing. At this later stage, the soil strength had increased by about 1.9 times, but the growth rate was insignificant compared to that at the stage of 7-day curing. The reasons for the variation in strength with respect to lignin contents were as follows. Lignin could enhance the interparticle bonding between soil particles and provide nucleation sites for forming some free calcium carbonate. After the soil was cemented and filled, it became more compact, correspondingly increasing the compressive strength of the soil. However, excessive incorporation could lead to lignin aggregation, which in turn increased the spacing between soil particles and reduced the attractive force between them, making the particles prone to sliding, thereby lowering the compressive strength of the soil.
From the trend of the curves, the compressive strength of the soil specimens first increased and then decreased as the axial strain increased. This was due to the fact that once the stress of the specimen reached its peak, damage began to occur, causing the specimen to yield. Moreover, the slope of the improved soil specimens’ curves during the elastic stage was greater than that of the untreated soil, indicating that its duration in the elastic stage is relatively short. Taking the example of specimens cured for 3 days, it was damaged after the soil strength reached its peak. The strength of both the untreated soil and the soil improved solely with lignin decreased slowly. However, the strength of the soil improved by EICP dropped rapidly, showing prominent brittle failure characteristics. As the curing duration was extended, the elastic stage of the co-consolidation curves significantly increased, and the decline rate after reaching the peak slowed down. This indicated that lignin gradually exerts its effect after a period of curing, effectively reducing the brittle failure of the soil.
Based on the UU and UCS tests, the results of the study showed that 0.75% lignin content combined with the EICP technique had a more significant upgrading effect on the improved soil; the cohesive strength of the optimally co-consolidated soil increased by 23%, 37% and 44% compared to that of the untreated soil after 3, 7 and 14 days of curing, respectively, and accordingly, the UCS was increased by 0.74, 1.65 and 1.9 times, which indicated that the extension of the maintenance time from 7 days to 14 days was not significant to the soil strength enhancement. Considering the construction time and cost, 7 days was chosen as the optimum curing time and 0.75% lignin content in combination with EICP technology for reinforcing silty clay.

3.3. Microtesting and Mechanism Analysis

3.3.1. SEM

The specimens were scanned by an electron microscope to observe the internal microstructural morphology and product distribution pattern, and to analyze the improvement effect of lignin combined with EICP on silty clay. Figure 9 shows the untreated soil magnified 200 times and the co-consolidated specimen magnified 2000 times after 14 days of curing, respectively.
In the figure, it can be observed that the spacing of the soil particles is large, the arrangement is loose, the inter-particle connectivity is poor, the porosity is high and the macroscopic performance is poor with regard to engineering properties. In contrast, the pore distribution of the co-consolidated specimen is less obvious, and the lignin-generated reticulate cement connects and fills the soil particles between particles. At the same time, the EICP-generated calcium carbonate is similar to the shape of a flake or flat form, part of which directly fills the pore space of the soil particles, and part of which is attached to the reticulate cement. This “calcium carbonate-lignin-soil particles” lattice space structure dramatically improves the soil’s compactness and bonding strength, thus effectively improving the soil’s overall characteristics.

3.3.2. XRD

In order to examine the mineral composition of the specimens improved by EICP, XRD tests were conducted to analyze the crystal types present in both untreated soil and EICP-improved specimens after different curing times. The results of these tests are depicted in Figure 10. The primary mineral crystal types identified were quartz, muscovite, calcite and vaterite. A comparison of the mineral fractions revealed no change in the types of minerals present in the EICP-amended soil specimens compared to the untreated soil. However, there was a change in the relative abundance of these minerals. Notably, the diffraction intensity of calcium carbonate crystals in the EICP-reinforced soil increased markedly, indicating the formation of new calcium carbonate in the treated soil. The diffraction peaks of the specimens that were maintained for 7 days showed the presence of vaterite and calcite crystals, whereas the specimens maintained for 14 days exhibited a significantly higher abundance of calcite crystals and a lower abundance of vaterite crystals. Correlation studies have indicated [36] that calcite crystals are the more stable and robust form of calcium carbonate, whereas vaterite crystals represent a metastable state. This suggests that, over time, some of the metastable forms of calcium carbonate crystals will transition to the stable form, thereby enhancing the strength of the soil. This finding also corroborates the analysis of the shear strength indicators.

