Research on Engineering Characteristics of Lignin–Cement-Stabilized Lead-Contaminated Lateritic Clay

Chen, Junhua; Wei, Xiulin; Huang, Bocheng; Chen, Aijun; Shi, Xiong; Li, Shouqian; Xiao, Ying; Liao, Xiao; Zhao, Liuxuan

doi:10.3390/buildings15244433

Open AccessArticle

Research on Engineering Characteristics of Lignin–Cement-Stabilized Lead-Contaminated Lateritic Clay

by

Junhua Chen

^1,2,

Xiulin Wei

^2,*

,

Bocheng Huang

²,

Aijun Chen

²,

Xiong Shi

²,

Shouqian Li

²,

Ying Xiao

²,

Xiao Liao

² and

Liuxuan Zhao

²

¹

School of Civil Engineering, Central South University, Changsha 410075, China

²

School of Architecture and Transportation Engineering, Guilin University of Electronic Technology, Guilin 541004, China

^*

Author to whom correspondence should be addressed.

Buildings 2025, 15(24), 4433; https://doi.org/10.3390/buildings15244433

Submission received: 25 September 2025 / Revised: 26 November 2025 / Accepted: 28 November 2025 / Published: 8 December 2025

(This article belongs to the Section Building Structures)

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Abstract

This study systematically investigates the engineering characteristics of lead-contaminated red clay stabilized by calcium lignosulfonate and ordinary Portland cement composite binders. A series of experiments were conducted to evaluate the effects of lignosulfonate contents (0%, 0.25%, 0.5%, 1%, 2%), cement content (4%, 6%, 8%, 10%), and lead ion concentration (0%, 0.1%, 1%) on the mechanical properties, permeability characteristics, and leaching behavior. Key findings include the following. (1) Based on the highest mean UCS values observed in this study, the best-performing formulations were 1% lignosulfonate + 4% cement for uncontaminated soil, 0.5% lignosulfonate + 4% cement for 0.1% lead, and 0.25% lignosulfonate + 10% cement for 1% lead. (2) The permeability coefficient initially decreases and then increases with lignosulfonate addition, with maximum reductions of 65.9% and 44.4% for 0.1% and 1% lead contamination under their respective best-performing formulations under these specific test conditions. (3) The leaching concentration of 0.1% lead-contaminated soil met the national standard (<5 mg/L). Critically, however, the 1% lead-contaminated soil failed the TCLP test, with a leaching concentration of 37.3 mg/L, vastly exceeding the regulatory limit. This constitutes a treatment failure for environmental safety purposes, rendering the concurrent mechanical strength improvement irrelevant. (4) Microstructural and X-Ray Diffraction analyses (SEM and XRD) suggest that lignosulfonate improves soil structure by promoting the formation of C-S-H gel and ettringite (3CaO·Al₂O₃·3CaSO₄·32H₂O), whereas high lead concentrations inhibit ettringite formation. This research provides a theoretical foundation for the multi-criteria evaluation and application of lignosulfonate–cement composites in lead-contaminated soil remediation.

Keywords:

lead ion-contaminated soil; calcium lignosulfonate; cement stabilization; strength characteristics; permeability; toxicity characteristic leaching procedure concentration

1. Introduction

The escalating global issue of soil heavy metal contamination [1], driven by industrialization and urbanization, necessitates the development of efficient and sustainable remediation technologies. Among various heavy metals, lead (Pb) poses a particularly severe threat due to its high toxicity, mobility, and persistence, which disrupts soil ecosystems and endangers human health through bioaccumulation.

Solidification/stabilization (S/S) has emerged as a predominant approach for treating heavy metal-contaminated soils, with cement-based materials being the conventional cornerstone due to their high efficiency and cost-effectiveness [2,3,4]. These materials immobilize heavy metals primarily through physical encapsulation and chemical precipitation [5]. However, this conventional approach faces a critical dilemma: its substantial carbon footprint undermines its long-term sustainability, and its performance can be inefficient for treating soils with low to moderate levels of contamination.

This dilemma has spurred the search for sustainable, biomass-derived supplementary materials. Calcium lignosulfonate, an abundant industrial by-product, emerges as a promising candidate. It is rich in functional groups (e.g., hydroxyl, methoxy) that can chelate heavy metal ions and, notably, has been reported to potentially enhance cement hydration reactions [6,7,8,9,10,11]. This dual functionality suggests a potential synergistic effect when used in conjunction with cement.

Substantial research efforts have been dedicated to optimizing cement-based S/S systems. Studies have successfully incorporated supplementary materials such as red mud, slag, fly ash, and biochar to enhance solidification efficacy for specific contaminants, like copper and cadmium [12,13,14,15,16,17]. Concurrently, the independent utility of lignin in geotechnical engineering and environmental remediation has been well-documented. It has been proven to improve the structure of loess [18], enhance silty clays when combined with enzyme-induced carbonate precipitation [19], suppress swelling in expansive soils [20], and immobilize heavy metals, like Cd and Pb, through adsorption and complexation [21,22,23,24].

Despite these parallel advancements, a critical knowledge gap persists at their intersection. The synergistic mechanisms of a lignin cement composite system specifically designed for Pb-contaminated soil remain inadequately explored and poorly quantified. Key questions regarding the adaptability of this synergy to varying Pb contamination levels, its manifestation in comprehensive engineering properties (strength, permeability, and leaching behavior), and the underlying microstructural evolution are yet to be systematically answered.

Therefore, this study aims to fill this precise research gap by conducting a systematic experimental investigation on the engineering characteristics of lignin–cement-stabilized lead-contaminated lateritic clay. This work primarily focuses on the short-term performance under standard curing conditions. This article focuses on elucidating the effects of lignin content, cement content, and lead ion concentration on the unconfined compressive strength, permeability, and leaching toxicity after a 28-day curing period. Furthermore, scanning electron microscopy (SEM) and X-Ray Diffraction (XRD) are employed to unravel the microstructural mechanisms governing the stabilization process. The overarching goal is to provide a preliminary theoretical foundation and practical insights for the application of this sustainable composite binder in the remediation of lead-contaminated soils. It is important to note that the long-term durability and field performance under environmental cycles are beyond the scope of this initial investigation and must be validated in future work.

2. Experimental Program Design

2.1. Materials and Their Characterization

(1) The red clay investigated in this study was collected from Guilin, Guangxi, with its fundamental physicochemical properties detailed in Table 1. According to the “Soil Environmental Quality Risk Control Standard for Soil Contamination of Development Land” (GB 36600-2018) [25], the regulatory limit for lead in Class I land is 800 mg/kg and 2500 mg/kg for Class II land in China. Based on the Class I limit, two contamination levels were set, 1000 mg/kg and 10,000 mg/kg, corresponding to 0.1% and 1% by mass, respectively. It is important to note that even the lower contamination level (0.1%) already exceeds the Class I regulatory limit by 25%, while the higher level (1%) reaches 12.5 times the standard. These two levels were selected to represent contamination scenarios of 1.25 times and 12.5 times the Class I regulatory threshold, simulating low and high levels of lead-contaminated soil, respectively. Lead nitrate (Pb(NO₃)₂), ≥99.0% purity, supplied by Guangdong Yuneng Laboratory Equipment Technology Co., Ltd., Foshan, China, was used as the contaminant source in this study to simulate a scenario of soluble lead contamination. The lead nitrate solution was thoroughly mixed with air-dried and sieved red clay to ensure that lead existed primarily in the exchangeable and soluble forms within the soil matrix. This approach represents a “worst-case” scenario in terms of lead mobility and reactivity, which is highly relevant for assessing the immobilization capacity of the binder against the most challenging fraction of contaminants.

However, it is critically important to note that in actual contaminated sites, lead undergoes long-term aging processes. During aging, soluble lead can transform into more stable fractions through specific sequestration mechanisms with soil components, such as adsorption onto clay mineral surfaces and iron–aluminum oxides, incorporation into carbonate minerals, or formation of organic complexes. These aged lead species are typically less reactive and mobile. Therefore, the fixation mechanisms, kinetics, and ultimate remediation efficacy in real field conditions, where the binder must target these more stable and less accessible lead pools, may differ significantly from those observed under the experimental conditions of this study. Consequently, the stabilization efficiency reported here might represent an upper limit for this specific binder system.

(2) The cement contents of 4%, 6%, 8%, and 10% (by mass of dry soil) were systematically determined through preliminary tests and comprehensive consideration of stabilization efficiency and economic feasibility. The 4% threshold was established as the minimum content providing significant strength improvement, while the selected gradient enables analysis of the dose–response relationship from marginal to substantial enhancement. The 10% upper limit balances stabilization efficacy with economic and environmental considerations, preventing diminished advantages of the composite binder through excessive cement usage. The cement used was Type P·O 42.5 ordinary Portland cement produced by Shandong Fuzhu Building Materials Co., Ltd., Weifang, China, with its key technical parameters detailed in Table 2.

