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Article

The Strength, Permeability, and Microstructure of Cement–Bentonite Cut-Off Walls Enhanced by Polypropylene Fiber

by
Zonghan Yang
,
Yajun Zhang
,
Yuhuan Zhu
,
Yuxin Li
and
Rongbing Fu
*
Centre for Environmental Risk Management and Remediation of Soil and Groundwater, College of Environmental Science and Engineering, Tongji University, Shanghai 200092, China
*
Author to whom correspondence should be addressed.
Sustainability 2025, 17(8), 3656; https://doi.org/10.3390/su17083656
Submission received: 3 April 2025 / Revised: 15 April 2025 / Accepted: 16 April 2025 / Published: 18 April 2025
(This article belongs to the Section Pollution Prevention, Mitigation and Sustainability)
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Versions Notes

Abstract

:
Cement–bentonite cut-off walls are widely used in geoenvironmental engineering such as landfill liners and contaminated site remediation, due to their low permeability and structural stability. However, excessive cement use reduces the swelling capacity of bentonite and increases environmental burdens. This study proposes incorporating polypropylene fibers (PPFs) into cement–bentonite cut-off walls to improve their performance under lower cement dosages. A total of 16 formulations were tested with different cement and fiber contents. Unconfined compressive strength (UCS), direct shear, and falling head permeability tests were conducted over 7, 14, and 28 days, respectively. Microstructural changes were examined using X-ray diffraction (XRD) and scanning electron microscopy (SEM). The results showed that compared to the conventional high-cement mixture without fibers, a formulation with moderate cement content and 2% PPF achieved higher compressive strength, comparable shear strength, and significantly lower permeability. Microstructural analysis confirmed that fiber addition enhanced cement hydration and preserved bentonite, forming a compact microstructure with reduced porosity. Furthermore, cost and carbon emission analyses revealed that the above optimized formulation reduced both material cost and embodied carbon by approximately 12.5% and 22.3%. These findings provide a sustainable and cost-effective approach to improve the mechanical and hydraulic performance of cement–bentonite cut-off walls.