3.3.3. Mechanism Analysis

Based on observations at the micro-morphological level, an in-depth analysis of the improvement mechanisms of lignin and EICP was conducted, as illustrated in Figure 11. When EICP is incorporated alone, carbonate ions produced by urease-catalyzed urea hydrolysis combine with exogenous calcium ions to form calcium carbonate precipitates. Due to their small size and uneven distribution, some precipitates fail to effectively adhere to soil particle surfaces, while others interconnect and fill soil pores, forming a “calcium carbonate-soil particle” spatial structure. Although soil strength increases under these conditions, brittle failure is prone. When lignin is incorporated alone, it compresses the double electric layer of clay minerals, resulting in reduced spacing between soil particles and a more compact soil mass. Additionally, lignin undergoes protonation with water molecules to form lignin polymers [37], creating a cobweb-like connection and filling among soil particles, thereby constructing a “lignin-soil particle” skeletal structure. Under these circumstances, the soil exhibits good ductility. The combined action of both lignin and EICP leverages their respective advantages, achieving a reinforcement effect where “1 + 1 > 2”. Untreated soil particles are loose due to wide spacing and high porosity. The addition of lignin not only promotes the aggregation of soil particles into clusters, filling soil pores, but also provides nucleation sites for calcium carbonate, allowing some free calcium carbonate to deposit on its surface. The resulting “calcium carbonate-lignin-soil particle” networked spatial structure further fills and strengthens the internal connections of the soil, making the soil structure more compact and enhancing soil strength. However, excessive incorporation of lignin can lead to its own preferential binding, reducing the matric suction between soil particles, increasing soil porosity and spacing between particles, making them prone to sliding and weakening frictional strength, thereby reducing mechanical properties [27].

4. Discussion

The results of the aforementioned experimental analysis confirm the feasibility of improving silty clay using lignin combined with EICP. This method not only effectively mitigates the brittle failure of bio-solidified soil but also significantly enhances its mechanical properties, aligning with the conclusions drawn by Song et al. [38] regarding the use of different admixtures in combination with EICP for stabilizing red clay. Further microscopic testing reveals that lignin provides nucleation sites for the disordered calcium carbonate particles produced by EICP, causing the calcium carbonate to transition from a random coating distribution on the soil particle surfaces to a cohesive distribution between soil particles, forming a more tightly structured cementitious filler. This is the primary reason for the improvement in the specimen’s integrity and strength. This finding offers theoretical support for the resourceful utilization of lignin as a soil modifier and holds practical significance for the sustainable development of lignin and environmental protection efforts. Furthermore, this paper provides insights into the effects and mechanisms of lignin combined with EICP for improving silty clay, offering theoretical support and practical guidance for infrastructure construction in silty clay regions.
However, despite the promising results achieved in laboratory settings, this method may be subject to interference from dynamic loads in actual working conditions. To assess this potential impact, future experiments should consider testing under dynamic load cycling conditions to more accurately evaluate the long-term stability and durability of bio-reinforced materials. Additionally, more in-depth research on durability is needed to ensure that this technology maintains its reinforcing effect under various environmental conditions. Currently, research on bio-reinforcement technologies is still primarily focused on indoor environments, with limited reports on practical engineering applications. Therefore, future efforts should strengthen their application to real-world projects.

5. Conclusions

A novel method that combined EICP with lignin was investigated in detail to improve the undesirable properties of low-liquid-limit silty clay. The following conclusions were drawn from analyzing the stress–strain curves, peak shear strength, elastic modulus shear-strength indicators and unconfined compressive strength of mechanical tests by applying lignin as an admixture in the EICP improvement of silty clay, combined with microscopic methods, to explore the improvement mechanism.
  • The stress–strain curves of triaxial compression tests exhibits weak strain softening under low confining pressures and weak strain hardening under higher confining pressures. The combination of lignin and EICP can effectively enhance the mechanical properties of the soil, with the overall mechanical performance of the co-consolidated soil specimens being superior to that of singly consolidated and untreated soil specimens. Soil consolidated solely by EICP undergoes brittle failure, while the incorporation of lignin can enhance the ductility of the soil and weaken its brittle characteristics.
  • The incorporation of lignin enhances the effect of EICP in reinforcing silty clay. As the lignin content increases, the peak shear strength, elastic modulus, cohesion and UCS first increase and then decrease, reaching an optimum at a lignin content of 0.75%. The incorporation of lignin significantly improves cohesion but has a minor impact on the friction angle.
  • Extending the curing duration can effectively improve the mechanical properties of the improved soil. As the curing duration increases, the mechanical indicators of the soil improved, with the combination of lignin and EICP gradually increasing overall and tending to stabilize after 7 days of curing. Therefore, the optimal curing time is 7 days.
  • Lignin provides nucleation sites for the disordered calcium carbonate, and the synergistic effect of the two forms a network spatial structure of “calcium carbonate-lignin-soil particles” that fills and cements the soil particles, making the soil structure more compact and increasing its strength. As the curing duration extends, the type of calcium carbonate crystals gradually transforms into stable rhombic calcite, further enhancing the soil strength.

Author Contributions

Conceptualization, C.P. and H.Z.; formal analysis, J.Z.; data curation, B.D.; writing—original draft, C.P. and H.Z.; writing—review and editing, C.P. and H.Z.; supervision, B.D. and D.W.; funding acquisition, C.P. and B.D. All authors have read and agreed to the published version of the manuscript.

Funding

This study was supported by the Youth Science Foundation of the National Natural Science Foundation of China (Grant No. 52308356) and the Open Fund Grant of the Key Laboratory of Earth and Rock Dam Damage Mechanism and Prevention and Control Technology of the Ministry of Water Resources (Grant No. YK319008).