Calcium lignosulfonate was procured from Wengjiang Biotechnology Co., Ltd., Shaoguan, China. The received lignin was in powder form and was used as received without any further chemical purification. However, prior to sample preparation, it was dried in an oven at 60 °C for 24 h to remove absorbed moisture and ensure consistent dosage calculations. The molecular formula of the lignin used in this paper is C₂₀H₂₄CaO₁₀S₂, with a molecular weight of 528.61, a purity of 96%, and a sulfonation degree of approximately 1.8 meq/g, and it contains about 6% reducing sugar components. Calcium lignosulfonate used in the experiment is shown in Figure 1. For terminological consistency, this compound is hereafter abbreviated as “lignin” in subsequent sections. The preliminary experimental results demonstrated that lignin incorporation induced a reduction in unconfined compressive strength of cement-stabilized soils (Figure 2). Consequently, lignin content was restricted to <4%, with five experimental levels established: 0%, 0.25%, 0.5%, 1%, and 2%.

A coded formulation system was adopted for clarity, where “C” denotes cement content (%) and “L” represents lignin content (%). Examples include C0 (virgin soil, 0% cement), L6 (6% lignin alone), and C4L0 (the cement content is 4% + the lignin content is 0%), with other formulations following this convention.

(3) The prepared lead-contaminated soil samples were compacted into standard specimens according to designated formulations and subjected to 28-day curing in a humidity-controlled chamber (20 ± 1 °C, RH ≥ 95%). Following curing, a comprehensive evaluation was conducted through unconfined compressive strength (UCS) tests, permeability tests, and toxicity characteristic leaching procedure (TCLP) tests. To ensure reliability, triplicate tests were performed for each formulation, and the experimental data presented in this article are the average values, and their variability is expressed as standard deviation (±). These tests aimed to systematically investigate the influence mechanisms of lignin content and cement content on the solidification effectiveness of lead-contaminated soils.

2.2. Experimental Methodology

2.2.1. UCS Test

The UCS tests were performed in compliance with the Standard for Geotechnical Testing Methods, GB/T 50123-2019 [26], utilizing a compaction method for specimen preparation. The detailed procedure was as follows. The prepared lead-contaminated soil was compacted in five layers into a cylindrical mold (φ50 mm × H100 mm). Each layer was compacted to achieve a 95% compaction degree (relative to the maximum dry density). Upon completion of compaction, the specimens were extracted from the mold using an electric demolding apparatus to preserve structural integrity. They were immediately wrapped with airtight plastic film to prevent moisture loss. They were cured for 28 days in a SHBY-90B standard temperature- and humidity-controlled curing chamber (20 ± 2 °C, relative humidity ≥ 95%). The specific parameters of the test plan are shown in Table 3.

The UCS specimens were loaded using a universal testing machine (Shanghai Kanxiang Instruments Co., Ltd., Shanghai, China) at a constant displacement rate of 0.1 mm/min. The experimental program was designed to systematically investigate the variation patterns of UCS in soils under varying lignin contents, cement contents, and lead ion concentrations.

2.2.2. Permeability Test

The permeability tests were conducted in accordance with the GB/T 50123-2019 Standard for Geotechnical Testing Methods [26]. Following the procedures below, a standard cutting ring (φ61.8 mm × H40 mm) was internally coated with petroleum jelly and fitted with a filter paper at the base. Pre-calculated masses of contaminated soil were compacted in layers using a hydraulic jack to achieve a 95% compaction degree. Compacted specimens underwent 28-day standard curing, after which they were covered with moistened filter paper and placed in a stackable saturation chamber. The chamber was sealed in a ZK-270 vacuum saturation apparatus for airtight vacuum treatment. They were subsequently immersed in water for 48 h under a sustained vacuum to complete saturation. Saturated samples were tested with the TST-55 penetrometer with a variable head. By measuring the permeability coefficients (k) of contaminated soils with different formulations, the influence of lignin content and cement content on soil permeability characteristics was systematically analyzed. Detailed experimental parameters are provided in Table 4.

2.2.3. TCLP Test

The TCLP tests were conducted in accordance with the HJ/T 300-2007 Solid Waste—Extraction Procedure for Leaching Toxicity—Acetic Acid Buffer Solution Method [27], with specific experimental parameters provided in Table 5. The procedure involved the following steps: 50 g specimens obtained from UCS tests were crushed, sieved through a 1 mm mesh, and sealed for analysis by a qualified third party testing agency. An acetic acid buffer solution was employed as the extractant, and the specimens were mixed with the solution at a specified liquid-to-solid ratio. The mixture was then placed in an automatic rotary oscillator and agitated under standardized frequency and duration as prescribed by the method. After oscillation, the leachate was analyzed by inductively coupled plasma optical emission spectrometry (ICP-OES) for target contaminant concentrations, thus evaluating the stabilization treatment’s effect on lead leaching toxicity. It is important to note that this standard crushing process exposes fresh, unweathered surfaces of the stabilized matrix to the extractant. This is prescribed by the regulatory method to create a consistent and aggressive testing condition. However, it may not fully represent the leaching behavior of an intact, monolithic stabilized mass in the field, where the surface area exposed to percolating water is significantly lower. Consequently, the leaching concentrations measured in this study might be considered as representative of a “worst-case” scenario.

2.2.4. Microscopic Test

Specimens for scanning electron microscopy (SEM) analysis were extracted from failed samples subjected to UCS tests. Due to the extensive quantity of specimens, comprehensive observation of all samples was impractical. Therefore, a subset of specimens was systematically selected to capture the key variations across the experimental design, ensuring representation of critical conditions. The selection included untreated soil (C0L0) as a baseline microstructural reference; 1% lead-contaminated soil without stabilization (C0L0 at 1% lead) to isolate the impact of high lead contamination; the best-performing high-cement formulation for 1% lead contamination (C10L0.25 at 1% lead) to examine the microstructure associated with the best mechanical performance under severe contamination; the same binder formulation for 0.1% lead contamination (C10L0.25 at 0.1% lead) to enable a direct comparison of the microstructural impact of lead concentration; and the cement-only formulations under both contamination levels (C10L0 at 0.1% and 1% lead) to assess the microstructural role of cement in the absence of lignin. This approach was designed to provide contrasting microstructural insights across contamination levels and stabilization treatments, focusing on the most informative formulations identified by macroscopic tests. The test was carried out using a tungsten filament scanning electron microscope of the model EM6200 produced by Beijing Zhongke Keqi Co., Ltd., Beijing, China. The fully automatic ion sputter coater used for gold coating of specimens is shown in Figure 3, and the gold-coated SEM specimens are presented in Figure 4.

The samples used in the X-Ray Diffraction (XRD) test were the same as those in the SEM test. In this test, an X-ray diffractometer of the model TD-3500 was used. The samples were ground and sieved before scanning. The scanning angle was 5° to 80°, and the scanning speed was 5°/min.

3. Results

This section presents the experimental results in a consolidated manner, including all key tables and figures with brief descriptions. The detailed interpretation and mechanistic discussion of these results are provided in Section 4.

3.1. Unconfined Compressive Strength (UCS)

The UCS of stabilized soils was significantly influenced by the lignin content, cement content, and lead ion concentration. The key results are summarized in Table A1, Table A2, Table A3, Table A4 and Table A5 and illustrated in Figure 5, Figure 6, Figure 7, Figure 8, Figure 9, Figure 10, Figure 11, Figure 12, Figure 13 and Figure 14. The empirical fitting parameters for the relationships between cement content and UCS (described by exponential or linear functions depending on lignin content) are provided in Table A1, Table A2, Table A3, Table A4 and Table A5 in Appendix A.

3.2. Permeability Characteristics

The permeability coefficient (k) of the stabilized soil was another critical performance indicator. The results demonstrating the effects of lignin and cement content are summarized in Table 6 and Figure 15, Figure 16, Figure 17, Figure 18, Figure 19, Figure 20 and Figure 21.

3.3. TCLP Leaching Characteristics

The leaching toxicity, a critical indicator of environmental safety, is presented in Figure 22, Figure 23, Figure 24 and Figure 25.

3.4. SEM Characteristics

SEM images provided visual evidence of the microstructural evolution, as shown in Figure 26, Figure 27, Figure 28, Figure 29, Figure 30 and Figure 31. These images reveal the formation of hydration products and changes in soil fabric. SEM was employed to qualitatively observe the microstructural evolution of the stabilized soils and to provide visual evidence supporting the macroscopic engineering properties. The following analysis focuses on identifying the formation of key hydration products (e.g., C-S-H gel, ettringite) and describing observable changes in soil fabric (e.g., porosity, cementation). It should be noted that this analysis is primarily descriptive; future work incorporating quantitative image analysis would provide a more rigorous microstructural quantification.

3.5. X-Ray Diffraction Characteristics

XRD tests can track the phase evolution caused by cement hydration and lead stabilization and determine any newly formed crystalline phases. The result is shown in Figure 32.

4. Discussion

This section provides a comprehensive analysis and mechanistic interpretation of the experimental results presented in Section 3.