1. Introduction

In geoenvironmental engineering, cut-off walls are primarily applied in landfills and contaminated site remediation to limit groundwater flow and pollutant transport [1,2,3,4,5,6,7,8,9]. With land development, especially in urban expansion areas, risk-based management strategies such as cut-off walls have gradually become the mainstream strategy for contaminated site remediation [10,11,12]. When land is not urgently needed for redevelopment, risk control measures can effectively prevent pollutant exposure and migration, while offering better economic feasibility, achieving a win–win outcome for both environmental protection and sustainable development.
Among various cut-off wall materials, cement–bentonite cut-off walls are widely recognized for their low permeability (about 1 × 10−6 cm/s), excellent stability, relatively high compressive strength, and low construction requirements [13,14,15]. The dominant advantages of cement–bentonite cut-off walls are that cement provides high compressive strength and a consolidated structure during cement hydration, while bentonite swells upon water absorption to fill the pores within the material, thus reducing seepage pathways and achieving excellent impermeability [3,16,17].
However, cement–bentonite cut-off walls also present certain shortcomings. While cement improves the compressive strength of the material, high cement content introduces a series of issues. Firstly, during the hydration process, a highly alkaline environment is generated along with a large amount of multivalent cations such as Ca2⁺ and Al3⁺, which can exchange with the interlayer cations in bentonite. Notably, the bentonite used in cement–bentonite cut-off walls is usually either a naturally occurring sodium bentonite or a calcium bentonite that has been sodium-activated. Once this ion exchange occurs, part of the sodium bentonite is converted into calcium bentonite, reducing the interlayer spacing of montmorillonite and partially destroying the bentonite particles, thereby diminishing its water absorption capacity and impermeability [18,19,20]. Additionally, high cement content can significantly increase costs and result in undesirable environmental impacts. Cement production is energy-intensive, releasing substantial amounts of carbon dioxide and other greenhouse gases, thereby contributing to global warming [21,22,23,24]. According to data from the Center for International Climate Research, cement production accounts for approximately 7–8% of global carbon emissions. Therefore, reducing cement content and introducing additives to maintain or improve the performance of cement–bentonite cut-off walls has become a key research focus [21].
To address these shortcomings, researchers have introduced various modification strategies to enhance the performance and sustainability of cement–bentonite cut-off walls [8,21,25,26,27,28,29,30]. A previous study involve partially replacing cement with supplementary cementitious materials such as ground granulated blast-furnace slag or fly ash [31]. Studies have shown that an appropriate slag replacement ratio, about 30%, as suggested by Jefferis et al. [32], can reduce the permeability coefficient of the cut-off material by an entire order of magnitude. Similarly, Cui et al. [30] reported that alkali-activated slag not only improves compressive strength and accelerates solidification, but also enhances seepage resistance. Liu et al. [33] and Ji et al. [34] found that replacing cement with fly ash significantly increases the chemical compatibility of cut-off materials in the presence of heavy metals such as Cd and Pb.
Alongside cement replacement strategies, another widely used strategy for improving cementitious materials is fiber reinforcement [35,36,37]. The incorporation of fibers effectively enhances the tensile strength, toughness, and crack resistance of cementitious composites, while also mitigates shrinkage and drying-induced cracking [38,39,40,41]. In recent years, various types of fiber-reinforced materials, including steel fibers, glass fibers, carbon fibers, and synthetic fibers, have been employed in different cement-based systems [42,43,44,45,46,47]. For instance, steel fibers are widely used in high-strength concrete structures, due to their superior toughness, while glass fibers offer enhanced corrosion resistance and fire resistance. Synthetic fibers, including polyethylene and polypropylene fibers, have gained increasing attention due to their good dispersion properties and chemical stability.
Among these, polypropylene fibers show the better mechanical and chemical stability, showing significant benefits in toughening and crack resistance in conventional cement-based materials [48,49,50,51,52,53]. Shen et al. [48] found that incorporating polypropylene fibers into concrete improved its tensile strength and flexural toughness, while Liu et al. reported that adding polypropylene fibers to ordinary cement mortar significantly reduced drying shrinkage effects [48]. However, there remains a lack of systematic research on the application of polypropylene fibers in cement–bentonite cut-off wall systems, and their reinforcement mechanisms within cement–bentonite matrices remain unclear. Therefore, the present study attempts to introduce polypropylene fibers into cement–bentonite slurries and systematically examine the variations in mechanical properties (such as unconfined compressive strength, internal friction angle, and cohesion) and seepage characteristics (permeability coefficient) under different cement contents and polypropylene fiber dosages. Additionally, X-ray diffraction and scanning electron microscopy analyses will be employed to investigate microstructural evolution, providing valuable insights for designing more efficient and sustainable underground cut-off walls.

2. Materials and Methods

2.1. Materials

The bentonite used in this study is sodium-based bentonite, with montmorillonite, quartz, and low albite as its primary mineral components, as identified by X-ray diffraction (XRD) analysis, as shown in Figure 1. The chemical compositions of bentonite were determined using X-ray fluorescence (XRF) spectroscopy, showing major constituents of SiO2 (65.746%), Al2O3 (15.203%), Na2O (3.424%), MgO (3.174%), and CaO (6.778%) (Table 1). As shown in Figure 2, the particle size distribution was analyzed using a Malvern laser particle size analyzer, with a median particle size d(5%) of 9.183 μm, d(10%) of 3.726 μm, and d(90%) of 28.558 μm. The specific surface area of bentonite is 1.19 m2/g. Additionally, the swelling volume of bentonite is 54 mL/g, determined according to GB/T 20973-2020 Bentonite [54].
The cement used is ordinary Portland cement (PO 42.5), which showed key components of CaO (46.322%), SiO2 (23.955%), Al2O3 (10.04%), Fe2O3 (5.997%), and SO3 (5.508%) (Table 1). The particle size distribution of cement is a median particle size d(5%) of 6.734 μm, d(10%) of 2.644 μm, and d(90%) of 18.722 μm. The specific surface area of cement is 0.88 m2/g (Figure 2).
The polypropylene fibers used in this study have a length of 6 mm and exhibit the following physical and mechanical properties: density of 0.91, tensile strength > 486 MPa, elastic modulus > 4.8 GPa, diameter of 18–48 μm, and elongation at break > 15% (Table 2). The high tensile strength and excellent elongation properties of polypropylene fibers contribute to improving the crack resistance and durability of cement–bentonite composites.