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.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Particle grading curve.
Figure 1. Particle grading curve.
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Figure 2. Flow chart of the test.
Figure 2. Flow chart of the test.
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Figure 3. Stress–strain curves of specimens under different curing times and confining pressures: (a) T = 3 d, σ3 = 200 kPa; (b) T = 7 d, σ3 = 200 kPa; (c) T = 14 d, σ3 = 200 kPa; (d) T = 14 d, σ3 = 100 kPa; and (e) T = 14 d, σ3 = 150 kPa.
Figure 3. Stress–strain curves of specimens under different curing times and confining pressures: (a) T = 3 d, σ3 = 200 kPa; (b) T = 7 d, σ3 = 200 kPa; (c) T = 14 d, σ3 = 200 kPa; (d) T = 14 d, σ3 = 100 kPa; and (e) T = 14 d, σ3 = 150 kPa.
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Figure 4. Stress–strain curves of lignin (0.75%) combined with EICP-amended soil: (a) T = 14 d; (b) σ3 = 200 kPa.
Figure 4. Stress–strain curves of lignin (0.75%) combined with EICP-amended soil: (a) T = 14 d; (b) σ3 = 200 kPa.
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Figure 5. Peak shear strength versus lignin content and curing time: (a) T = 3 d; (b) σ3 = 100 kPa.
Figure 5. Peak shear strength versus lignin content and curing time: (a) T = 3 d; (b) σ3 = 100 kPa.
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Figure 6. Elastic modulus versus conservation time: (a) T = 7 d; (b) σ3 = 150 kPa.
Figure 6. Elastic modulus versus conservation time: (a) T = 7 d; (b) σ3 = 150 kPa.
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Figure 7. Shear strength indicators at different curing times.
Figure 7. Shear strength indicators at different curing times.
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Figure 8. Stress–strain curves of unconfined compressive strength: (a) T = 3 d; (b) T = 7 d; (c) T = 14 d.
Figure 8. Stress–strain curves of unconfined compressive strength: (a) T = 3 d; (b) T = 7 d; (c) T = 14 d.
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Figure 9. SEM photographs of untreated soil and co-consolidated soil: (a) untreated soil magnified 200 times; (b) co-consolidated specimen magnified 2000 times after 14 days of curing.
Figure 9. SEM photographs of untreated soil and co-consolidated soil: (a) untreated soil magnified 200 times; (b) co-consolidated specimen magnified 2000 times after 14 days of curing.
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Figure 10. X-ray diffraction images of untreated soil and EICP-amended soils.
Figure 10. X-ray diffraction images of untreated soil and EICP-amended soils.
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Figure 11. Mechanism of lignin combined with EICP for clay amendment.
Figure 11. Mechanism of lignin combined with EICP for clay amendment.
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Table 1. Basic physical indicators of silty clay.
Table 1. Basic physical indicators of silty clay.
Liquid Limit
(%)		Plastic Limit
(%)		Plasticity IndexOptimum Moisture Content
(%)		Maximum Dry Density
(g/cm3)		Specific Gravity
33.216.716.516.81.832.72
Table 2. Basic properties of lignin.
Table 2. Basic properties of lignin.
AmendmentAppearanceLignin Content
(%)		Water-Insoluble Substances
(%)		pH
Calcium lignosulfonateYellow-brown powder>93≤0.54–6.5
Table 3. Test program.
Table 3. Test program.
Test
Number		TypeOptimum Moisture Content (%)UU Test Confining Pressure (kPa)Curing Time
(d)
TUntreated soil16.8100, 150, 2003, 7, 14
TMSoil + Lignin16.6
TESoil + Reaction liquid16.8
TME 0.25%Soil + Reaction liquid + 0.25% Lignin16.6
TME 0.5%Soil + Reaction liquid + 0.5% Lignin16.4
TME 0.75%Soil + Reaction liquid + 0.75% Lignin16.2
TME 1%Soil + Reaction liquid + 1% Lignin16.0
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Peng, C.; Zhou, H.; Deng, B.; Wang, D.; Zhu, J. Mechanical Properties Test and Microscopic Mechanism of Lignin Combined with EICP to Improve Silty Clay. Sustainability 2025, 17, 975. https://doi.org/10.3390/su17030975

AMA Style

Peng C, Zhou H, Deng B, Wang D, Zhu J. Mechanical Properties Test and Microscopic Mechanism of Lignin Combined with EICP to Improve Silty Clay. Sustainability. 2025; 17(3):975. https://doi.org/10.3390/su17030975

Chicago/Turabian Style

Peng, Cheng, Haiyan Zhou, Bo Deng, Dongxing Wang, and Jierong Zhu. 2025. "Mechanical Properties Test and Microscopic Mechanism of Lignin Combined with EICP to Improve Silty Clay" Sustainability 17, no. 3: 975. https://doi.org/10.3390/su17030975

APA Style

Peng, C., Zhou, H., Deng, B., Wang, D., & Zhu, J. (2025). Mechanical Properties Test and Microscopic Mechanism of Lignin Combined with EICP to Improve Silty Clay. Sustainability, 17(3), 975. https://doi.org/10.3390/su17030975

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