4.1. Effect of Lignin Content on UCS of Stabilized Soil

The evolution of failure modes provides critical insight into the fundamental mechanical behavior of the stabilized soil, particularly its transition between ductile and brittle responses. The transition from localized shear failure to Y-shaped failure signifies an enhancement in material ductility and structural integrity. As shown in Figure 5 and Figure 6, for different concentrations of contaminated soil, cement-stabilized soil mainly exhibits localized shear failure at low levels of lignin addition. As the lignin content increases, the samples first show Y-shaped failure and then continue to exhibit shear failure. Localized shear corresponds to lower peak stresses. As shown in Figure 7, the peak stresses of C4L0 and C4L2 at 0.1% Pb are 2.12 MPa and 1.45 MPa, respectively. But, under low-concentration lead contamination, C10L0.5 exhibited Y-type failure, with a peak stress of 6.69 MPa.

It can be observed by comparing Figure 5 and Figure 6 that when the lead pollution concentration is 1%, under the same cement content, the appearance of lignin–cement-stabilized soil samples follows a “deep-light-deep” pattern. The sample surface exhibits a certain deep red color at L2, which differs from the appearance of the stabilized soil corresponding to a lead ion content of 0.1%. The failure mode of the samples shows that at L0, the cement-stabilized soil mainly experiences localized shear failure at low cement content. As the cement content increases to 10%, the sample exhibits Y-shaped failure. At L2, as the cement content increases, the failure mode of the stabilized soil changes from shear failure to spalling.

The evolution of failure modes offers critical insight into the stabilized soil’s internal structural behavior. Systematic analysis reveals relationships between the binder formulation, soil fabric, and the resulting failure mechanism. Cement content governs the failure mode transition of the stabilized soil. Under low cement content (C4), failure occurs as a localized shear along a single dominant plane, indicating a preference for weak, poorly cemented zones. In contrast, a higher cement content (C10) promotes Y-shaped or multiple shear failures, which reflects a more homogeneous cementitious network and a transition to composite material behavior, typically associated with enhanced strength. The microstructural integrity is influenced by the lignin content. Recommended content fosters a dense, interlocked fabric, resulting in a Y-shaped failure that indicates improved toughness. As shown in Figure 7, the peak stress of C10L0.5 in 0.1% lead ion-contaminated soil is 6.69 MPa, and that in 1% lead ion-contaminated soil is 4.34 MPa. Conversely, excessive lignin (L2) causes spalling failure, which exposes a brittle and poorly cemented matrix whose cohesion is compromised by inhibited hydration. Furthermore, lead concentration affects the brittleness of failure. Under identical formulations, a high lead level (1%) promotes spalling, whereas a low level (0.1%) does not. This trend is consistent with microstructural observations, which revealed that high lead concentrations inhibit ettringite formation and ultimately weaken cementation. Therefore, the failure mode serves as a direct, qualitative indicator of the material’s mechanical performance. A Y-shaped failure correlates with superior strength and a robust microstructure. In contrast, localized shear and spalling failures reflect either inadequate stabilization or adverse interactions between the binder, soil, and contaminants.

Figure 8 illustrates the effect of lignin content on the UCS of cement-stabilized soils under varying lead ion concentrations. Across all cement contents, the UCS consistently showed an initial increase followed by a decrease with increasing lignin content. For lead ion-free soils (0% lead ion), the maximum UCS enhancement occurred at L1, establishing this as the best-performing content for uncontaminated soil stabilization. However, a further increase in lignin to 2% resulted in strength reduction, which is attributed to competitive adsorption between excess lignin and cement hydration products. This adsorption reduced the activity of the cement particle surfaces, slowing down and hindering the hydration reaction, thereby reducing the strength.

For 0.1% lead ion contamination, the UCS of cement-stabilized soils reached its maximum values at L0.5 across all tested cement contents. Therefore, the optimal lignin content identified in this study for the UCS of cement-stabilized soil contaminated with 0.1% lead ion is 0.5%. Under 1% lead ion contamination, the UCS of cement-stabilized soils reached its peak value at L0.25 across all tested cement contents. When the lignin content exceeded 0.25%, the UCS of cement-stabilized soils exhibited a gradual decline, with a significant reduction observed at L2. This strength degradation is attributed to the interference of excessive lignin in cement hydration processes.

The percentage increase in UCS of cement-stabilized soils at optimal lignin content identified in this study, relative to their lignin-free counterparts at the same cement content, calculated as [(UCS_with_lignin − UCS_without_lignin)/UCS_without_lignin] × 100%, is illustrated in Figure 9. As shown in Figure 9a, for uncontaminated soil at L1, the percentage increase in UCS exhibited a linear decreasing trend with increasing cement content. At the C4L1 proportioning, the UCS exhibited a maximum percentage increase of 66% compared to the lignin-free sample (C4). This enhancement is attributed to the synergistic interaction between lignin and cement at low cement contents. Therefore, for uncontaminated soil, the formulation C4L1 yielded the highest mean UCS in our tests and is put forward as a promising candidate. As shown in Figure 9b, for 0.1% lead ion-contaminated soil with L0.5, the percentage increase in UCS exhibited a trend similar to that observed in uncontaminated soil, decreasing progressively with increasing cement content. Thus, for 0.1% lead ion-contaminated stabilized soil, the proportioning C4L0.5 showed the highest mean UCS and is accordingly suggested. As shown in Figure 9c, for 1% lead ion-contaminated soil under L0.25, the percentage increase in UCS increased progressively with higher cement contents. Therefore, for 1% lead ion-contaminated soil, the proportioning C10L0.25 resulted in the highest mean UCS under these experimental conditions and is proposed for consideration.

Analysis of Figure 9, which illustrates the percentage increase in UCS relative to lignin-free samples, identifies the optimal lignin content identified in this study and corresponding formulations under varying lead ion concentrations. Two principal trends are observed: (1) the optimal lignin content identified in this study decreases as lead ion concentration rises and (2) the cement content required for maximum strength improvement percentage shifts from the low-cement zone (C4) to the high-cement zone (C10). This shift in the best-performing formulation under these specific test conditions suggests a competitive interaction between lead ions, lignin, and cement. Specifically, lead ions compete with lignin for adsorption sites on the surfaces of cement hydrates, potentially diminishing lignin’s ability to facilitate hydration. More critically, at high concentrations, lead ions likely consume sulfate ions (SO₄²⁻)—a key reactant supplied by calcium lignin and cement for ettringite formation—through precipitation and/or adsorb onto cement particles, thereby directly inhibiting the hydration process. Consequently, in soil with 1% lead ions, a higher cement content (C10) is required to compensate for this inhibition and provide sufficient hydration products for strength development. These results demonstrate that high lead concentrations notably attenuate the stabilizing efficacy of the lignin–cement composite system.

4.2. Effect of Cement Content on UCS of Stabilized Soil

For lignin–cement-stabilized soils, cement acts as the primary factor governing soil strength enhancement and plays a dominant role in the strength development of the stabilized material. As demonstrated by Figure 10, Figure 11, Figure 12, Figure 13 and Figure 14, under fixed lignin content conditions, the cement content exhibits a positive correlation with the UCS of stabilized soils. As shown in Figure 10, under L0, the UCS of stabilized soils exhibits correlations with both cement content and lead ion concentration.

A fitting analysis between cement content and UCS revealed a strong exponential functional relationship. The fitting equation is provided in Equation (1), with the fitted results summarized in Appendix A, Table A1, and the corresponding curve illustrated in Figure 10.

σ = a \times \exp (b C)

(1)

In the formula, σ is the unconfined compressive strength (kPa); C is the cement content in stabilized soil (%); and a and b are the fitting parameters between cement content and unconfined compressive strength.

It should be noted that the fitting parameters a and b in these equations are empirical constants derived from regression analysis. They serve to mathematically describe the observed trends within the tested range of variables but do not possess direct, standalone physical meanings. The primary value of these fits lies in quantitatively illustrating the dominant role of cement content and the modifying effects of lignin and lead.

Analysis of Figure 10 reveals that the UCS of stabilized soils under varying lead ion concentrations exhibits exponential growth with increasing cement content. When the cement content increases from 4% to 10%, the strength increment of high lead ion stabilized soils becomes slower within the same cement content variation range. This phenomenon is likely due to lead ions forming precipitates with anions under the alkaline conditions provided by cement, which can coat cement particles and impede hydration. These precipitates subsequently deposit on cement surfaces, which impedes the hydration reactions. This interference ultimately reduces the efficacy of the soil stabilization. Notably, this inhibitory effect intensifies with higher cement contents.

The influence of cement content on the UCS of stabilized soils under varying lead ion concentrations after lignin incorporation is illustrated in Figure 11, Figure 12, Figure 13 and Figure 14. As shown in Figure 11 and Figure 12, under L0.25 and L0.5, the cement content and UCS of stabilized soils across varying lead ion concentrations still exhibit a discernible exponential functional relationship. The incorporation of low lignin contents exhibits minimal impact on the variation pattern between cement content and UCS. The fitting relationships between cement content and UCS for stabilized soils under L0.25 and L0.5 are summarized in Appendix A, Table A2 and Table A3, respectively.

As shown in Figure 13 and Figure 14, under elevated lignin contents, the UCS of stabilized soils exhibits a linear relationship with increasing cement content. The fitting equations for L1 and L2 are presented in Appendix A, Table A4 and Table A5, respectively. This observed transition in the best-fit model—from an exponential function at lower lignin contents to a linear function at higher lignin contents—is a mathematical description of the data trend. While the change in model form is notable, it is based on a limited number of data points. The primary value of these fitting relationships lies in quantitatively illustrating the positive correlation between cement content and UCS and in highlighting the modifying effect of lignin on this relationship. The positive correlation between cement content and UCS for 1% lead ion-contaminated soil within this high lignin content range was less pronounced.