2.2. Sample Mixing Proportion Design and Preparation

In this study, 16 sets of experimental samples were designed to systematically analyze the influence of cement content and polypropylene fiber dosage on the mechanical and anti-seepage properties of cement–bentonite cut-off materials. The samples were classified based on cement content (50 kg/m3, 100 kg/m3, 150 kg/m3, and 200 kg/m3) and polypropylene fiber (PPF) dosage (0%, 1%, 2%, and 3%). The selection of cement dosages and PPF dosages was based on a comprehensive consideration of previous literature, preliminary tests, and the specific objectives of this study [8,13,49,55]. The sample nomenclature follows an A, B, C, D labeling system for different cement contents, with numbers 1–4 representing fiber dosages. The bentonite content was fixed at 100 kg/m3 in all samples to ensure data comparability. Table 3 presents the experimental mix proportions used in this study.
The preparation process of the cement–bentonite samples is illustrated in Figure 3. The preparation steps are as follows: First, 1 L of tap water was measured and poured into a 2 L beaker. The pre-weighed bentonite was then gradually added to the beaker and continuously stirred with a mechanical mixer until no visible clumps remained. The bentonite slurry was then sealed with plastic wrap and left to stand at 20 °C for 24 h to ensure full water absorption and swelling, improving its dispersion. Meanwhile, the pre-weighed polypropylene fibers and cement were dry-mixed under controlled conditions to ensure uniform fiber distribution and prevent fiber agglomeration. Next, the hydrated bentonite slurry was gradually poured into the pre-mixed cement and polypropylene fiber mixture, while pre-stirred with a mixing rod to achieve initial uniformity. The mixture was then subjected to a two-stage mechanical mixing process using a JJ-20H cement mortar mixer. A low-speed mixing phase was conducted for 1 min with a blade rotation speed of 140 ± 5 rpm and a planetary speed of 62 ± 5 rpm. It was followed by a high-speed mixing phase for 5 min, during which the blade rotated at 285 ± 10 rpm and the planetary motion operated at 125 ± 10 rpm. Visual observation throughout the mixing confirmed uniform dispersion of fibers and the absence of visible agglomerates. This procedure was consistently applied across all specimens to ensure reproducibility. After mixing, the slurry was immediately poured into standard molds (unconfined compressive strength test: φ40 × 100 mm; direct shear test: φ61.8 × 20 mm; falling head permeability test: φ61.8 × 40 mm), with the exposed surfaces covered with plastic wrap to prevent moisture loss. After 24–48 h, once the samples had solidified, the plastic wrap was removed. The specimens were placed in a curing chamber and then maintained at 20 ± 2 °C and relative humidity of above 95% for 7, 14, and 28 d.

2.3. Methods

2.3.1. Unconfined Compressive Strength Test

The unconfined compressive strength (UCS) test was conducted to evaluate the mechanical performance of specimens with different cement and polypropylene fiber contents. The test was performed using a YYW-II strain-controlled unconfined pressure apparatus, following GB/T 50123-2019 standards [56]. The specimen dimensions were φ40 mm × 100 mm, and tests were conducted after 7, 14, and 28 days of curing. A constant loading rate of 1 mm/min was applied, and the peak load was recorded to determine the UCS.

2.3.2. Direct Shear Test

The direct shear test was conducted to determine shear strength parameters (cohesion and internal friction angle). The test followed GB/T 50123-2019 standards and was performed using a ZJ-1B strain-controlled direct shear apparatus [56]. The specimen dimensions were φ61.8 mm × 20 mm, and tests were conducted after 28 days of curing. Shear tests were performed under 12.5, 25, 50, and 100 kPa normal stress conditions at a shear rate of 0.8 mm/min. The peak shear stress was recorded to determine cohesion © and internal friction angle (φ).

2.3.3. Falling Head Permeability Test

The falling head permeability test was conducted to determine the permeability coefficient and evaluate the cut-off performance of the materials. The test followed GB/T 50123-2019 standards, with specimen dimensions of φ61.8 mm × 40 mm, and was conducted after 7, 14, and 28 days of curing [56]. The test was performed using a TST-55 permeability apparatus, with a falling head device connected to the specimen top. Water level changes were recorded, and the permeability coefficient k was calculated using the following equation:
k = a L A t ln ⁡ h 1 h 2
where a is the cross-sectional area of the standpipe, L is the specimen height, A is the cross-sectional area of the specimen, t is the time interval, and h 1 and h 2 are the initial and final water heads, respectively.