4.3. Effect of Lignin Content on Permeability Coefficient of Stabilized Soil

Figure 15 and Figure 16 reveal the regulatory mechanisms of lignin content on the permeability characteristics of cement-stabilized soils under varying lead ion contamination concentrations. As shown in Figure 15, for soil contaminated with 0.1% lead ions, the permeability coefficient of cement-stabilized soil first decreased and then increased with rising lignin content. This minimum value was reached at L0.5, indicating it as the recommended content. As shown in Figure 16, in 1% lead ion-contaminated soil, the permeability coefficient of cement-stabilized soil reaches its minimum at L0.25. Beyond this optimal content, the permeability coefficient increases. Excessive lignin adsorbs onto cement particles more extensively, which significantly inhibits the cement hydration process. This inhibition results in a less dense matrix with reduced formation of pore-filling hydration products (like C-S-H gel), ultimately leading to higher overall porosity and connectivity. Notably, at L2, the combined effect of this mechanism causes the permeability coefficient to exceed that of the lignin-free system, indicating a reversal of the improvement achieved at optimal content. This suggests a concentration threshold effect of lignin on permeability optimization. For low-contamination soil (0.1% lead ion), the optimal lignin content identified in this study is 0.5%; however, for high-contamination soil (1% lead ion), the content must be strictly limited to ≤0.25%.

Figure 17 suggests that under 0.1% lead ion concentration, the reduction in the permeability coefficient of cement-stabilized soil with the optimal lignin content identified in this study (0.5%) diminishes progressively as cement content increases. This macroscopic phenomenon suggests a microstructural evolution that can be inferred as follows: at an optimal lignin content, it is plausible that lignin particles contribute to partial pore-filling while simultaneously promoting the formation of additional C-S-H gel that likely further refines the pore network. This is consistent with the observation in SEM that when the cement content is high (C10), the pore structure is densely filled with a large amount of cement hydration products. The combined effect of these hypothesized processes is consistent with the observed permeability reduction of 69.1%. However, this interpretation, based on macroscopic data and consistent with qualitative SEM observations, requires validation through quantitative pore structure analysis to conclusively confirm the pore-filling effect and pore size distribution evolution.

Under 1% lead ion concentration, the reduction magnitude of the permeability coefficient in cement-stabilized soil with L0.25 and the increase magnitude in cement-stabilized soil with L2 are illustrated in Figure 18. At L0.25, the maximum reduction magnitude of the permeability coefficient in cement-stabilized soil reaches 18 ± 1.1% (C4), whereas L2 results in an increase magnitude of the permeability coefficient up to 44 ± 2.2% (C10). This reveals that excessive lignin content is correlated with a predominantly negative effect on the permeability of highly lead ion-contaminated soil. It can be inferred from this correlation that excessive lignin (2%) likely inhibits cement hydration, potentially leading to a less dense matrix with reduced cementation products and consequently weakened pore-filling effects, which could account for the enhanced pore water migration rates. Notably, as shown in Figure 18, C6 serves as a critical inflection point. At the low content range (≤6%), lignin exhibits permeability regulation, whereas at higher contents (>6%), cementitious bonding dominates the pore structure, and the influence of lignin gradually diminishes.

A comparison of Figure 17 and Figure 18 reveals distinct optimal conditions for different contamination levels. For soil with 0.1% lead ions, the maximum permeability reduction of 69.1 ± 1.5% occurs at C4. In contrast, for soil with 1% lead ions, the most significant reduction is observed at C10. This discrepancy stems from the differential impact of lead ion concentration. Higher levels of lead ions are more readily adsorbed onto the surfaces of cement and lignin particles, competing with lignin for functional sites. This adsorption, coupled with the potential precipitation of lead with sulfate and other ions, subsequently hinders the ability of lignin to enhance the cement hydration process. Furthermore, as shown in Figure 19, a comparative analysis between permeability and the UCS results reveals a clear negative correlation between the permeability coefficient and UCS values across different formulations. This consistent trend strongly suggests that the microstructural modifications induced by the lignin–cement composite are the common underlying cause for the simultaneous improvement in mechanical strength and reduction in permeability.

4.4. Effect of Cement Content on the Permeability Coefficient of Stabilized Soil

Figure 20 reveals the regulatory mechanism of cement content on the permeability coefficient in lignin-free systems. It shows that, regardless of lead ion contamination levels, the permeability coefficient exhibits a negative correlation with cement content. As cement content increased from 4% to 10%, it is expected that the production of C-S-H gels was enhanced. This is widely understood to induce a pore-filling effect, which provides a coherent explanation for the systematic decline in the permeability coefficient observed in this study. This trend was consistent across soils contaminated with both 0.1% and 1.0% lead ion concentrations. This trend indicates that permeability evolution is governed by the physical sealing effects of cement hydration products. Furthermore, the efficacy of this control demonstrates a nonlinear strengthening characteristic as the cement content increases.

A fitting analysis was performed between cement content and the permeability coefficient. The results show that their relationship for soils with both lead ion concentrations can be expressed by Equation (2). The fitting results are shown in Figure 20 and Figure 21, with good correlation coefficients.

k_{m} = a \ln C + b

(2)

In the formula, k is the permeability coefficient (cm/s); m represents the lead ion concentration; C denotes the cement content (%); and a and b are fitting parameters.

Figure 21 indicates that within the L0.25 to L2 range, the cement content exhibits a negative correlation with the permeability coefficient. This relationship is quantified by Equation (2), which confirms that cement content remains the dominant factor controlling permeability. Comparative analysis of contamination levels reveals that the permeability coefficient of 1% lead ion-contaminated soil is consistently higher than that of 0.1% lead ion-contaminated soil systems. This divergence can be interpreted by considering two plausible mechanisms. (1) From a microstructural perspective, it is possible that high lead ion concentrations adversely affect the cementation network, potentially leading to a more porous structure. (2) Regarding hydration, it is inferred that in 1% lead ion-contaminated soil, heavy metal ions may inhibit C-S-H gel formation, thereby reducing the efficiency of pore-filling. The specific manifestation and relative contribution of these mechanisms warrant further investigation with targeted microstructural techniques. Comparatively, in 0.1% lead ion-contaminated soil, limited lead ion enhances compactness through precipitation reactions, thereby reducing the permeability coefficient under the same content.

Higher cement content results in relatively lower permeability coefficients for cement-stabilized soils with varying lead ion concentrations. The fitting correlation coefficients are listed in Table 6.

Figure 21 suggests that under L0, as the cement content gradually increases, the fitting curves between the two conditions converge. This convergence indicates that higher cement contents facilitate more effective pore structure filling. However, with increasing lignin content, the differences in permeability coefficients between soils with 0.1% and 1% lead ion concentrations under the same cement content gradually intensify. Notably, at L2, the permeability coefficient of 1% lead ion-contaminated soil notably exceeds that of 0.1% lead ion-contaminated soil. This discrepancy arises from distinct mechanistic interactions between the lead ion and stabilization agents under varying contamination levels.

4.5. Effect of Individual Addition of Lignin and Cement on TCLP Concentration of Stabilized Soil

Figure 22 and Figure 23 illustrate the influence of sole lignin or cement addition on the TCLP concentration of cement-stabilized soil under varying lead ion concentrations. Comparative analysis of the two figures reveals that as the content of lignin or cement increases, the lead ion leaching concentration consistently decreases. This trend demonstrates the solidification capabilities of both materials. However, under identical content conditions, the leaching concentration of the lignin system remains higher than that of the cement system, indicating superior solidification efficiency of cement. This divergence arises from their distinct action mechanisms. On one hand, lignin functions by physically adsorbing lead ions. On the other hand, cement provides long-term stabilization via ion exchange and multi-faceted physicochemical solidification, including C-S-H gel encapsulation and the formation of heavy metal mineral phases.

4.6. Effect of Lignin Content on TCLP Concentration of Cement-Stabilized Soil

Figure 24 and Figure 25 reveal the influence patterns of lignin content on leaching concentration under varying lead ion contamination levels. For 0.1% lead ion-contaminated soil, the leaching concentration initially decreases and subsequently increases with lignin content. The minimum value of 3.6 ± 0.1 mg/L is achieved at L0.5, representing a 41.0% reduction compared to the L0 baseline. This result complies with the hazardous waste standard (<5 mg/L) specified in the GB 5085.3-2007 Identification Standards for Hazardous Waste Leaching Toxicity [28]. For the 1% lead-contaminated soil, the leaching concentration was reduced to 37.3 ± 1.9 mg/L at C10L0.25, which still notably exceeds the regulatory limit of 5 mg/L. The failure to effectively immobilize lead at this high contamination level can be attributed to the overwhelming of the stabilization capacity of the composite binder. The system’s capacity for chemical encapsulation via C-S-H gel formation and precipitation in alkaline conditions is likely saturated. Furthermore, the TCLP employs an acetic acid buffer (pH ~2.88–4.93), which aggressively attacks the cementitious matrix. Under this acidic environment, the alkaline stabilization products are dissolved, and the stability of C-S-H gel is compromised, potentially leading to the re-mobilization of previously precipitated or adsorbed lead. As discussed in Section 4.8, the observed inhibition of ettringite formation by high lead concentrations also removes a potential secondary immobilization pathway. While lignin provides additional adsorption sites, its capacity is also finite and may be insufficient to cope with the immense quantity of soluble lead ions present in the 1% contaminated soil. Furthermore, the effectiveness of cementitious encapsulation is concurrently compromised by acid attack, further diminishing the overall immobilization capacity.

The failure of the stabilized, high-lead (1%) soil to pass the TCLP test has critical implications for its long-term environmental safety in the field. The aggressive acidic conditions of the TCLP (pH ~2.88–4.93) are not merely a regulatory benchmark but simulate a worst-case scenario of chemical stress, such as that imposed by strong acid rain or contact with acidic industrial waste. The re-mobilization of lead under these conditions indicates that the stabilization achieved under standard curing is not sufficiently robust to withstand severe acidic challenges. This raises concerns about the long-term stability of these treated soils in environments prone to acidification. To precisely understand which lead fractions were preferentially leached during the TCLP and are, therefore, vulnerable, a sequential extraction procedure on the post-leaching solids would be required in future work.

4.7. Effect of Cement Content on Microstructural Characteristics of Stabilized Soil

Figure 26 and Figure 27 present a comparative SEM analysis of uncontaminated soil and 1% lead ion-contaminated soil after 28-day curing. A comparative analysis of the SEM images provides a qualitative explanation for the macroscopic performance decline. The uncontaminated soil (Figure 26) exhibits a relatively denser structure. In contrast, the 1% lead-contaminated soil (Figure 27) shows visible signs of structural deterioration, appearing to have higher porosity and less cementation. This observed microstructural weakening is consistent with the notably lower UCS and higher permeability coefficient measured for the highly lead-contaminated soil.

As shown in Figure 28, stabilization with 10% cement (C10) led to the development of a visibly more compacted microstructure. Hydration products, primarily C-S-H gel, appear to extensively cover soil particles and form a continuous cementation network. This observed microstructure directly corresponds to the high UCS and low permeability coefficient measured for this formulation. As the core product of cement hydration, the generation of C-S-H gel positively correlates with cement content. Enhanced cementation effects due to increased C-S-H content result in improved UCS and reduced permeability coefficient for 1% lead ion-contaminated soil.

For the soil contaminated with 0.1% lead ions and stabilized with 10% cement (C10L0), the SEM analysis revealed a well-developed and densely packed microstructure, as shown in Figure 29. The hydration products, predominantly C-S-H gel, extensively covered the soil particles and formed a cohesive cementitious matrix that effectively bonded the soil fabric. This dense microstructure, resulting from uninterrupted cement hydration under low lead contamination, provides a direct explanation for the high UCS and low permeability coefficient measured for this formulation. The observed structural integrity of the C10L0 sample under 0.1% lead contamination stands in stark contrast to the visibly more porous and less cemented structure of its counterpart under 1% lead contamination (Figure 28), underscoring the detrimental impact of high lead concentration on the cementation process.

4.8. Effect of Lignin Content on Microstructural Characteristics of Stabilized Soil

Figure 30 and Figure 31, respectively, illustrate the microstructural characteristics of C10L0.25-stabilized soils contaminated with 0.1% and 1% lead ions. The SEM images reveal a distinct difference in microstructure that aligns with the macroscopic engineering properties measured in Section 4.1, Section 4.2, Section 4.3 and Section 4.4. In the 0.1% lead-contaminated soil (Figure 29), the alkaline environment provided by cement hydration, coupled with sulfate ions (SO₄²⁻) supplied by calcium lignin and aluminum phases (Al³⁺) from the cement and clay, creates ideal conditions for the simultaneous formation of C-S-H gel and ettringite. The presence of abundant needle-like crystals, characteristic of ettringite (3CaO·Al₂O₃·3CaSO₄·32H₂O) [29,30], is observed, interlocking with the C-S-H gel to create a complex microstructure. This observed enhancement in microstructural complexity and the apparent reduction in porosity provide a qualitative explanation for the superior UCS, lower permeability, and improved leaching resistance documented for this formulation (C10L0.25 under 0.1% lead ion).

In contrast, for the 1% lead-contaminated soil stabilized with the same binder formulation (C10L0.25, Figure 31), the microstructure is markedly different. The needle-like ettringite crystals are notably scarce, and the structure seems to be primarily composed of C-S-H gel with visually coarser and more porous features. This microstructural disparity is consistent with the inferior mechanical and permeability performance of the highly contaminated soil compared to its low-contamination counterpart. The virtual absence of ettringite in the high-lead system suggests that lead ions notably inhibit its formation, potentially through competitive precipitation reactions. Lead ions (Pb²⁺) compete with aluminum phases (Al³⁺) for the sulfate ions (SO₄²⁻) to form less soluble PbSO₄ precipitates, thereby depleting the sulfate source essential for ettringite (3CaO·Al₂O₃·3CaSO₄·32H₂O) crystallization. This competitive precipitation is a plausible primary reason for the altered stabilization mechanism and inferior macroscopic performance under high lead contamination. This interpretation, while consistent with the SEM observations and chemical principles, remains speculative without direct mineralogical or spectroscopic confirmation and highlights a key area for future investigation. Consequently, the stabilization mechanism shifts from a dual enhancement (ettringite interlocking + C-S-H cementation) under low lead concentrations to a primarily gel-dominated cementation under high lead concentrations. This altered mechanism is less effective in enhancing soil strength and reducing permeability.

4.9. XRD Mineral Composition Analysis

To thoroughly investigate the phase evolution law of the lignin–cement composite system in stabilizing lead-contaminated red clay, XRD analysis was conducted on the samples. In XRD testing, the phase composition is analyzed by analyzing the peak position and intensity of the diffraction pattern and combining it with the Jade analysis software. Based on the standard card library of the International Diffraction Data Center (ICDD), the crystal plane spacing (d value) corresponding to the diffraction peaks and the intensity of the characteristic peaks are matched to identify the minerals and gelation products present in the sample. The results are presented in Figure 32.

The analysis indicated that the mineral composition of the pristine red clay was primarily composed of native aluminosilicates such as quartz (SiO₂), montmorillonite (Al₂Si₄O₁₀(OH)₂·nH₂O), and illite (KAl₂(AlSi₃O₁₀)(OH)₂). After contamination with 1% lead ions, no new distinct crystalline peaks appeared in its XRD pattern, suggesting that the externally introduced lead primarily existed in the soil in amorphous adsorbed, complexed, or trace precipitate forms, without forming independent lead minerals with a long-range ordered structure.

After stabilization with 10% cement, the XRD patterns of the samples at both contamination levels changed. In the 0.1% lead + 10% cement sample, distinct broad diffraction peaks corresponding to calcium silicate hydrate (C-S-H) gel were detected, along with characteristic peaks of cement phases, such as portlandite (Ca(OH)₂). Most importantly, characteristic peaks of ettringite (3CaO·Al₂O₃·3CaSO₄·32H₂O) were present in the pattern, demonstrating that the cement hydration system remained intact under low lead contamination, allowing sulfate and aluminum phases to react successfully to form needle-like ettringite, which plays a crucial role in pore filling and strength enhancement. However, in the 1% lead + 10% cement sample, the diffraction peak intensity of ettringite was significantly weakened. This finding is entirely consistent with the lack of needle-like crystals observed via SEM in the high-lead samples in this study.

Following the additional incorporation of 0.25% lignin, the phase composition evolved differently. For the 0.1% lead + 10% cement + 0.25% lignin sample, the diffraction peaks of the C-S-H gel became broader, suggesting that the addition of lignin potentially promoted the formation of hydration products, resulting in a more abundant gel phase. Simultaneously, the diffraction peaks of ettringite remained clearly visible, indicating that lignin did not adversely affect the crystallization of ettringite in the low-lead environment. Its potential coagulation-promoting effect, synergizing with the concomitant formation of ettringite, collectively optimized the microstructure.

4.10. Performance Comparison and Practical Implications of the Best-Performing Formulations

The formulations identified as best-performing in Section 4.1 and Section 4.2, C4L0.5 for 0.1% lead and C10L0.25 for 1% lead, were primarily selected based on their superior UCS performance. To evaluate their overall practicality, this section compares their key engineering and environmental properties, as summarized in Table 7. In practice, the selection of a stabilization formulation often involves dominant criteria. The following discussion adopts this perspective to assess the applicability of the suggested formulations.

Analysis of Table 7 reveals that the applicability of the suggested formulations is governed by different dominant criteria under varying contamination levels.

For the 0.1% lead-contaminated soil, the formulation C4L0.5 demonstrates a favorable alignment of all key criteria. It provides substantial strength enhancement, a significant reduction in permeability, and—most critically—reduces the leaching concentration to a level (3.6 mg/L) that complies with the regulatory standard (<5 mg/L). Furthermore, it achieves this with the lowest cement content (4%), making it a cost-effective and environmentally compliant option. In this case, no single criterion presents a major barrier, making C4L0.5 a robust and practical suggestion.