2.3.4. Microstructural Analysis

XRD and scanning electron microscopy (SEM) analysis were carried out to study the mineral composition and microstructural characteristics of the specimens cured for a standard period of 28 days.

3. Results and Discussion

3.1. Unconfined Compressive Strength (UCS)

Figure 4 illustrates the variations in the unconfined compressive strength (UCS) of specimens with different cement dosages, fiber contents, and curing ages. Overall, the UCS of all samples increased with curing age, reflecting the progressive hydration of cement and the development of internal strength. Groups A, B, and C, with relatively low cement dosages (50 kg/m3, 100 kg/m3, and 150 kg/m3, respectively), exhibited low UCS values throughout the curing period. Even with the addition of polypropylene fibers, the 28-day strength of Groups A and B remained below 100 kPa, indicating limited mechanical development and inadequate structural integrity for most geoengineering applications. In contrast, Group D, with higher cement dosages (200 kg/m3), exhibited relatively high structural bearing capacity even without fiber reinforcement. After 28 days, UCS values reached 176.40 kPa (D-1). More notably, with 2% polypropylene fiber incorporation, the UCS of the C and D groups increased significantly to 231.17 kPa (C-3) and 239.53 kPa (D-3), both well exceeding the 100 kPa threshold, indicating that adding appropriate fiber to cement systems effectively enhances mechanical performance and meets practical engineering requirements.
The increase in UCS percentage (Figure 5) clearly showed the strengthening influence of fiber addition under different conditions. Group C (cement dosage of 150 kg/m3) with 2% fiber content exhibited the most significant improvement, with UCS increases of 147.95%, 122.22%, and 194.92% after 7, 14, and 28 days, respectively. The result suggested that a moderate cement content ensures sufficient hydration product formation while preserving bentonite’s swelling capacity, thus optimizing the reinforcing effects of fiber.
The observed strength improvements were further confirmed by statistical analysis. A three-way ANOVA was conducted to evaluate the effects of fiber content, curing age, and cement dosage on UCS. The results revealed that all three factors significantly influenced UCS (p < 0.01), with cement dosage being the most dominant factor (F = 29.98, p < 0.001), followed by curing time (F = 18.56, p < 0.001) and fiber content (F = 5.25, p = 0.0039). These findings statistically validate the trends observed in Figure 4 and Figure 5, indicating that the incorporation of polypropylene fibers and optimization of cement dosage significantly enhance the mechanical performance of cement–bentonite barrier materials.
In comparison, Group A (lowest cement dosage) still showed notable strength improvements with fiber addition, especially after 28 days, indicating that the ability of fibers to compensate limited hydration reactions. Group D (highest cement dosage) showed minimal or even negative enhancement effects at later stages. This might be attributed to poor fiber dispersion in high-cement matrices. In addition, the abundant Ca2⁺ generated during cement hydration replaces Na⁺ in bentonite, disrupting its layered structure and reducing its colloidal stability. As a result, bentonite loses its ability to form a cohesive network with fibers, which weakened the fiber–matrix interface and limited the reinforcement effect.
Additionally, the data confirms that the reinforcing effect of fibers becomes more prominent over time, especially in Group C, indicating that fiber reinforcement continues to improve microstructural densification and long-term strength development.