In stark contrast, for the 1% lead-contaminated soil, the formulation C10L0.25 faces a critical trade-off. While it delivers the highest UCS and reduced permeability, it fails the paramount environmental safety criterion, with a leaching concentration (37.3 mg/L) vastly exceeding the regulatory limit. From an environmental compliance perspective, which is often the non-negotiable starting point for remediation, this formulation is not viable. The high cement content (10%) also renders it less cost-effective. Therefore, while identified based on strength, the C10L0.25 formulation may not be suitable for practical application under the tested conditions. This highlights the limitation of the lignin–cement system for highly contaminated scenarios and underscores that mechanical improvement does not guarantee environmental safety.

5. Limitations of the Study

This study acknowledges several limitations that warrant consideration. The findings are based exclusively on a specific lateritic clay, which may constrain the generalizability of the best-performing formulations to other soil types. The evaluation was conducted after a standard 28-day curing period, leaving the long-term durability and field performance under environmental cycles unverified. Furthermore, the potential for microbial degradation of the lignin component over the long term represents a biological vulnerability that could compromise the stability of the stabilized soil and the immobilization of contaminants, and this was not assessed in the present study. The use of soluble lead nitrate may not fully represent the behavior of aged contaminants in real sites, and the stabilization efficacy for multi-contaminant systems remains unexplored. While the TCLP results clearly indicate the risk of lead re-mobilization under acidic conditions, the study did not perform sequential chemical extraction analysis on the leached residues. This limits a more mechanistic understanding of the stabilization and subsequent failure processes. The microstructural analysis relied on qualitative SEM observations without quantitative metrics, such as porosity and pore size distribution analysis from image processing. Additionally, the mechanistic discussions regarding permeability changes are primarily inferential and based on macroscopic test results and qualitative SEM observations. The study lacks quantitative pore structure analysis to directly characterize how the pore size distribution and connectivity evolve with different lignin and cement formulations. These limitations highlight the need for future research involving diverse soils, long-term performance validation, complex contamination scenarios, advanced quantitative microstructural techniques, and quantitative pore structure analysis.

6. Conclusions

This study systematically investigated the short-term effectiveness and mechanisms of a lignin–cement composite binder for stabilizing lead-contaminated lateritic clay under standard 28-day curing conditions. The experimental findings directly address the key research gaps identified at the outset, namely, the poorly quantified synergistic mechanisms of the lignin–cement system for lead contamination and its adaptability to varying contamination levels across comprehensive engineering properties. The conclusions of this study are firmly supported by experimental data derived from triplicate tests. The reported standard deviations indicate data variability, confirming that the observed trends and identified optimal formulations under these specific laboratory conditions are reliable. However, the long-term stability and durability of these formulations require further investigation. The main conclusions are as follows.

(1) Lignin effectively enhanced the strength and significantly reduced the permeability of the stabilized soil. However, the optimal formulation was dependent on the lead concentration. Based on the trends observed in this study, the formulation C4L0.5 was identified as the best-performing for low-concentration (0.1%) contaminated soil and C10L0.25 for high-concentration (1%) contaminated soil under the specific experimental conditions of this study. This dependency explicitly resolves the question of adaptability raised in the introduction, demonstrating a competitive interaction between lead ions and the binder components. These interactions primarily involve competition for adsorption sites on hydration products and competition for key chemical reactants necessary for the formation of strength-giving phases like ettringite.

(2) A critical boundary condition for the proposed method was identified. The composite binder demonstrated stabilization efficacy for low-concentration contaminated soil, reducing the leaching concentration to 3.6 ± 0.1 mg/L, which complies with the national standard. But, it fundamentally failed to immobilize the high-concentration (1%) lead-contaminated soil from an environmental safety perspective. The TCLP leaching concentration remained unacceptably high at 37.3 ± 1.9 mg/L, constituting a treatment failure. Under these conditions, the observed mechanical improvements are secondary, as the treated material does not meet the primary criterion of environmental safety.

(3) SEM and XRD analyses indicated that lignin promoted the formation of hydration products consistent with the typical morphologies of C-S-H gel and ettringite. Conversely, high lead concentrations inhibited ettringite formation, as confirmed by XRD, providing a plausible explanation for the observed macroscopic performance variations. It is important to note that these mechanistic interpretations are based on morphological evidence and require further confirmation by quantitative phase analysis.

(4) This research underscores the necessity of a multi-criteria evaluation framework as envisioned initially. The C4L0.5 formulation presents a viable solution for low-concentration contamination, balancing all criteria. However, the clear failure to pass the TCLP test at the 1% Pb level definitively establishes the upper applicability limit of the lignin–cement system for lead contamination. For heavily contaminated scenarios, more potent stabilization strategies or pre-treatment steps are essential.

The findings of this study are based on specific experimental conditions, and the identified best-performing formulations and trends should be considered indicative. It is critical to note that the optimal formulations identified in this study (e.g., C4L0.5 for 0.1% lead) are specific to the mineralogy and properties of the lateritic clay from Guilin. To transition this technology towards practical application, future work must include robust statistical validation and long-term performance assessment. Furthermore, investigations must expand to real-site scenarios with complex lead speciation.

Author Contributions

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

Funding

The research described in this paper was financially supported by the Natural Science Foundation of Guangxi Zhuang Autonomous Region (2022GXNSFAA035485), the Specific Research Project of Guangxi for Research Bases and Talents (GUIKE AD21220051), and the Natural Science Foundation of Guangxi Zhuang Autonomous Region (2025GXNSFAA069214).

Data Availability Statement

Some or all data, models, or code that support the findings of this study are available from the corresponding author upon reasonable request.

Conflicts of Interest

The authors declare no conflicts of interest.

Appendix A

Table A1. Fitting relationships between cement content and UCS of stabilized soils under varying lead ion concentrations at L0.

Lead Ion Concentrations (%)	a	b	Fitting Equation	R²
0	1611.64	0.1363	$σ = 1611.64 \times e^{0.1363 C}$	0.9669
0.1	1276.83	0.1220	$σ = 1276.83 \times e^{0.1220 C}$	0.9890
1	1274.20	0.0876	$σ = 1274.20 \times e^{0.0876 C}$	0.9895

Table A2. Fitting relationships between cement content and UCS under varying lead ion concentrations at L0.25.

Lead Ion Concentrations (%)	a	b	Fitting Equation	R²
0	2580.21	0.9560	$σ = 2580.21 \times e^{0.9560 C}$	0.9929
0.1	1299.00	0.1196	$σ = 1299.00 \times e^{0.1196 C}$	0.9985
1	1257.72	0.1235	$σ = 1257.72 \times e^{0.1235 C}$	0.9454

Table A3. Fitting relationships between cement content and UCS under varying lead ion concentrations at L0.5.

Lead Ion Concentrations (%)	a	b	Fitting Equation	R²
0	498.98	2162.34	$σ = 498.98 \times e^{2162.34 C}$	0.9752
0.1	2018.84	0.1146	$σ = 2018.84 \times e^{0.1146 C}$	0.9729
1	1390.87	0.0976	$σ = 1390.87 \times e^{0.0976 C}$	0.9450

Table A4. Fitting relationships between cement content and UCS under varying lead ion concentrations at L1.

Lead Ion Concentrations (%)	a	b	Fitting Equation	R²
0	423.82	3094.33	$σ = 423.82 C + 3094.33$	0.9681
0.1	626.66	−218.52	$σ = 626.66 C - 218.52$	0.9633
1	136.22	1163.94	$σ = 136.22 C + 1163.94$	0.9244

Table A5. Fitting relationships between cement content and UCS under varying lead ion concentrations at L2.

Lead Ion Concentrations (%)	a	b	Fitting Equation	R²
0	417.41	2898.11	$σ = 417.41 C + 2898.11$	0.9553
0.1	605.37	−474.69	$σ = 605.37 C - 474.69$	0.9209
1	87.46	998.95	$σ = 87.46 C + 998.95$	0.9133

References

Song, P.; Xu, D.; Yue, J.; Ma, Y.; Dong, S.; Feng, J. Recent advances in soil remediation technology for heavy metal contaminated sites: A critical review. Sci. Total Environ. 2022, 838, 156417. [Google Scholar] [CrossRef]
Calgaro, L.; Contessi, S.; Bonetto, A.; Badetti, E.; Ferrari, G.; Artioli, G.; Marcomini, A. Calcium aluminate cement as an alternative to ordinary Portland cement for the remediation of heavy metals contaminated soil: Mechanisms and performance. J. Soils Sediments 2021, 21, 1755–1768. [Google Scholar] [CrossRef]
Li, W.; Ni, P.; Yi, Y. Comparison of reactive magnesia, quick lime, and ordinary Portland cement for stabilization/solidification of heavy metal-contaminated soils. Sci. Total Environ. 2019, 671, 741–753. [Google Scholar] [CrossRef] [PubMed]
Firouzbakht, S.; Gitipour, S.; Baghdadi, M. Impact of hydrochar in stabilization/solidification of heavy metal-contaminated soil with Portland cement. Environ. Geochem. Health 2024, 46, 512. [Google Scholar] [CrossRef] [PubMed]
Chi, L.; Huang, C.; Li, Z.; Ruan, S.; Peng, B.; Li, M.; Liang, Q.; Yin, K.; Lu, S. Heavy metals immobilization of LDH@biochar-containing cementitious materials: Effectiveness and mechanisms. Cem. Concr. Compos. 2024, 152, 105667. [Google Scholar] [CrossRef]
Liu, Q.; Deng, Y.; Tang, J.; Chen, D.; Li, X.; Lin, Q.; Yin, G.; Zhang, M.; Hu, H. Potassium lignosulfonate as a washing agent for remediating lead and copper co-contaminated soils. Sci. Total Environ. 2019, 658, 836–842. [Google Scholar] [CrossRef]
Xie, L.; Chen, Q.; Liu, Y.; Ma, Q.; Zhang, J.; Tang, C.; Duan, G.; Lin, A.; Zhang, T.; Li, S. Enhanced remediation of Cr(VI)-contaminated soil by modified zero-valent iron with oxalic acid on biochar. Sci. Total Environ. 2023, 905, 167399. [Google Scholar] [CrossRef]
Chen, Y.; Yang, W.; Zou, Y.; Wu, Y.; Mao, W.; Zhang, J.; Zia-Ur-Rehman, M.; Wang, B.; Wu, P. Quantification of the effect of biochar application on heavy metals in paddy systems: Impact, mechanisms and future prospects. Sci. Total Environ. 2024, 912, 168874. [Google Scholar] [CrossRef] [PubMed]
Medha, I.; Chandra, S.; Vanapalli, K.R.; Samal, B.; Bhattacharya, J.; Das, B.K. (3-Aminopropyl) triethoxysilane and iron rice straw biochar composites for the sorption of Cr (VI) and Zn (II) using the extract of heavy metals contaminated soil. Sci. Total Environ. 2021, 771, 144764. [Google Scholar] [CrossRef]
Hu, T.; Zhang, M.; Wei, X.; Xu, Z.; Li, D.; Deng, J.; Li, Y.; Zhang, Y.; Lin, X.; Wang, J. Efficient Pb(II) removal from contaminated soils by recyclable, robust lignosulfonate/polyacrylamide double-network hydrogels embedded with Fe₂O₃ via one-pot synthesis. J. Hazard. Mater. 2024, 479, 135712. [Google Scholar] [CrossRef]
Liu, S.; Xie, Z.; Zhu, Y.; Zhu, Y.; Jiang, Y.; Wang, Y.; Gao, H. Adsorption characteristics of modified rice straw biochar for Zn and in-situ remediation of Zn contaminated soil. Environ. Technol. Innov. 2021, 22, 101388. [Google Scholar] [CrossRef]
Liang, L.; Zhang, J.; Fang, P.; Suo, C. Study on Properties of Copper-Contaminated Soil Solidified by Solid Waste System Combined with Cement. Sustainability 2022, 14, 5604. [Google Scholar] [CrossRef]
Shamsad, A.; Bahraq, A.A.; Khalid, H.R.; Alamutu, L.O. Stabilization/solidification of heavy metal-contaminated marl soil using a binary system of cement and fuel fly ash. Environ. Monit. Assess. 2023, 195, 1557. [Google Scholar]
Li, G.; Li, H.; Li, Y.; Chen, X.; Li, X.; Wang, L.; Zhang, W.; Zhou, Y. Stabilization/Solidification of Heavy Metals and PHe Contaminated Soil with β-Cyclodextrin Modified Biochar (β-CD-BC) and Portland Cement. Int. J. Environ. Res. Public Health 2022, 19, 1060. [Google Scholar] [CrossRef]
Meng, S.; Ge, C.; Zhao, G.; Hou, S. Sustainable Stabilization of Cadmium (Cd)-Contaminated Soil with Biochar-Cement: Evaluation of Strength, Leachability, Microstructural Characteristics, and Stabilization Mechanism. Soil Sediment Contam. Int. J. 2025, 34, 963–980. [Google Scholar] [CrossRef]
Li, X.; Yu, X.; Liu, L.; Yang, J. Cyclic drying and wetting tests on combined remediation of chromium-contaminated soil by calcium polysulfide, synthetic zeolite and cement. Sci. Rep. 2021, 11, 91282–91284. [Google Scholar] [CrossRef] [PubMed]
Zha, F.; Liu, J.; Xu, D.; Kang, B.; Yang, C.; Deng, Y. Solidification/Stabilization (s/s) of Zinc Contaminated Soils Using Cement Blended with Soda Residue. Environ. Geotech. 2020, 10, 76–87. [Google Scholar] [CrossRef]
Liu, D.; Zhang, Y.; Gao, X.; Liu, Z.; Liu, C. The Structural Mechanism of Calcium Lignosulfonate-Modified Loess Stabilization. J. Lanzhou Univ. (Nat. Sci.) 2023, 59, 468–475. [Google Scholar]
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. [Google Scholar] [CrossRef]
Cai, Y.; Ou, M. Experimental Study on Expansive Soil Improved by Lignin and Its Derivatives. Sustainability 2023, 15, 8764. [Google Scholar] [CrossRef]
Deng, J.; Wang, P.; Xu, Z.; Hu, T.; Li, D.; Wei, X.; Chen, C.; Li, Y.; Zhang, Y. Contaminated soil remediation with nano-FeS loaded lignin hydrogel: A novel strategy to produce safe rice grains while reducing cadmium in paddy field. J. Hazard. Mater. 2024, 469, 133965. [Google Scholar] [CrossRef] [PubMed]
He, Y.; Mi, B.; Luo, C.; Tao, H.; Zhang, X.; Yu, J.; Mo, X.; Hu, J.; Chen, L.; Tu, N.; et al. A novel strategy employing lignin biochar to simultaneously promote remediation and safe crop production in Cd-contaminated soil. Chem. Eng. J. 2024, 499, 156237. [Google Scholar] [CrossRef]
Liu, Q.; Luo, J.; Tang, J.; Chen, Z.; Chen, Z.; Lin, Q. Remediation of cadmium and lead contaminated soils using Fe-OM based materials. Chemosphere 2022, 307, 135853. [Google Scholar] [CrossRef] [PubMed]
Lee, L.N.; Guo, X.; Chng, M.L.; Ibrahim, M.; Bashir, M.J. Effect of Lignin and Commercial Plant Extract on the Fractionation of Heavy Metals in Multi-Metal Contaminated Soil. Int. J. Environ. Sci. Dev. 2024, 15, 1469. [Google Scholar] [CrossRef]
GB 36600-2018; Soil Environmental Quality-Risk Control Standard for soil Contamination of Development Land. Ministry of Ecology and Environment & State Administration for Market Regulation, China Environment Publishing Group: Beijing, China, 2018.
GB/T 50123-2019; Standard for Geotechnical Testing Methods. Ministry of Housing and Urban-Rural Development of the People’s Republic of China, China Planning Press: Beijing, China, 2019.
HJ/T 300-2007; Solid Waste-Extraction Procedure for Leaching Toxicity-Acetic Acid Buffer Solution Method. Institute of Solid Waste Pollution Control Technology, Chinese Research Academy of Environmental Sciences, Environmental Protection Industry Standards of the People’s Republic of China: Beijing, China, 2007.
GB 5085.3-2007; Identification Standards for Hazardous Wastes-Identification for Extraction Toxicity. Institute of Solid Waste Pollution Control Technology, Chinese Research Academy of Environmental Sciences, General Administration of Quality Supervision, Inspection and Quarantine of China: Beijing, China, 2007.
Guo, J.; Xu, M.; Zhou, J.; Li, H.; Zhang, Z.; Chen, Z. Growth and morphology of ettringite fibres. J. Hebei Univ. Technol. 2023, 52, 82–89. [Google Scholar]
Li, Q.; Sun, W.K.; Chen, L.Q. Research on the Mechanisms of Soil Agent Solidifying Buildings Residues. Adv. Mater. Res. 2011, 1450, 1227–1230. [Google Scholar] [CrossRef]

Figure 1. Calcium lignosulfonate molecular formula.

Figure 2. UCS of L4 combined with cement of different contents and cement alone.

Figure 3. Fully automatic ion sputter coater.

Figure 4. Gold-coated SEM specimen.

Figure 5. Morphological Alterations and Failure Modes of Lignin-Cement Stabilized Soil under 0.1% Lead Ion Concentrations.

Figure 6. Morphological Alterations and Failure Modes of Lignin-Cement Stabilized Soil under 1% Lead Ion Concentrations.

Figure 7. Stress-Strain Curves of Solidified Soil under Different Lignin and Cement Dosages ((a,b) are under the condition of 0.1% lead ion contaminated soil, while (c,d) are under the condition of 1% lead ion contaminated soil).

Figure 8. UCS of Different Lignin Effects on Different Cement Contents and Lead Ion Contents.

Figure 9. UCS Percentage Increase Relative to Lignin-Free Soil for Stabilized Soils with Best-Performing Lignin Content. Note: The percentage increase was calculated as: [(UCS_with_lignin − UCS_without_lignin)/UCS_without_lignin] × 100%).

Figure 10. Relationship between Cement Contents and the UCS of Stabilized Soils under Varying Lead Ion Concentrations at L0.

Figure 11. Relationship between Cement Contents and the UCS of Stabilized Soils under Varying Lead Ion Concentrations at L0.25.

Figure 12. Relationship between Cement Contents and the UCS of Stabilized Soils under Varying Lead Ion Concentrations at L0.5.

Figure 13. Relationship between Cement Contents and the UCS of Stabilized Soils under Varying Lead Ion Concentrations at L1.

Figure 14. Relationship between Cement Contents and the UCS of Stabilized Soils under Varying Lead Ion Concentrations at L2.

Figure 15. Permeability coefficients of cement-stabilized soil with varying lignin contents at 0.1% lead ion contamination.

Figure 16. Permeability coefficients of cement-stabilized soil with varying lignin contents at 1% lead ion contamination.

Figure 17. Reduction magnitude of the permeability coefficient in cement-stabilized soil under optimal lignin content identified in this study.

Figure 18. Reduction and Increase Magnitude of Permeability Coefficient in Cement-Stabilized Soil by Lignin.

Figure 19. Relationship between the UCS of Solidified SSoil and its Rermeability Coefficient (log(k)). Note: (a) represents the lead ion content at 0.1% and (b) represents the lead ion content at 1%. The cement content at the five points on each line from left to right is C4, C6, C8, and C10 in sequence.

Figure 20. Permeability Coefficient of Solidified Soil with Varying Cement Contents at L0.

Figure 21. Permeability Coefficient of Solidified Soil with Varying Cement Contents under Varying Lignin Contents.

Figure 22. TCLP of Solidified Soil with 0.1% Lead Ion Contamination + Single-Added Lignin and Cement.

Figure 23. TCLP of Solidified Soil with 1% Lead Ion Contamination + Single-Added Lignin and Cement.

Figure 24. TCLP of Solidified Soil with C10 at Varying lignin Contents under 0.1% Lead Ion Contamination.

Figure 25. TCLP of Solidified Soil with C10 at Varying lignin Contents under 1% Lead Ion Contamination.

Figure 26. Uncontaminated Soil (4000× Magnification) 28d.

Figure 27. 1% lead ion Contaminated Soil (4000× Magnification) 28d.

Figure 28. 1% Lead Ion Contamination, C10L0 (4000× Magnification) 28d.

Figure 29. 0.1% lead ion Contamination, C10L0 (4000× Magnification) 28d.

Figure 30. 0.1% lead ion Contamination, C10L0.25 (4500× Magnification) 28d.

Figure 31. 1% lead ion Contamination, C10L0.25 (4000× Magnification) 28d.

Figure 32. XRD pattern of lignin-cement solidified lead-contaminated soil.

Table 1. Index properties of test soil.

Natural Moisture Content (%)	Liquid Limit (%)	Plastic Limit (%)	Specific Gravity	Maximum Dry Density (g/cm³)	Optimum Moisture Content (%)	PH
28.2	51.2	26.1	2.76	1.75	19.1	6.2

Table 2. Chemical composition and technical parameters of cement.

Bulk Density (g·m⁻³)	Specific Surface Area (m²·kg⁻¹)	Initial Setting Time	Final Setting Time	Compressive Strength (MPa)		Soundness
Bulk Density (g·m⁻³)	Specific Surface Area (m²·kg⁻¹)	Initial Setting Time	Final Setting Time	3d	28d	Soundness
1.65	300	160	332	23.8	45.4	Qualified

Table 3. UCS test scheme.

Lead Ion Content (%)	Lignin Content (%)	Cement Content (%)	Curing Age (d)	Number of Test Groups
0	0, 0.25, 0.50, 1, 2	4, 6, 8, 10	28	60
0.1	0, 0.25, 0.50, 1, 2	4, 6, 8, 10
1	0, 0.25, 0.50, 1, 2	4, 6, 8, 10

Table 4. Penetration test scheme.

Lead Ion Content (%)	Lignin Content (%)	Cement Content (%)	Curing Ag (d)
0.1	0, 0.25, 0.5, 1,2	4, 6, 8, 10	28
1	0, 0.25, 0.5, 1,2	4, 6, 8, 10	28

Table 5. Experimental design for TCLP.

Test Series	Lead Ion Content (%)	Lignin Content (%)	Cement Content (%)	Curing Period (d)
Lignin-only Cement-only	0.1, 1	4, 6, 8, 10	0	28
Lignin-only Cement-only	0.1, 1	0	4, 6, 8, 10
Lignin–Cement Composite	0.1	0, 0.25, 0.5, 1, 2	10
Lignin–Cement Composite	1	0, 0.25, 0.5, 1, 2	10

Table 6. Fitting relationships between cement content and permeability coefficient of cement-stabilized soil.

Lignin Content (%)	Lead Ion Concentration (%)	Parameter a	Parameter b	R²
0.0	0.1	−4.612	11.947	0.9795
0.0	1.0	−6.122	15.751	0.9426
0.25	0.1	−5.051	12.673	0.9551
0.25	1.0	−3.832	9.655	0.9557
0.5	0.1	−1.239	3.430	0.9677
0.5	1.0	−5.248	13.748	0.9544
1.0	0.1	−2.532	6.450	0.9784
1.0	1.0	−6.181	18.471	0.9749
2.0	0.1	−2.837	23.360	0.9496
2.0	1.0	−7.431	23.360	0.9133

Note: The fitting equation is

l o g (k) = a \cdot C + b

, where k is the permeability coefficient (cm/s) and C is cement content (%).

Table 7. Summary of key performance indicators for the strength-based best-performing formulations.

Lead Concentration	Best-Performing Formulation	UCS (MPa)	Permeability	TCLP Leaching Concentration (mg/L)	Cost-Effectiveness (Material Cost)
0.1%	C4L0.5	2361.5 ± 95.4	reduction rate reached 69.1%	3.6 ± 0.1 (<5)	Low cement content
1%	C10L0.25	3604.7 ± 110.9	reduction rate reached 76.78%	37.3 ± 1.9 (>5)	High cement content

Note: The formulations were primarily selected based on the UCS results. This table provides a comparative overview of their multi-faceted performance for practical assessment.

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Chen, J.; Wei, X.; Huang, B.; Chen, A.; Shi, X.; Li, S.; Xiao, Y.; Liao, X.; Zhao, L. Research on Engineering Characteristics of Lignin–Cement-Stabilized Lead-Contaminated Lateritic Clay. Buildings 2025, 15, 4433. https://doi.org/10.3390/buildings15244433

AMA Style

Chen J, Wei X, Huang B, Chen A, Shi X, Li S, Xiao Y, Liao X, Zhao L. Research on Engineering Characteristics of Lignin–Cement-Stabilized Lead-Contaminated Lateritic Clay. Buildings. 2025; 15(24):4433. https://doi.org/10.3390/buildings15244433

Chicago/Turabian Style

Chen, Junhua, Xiulin Wei, Bocheng Huang, Aijun Chen, Xiong Shi, Shouqian Li, Ying Xiao, Xiao Liao, and Liuxuan Zhao. 2025. "Research on Engineering Characteristics of Lignin–Cement-Stabilized Lead-Contaminated Lateritic Clay" Buildings 15, no. 24: 4433. https://doi.org/10.3390/buildings15244433

APA Style

Chen, J., Wei, X., Huang, B., Chen, A., Shi, X., Li, S., Xiao, Y., Liao, X., & Zhao, L. (2025). Research on Engineering Characteristics of Lignin–Cement-Stabilized Lead-Contaminated Lateritic Clay. Buildings, 15(24), 4433. https://doi.org/10.3390/buildings15244433

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Research on Engineering Characteristics of Lignin–Cement-Stabilized Lead-Contaminated Lateritic Clay

Abstract

1. Introduction

2. Experimental Program Design

2.1. Materials and Their Characterization

2.2. Experimental Methodology

2.2.1. UCS Test

2.2.2. Permeability Test

2.2.3. TCLP Test

2.2.4. Microscopic Test

3. Results

3.1. Unconfined Compressive Strength (UCS)

3.2. Permeability Characteristics

3.3. TCLP Leaching Characteristics

3.4. SEM Characteristics

3.5. X-Ray Diffraction Characteristics

4. Discussion

4.1. Effect of Lignin Content on UCS of Stabilized Soil

4.2. Effect of Cement Content on UCS of Stabilized Soil

4.3. Effect of Lignin Content on Permeability Coefficient of Stabilized Soil

4.4. Effect of Cement Content on the Permeability Coefficient of Stabilized Soil

4.5. Effect of Individual Addition of Lignin and Cement on TCLP Concentration of Stabilized Soil

4.6. Effect of Lignin Content on TCLP Concentration of Cement-Stabilized Soil

4.7. Effect of Cement Content on Microstructural Characteristics of Stabilized Soil

4.8. Effect of Lignin Content on Microstructural Characteristics of Stabilized Soil

4.9. XRD Mineral Composition Analysis

4.10. Performance Comparison and Practical Implications of the Best-Performing Formulations

5. Limitations of the Study

6. Conclusions

Author Contributions

Funding

Data Availability Statement

Conflicts of Interest

Appendix A

References

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