3.2. Shear Strength

The shear strength test results are shown in Figure 6. In the D-group (200 kg/m3 cement + 0~3% fiber) samples, cohesion increased from approximately 55 kPa (0% fiber) to 90 kPa (2% fiber), followed by a slight decrease at 3% fiber content. A similar trend was observed in the C-group samples, although the overall cohesion was lower, with a peak value of approximately 60 kPa. The result suggested that moderate fiber content enhanced the bonding between the matrix and soil particles via the bridging effects, improving shear strength. However, when fiber content exceeded 2%, fiber agglomeration might lead to matrix heterogeneity, causing a slight reduction in cohesion.
In contrast, the variation in the internal friction angle (φ) is more complex. In the C-group (150 kg/m3 cement + 0~3% fiber) samples, φ increased consistently with fiber content from 28° (0% fiber) to 32° (3% fiber), indicating that fiber addition improved mechanical interlocking between particles, enhancing shear resistance at lower cement contents. However, in the D-group samples, φ initially decreased, reaching its lowest value (~24°) at 2% fiber content, before increasing again to 27° at 3% fiber content. This phenomenon might be attributed to the high cement content creating a denser matrix, where fiber inclusion disrupted the original interparticle contacts, reducing internal friction. The observed differences in cohesion and internal friction angle were statistically evaluated using one-way ANOVA. For cohesion in Group C, fiber content showed a significant effect (p = 0.0148, F = 6.60), while for the internal friction angle in Group D, the effect was highly significant (p < 0.001, F = 153.71). These results confirm that fiber addition notably influences shear strength parameters, especially under varying cement dosages, aligning with the trends observed in Figure 6.
Overall, 2% fiber content optimally improved cohesion and maintained a reasonable internal friction angle, enhancing the overall shear strength. Therefore, for engineering applications, 2% fiber content is recommended, as it enhanced shear resistance and prevented from the matrix heterogeneity issues associated with excessive fiber addition. This makes it particularly suitable for landfill and contaminated site cut-off walls.

3.3. Hydraulic Conductivity (k)

The falling head permeability test results are shown in Figure 7. The results indicated that after 28 days, C-group samples (low cement content) exhibited lower permeability than D-group samples (high cement content). The lowest permeability was recorded at 4.17 × 10−6 cm/s in C-group samples with 2% fiber content, compared to D-1 (200 kg/m3 cement, no fiber) at 8.49 × 10⁻⁶ cm/s. This significant difference suggested that reducing cement dosage and increasing bentonite proportion were more effective in achieving long-term impermeability.
This enhancement was likely attributed to the swelling and void-filling behavior of bentonite, which created a more uniform and compact pore structure. In contrast, excessive cement content in D-group samples generated abundant Ca2⁺ during hydration, which replaced interlayer Na⁺ in bentonite, transforming sodium bentonite into calcium bentonite. The exchange reduced swelling capacity and increased pore connectivity, ultimately impairing the cut-off function of the material.
The influence of fiber content also varied between groups. In the C group, permeability generally decreased with increasing fiber content, indicating that fiber-induced crack bridging and matrix densification were more effective in low-cement systems. However, at 3%, slight fiber agglomeration might lead to micro-defects and localized porosity, as well as slightly increased permeability. A similar trend was observed in the D group, where 2% fiber also led to the lowest permeability (5.82 × 10⁻⁶ cm/s), followed by a minor increase at 3%, likely due to matrix heterogeneity.
Overall, permeability decreases with curing time for all specimens, indicating that progressive hydration contributed to pore refinement. However, the superior impermeability of the C group, especially C-3 (150 kg/m3 cement + 2% fiber), after 28 days further confirmed that a low-cement and high-bentonite formulation combined with fiber reinforcement were more effective in achieving long-term cut-off performance. These findings supported the application feasibility of the above composites in landfill liners and contaminated site remediation.
Although the 28-day results demonstrate that low-cement and fiber-reinforced mixtures exhibit superior impermeability, the long-term durability of this performance under environmental exposure requires further consideration. In particular, high fiber contents (e.g., 3%) may lead to localized micro-defects or weak interfaces that could evolve over time due to environmental factors such as wet–dry cycling, freeze–thaw actions, or chemical attacks (e.g., sulfates or chlorides). These conditions may exacerbate pore connectivity or fiber–matrix debonding, potentially increasing permeability in the long term. Therefore, while 2% fiber appears optimal for short-term impermeability, the stability of this behavior over extended periods and under field-relevant conditions warrants further investigation.

3.4. Microstructural

3.4.1. XRD Analysis

XRD analysis indicated that the representative sample C-1 (150 kg/m3 cement + 0% fiber) (Figure 8) was composed of quartz, montmorillonite, ettringite (AFt), calcium silicate hydrate (C–S–H), and calcite.
Figure 9 presents the XRD patterns of Group C (150 kg/m3 cement + 0~3% fiber) and D (150 kg/m3 cement + 0~3% fiber) specimens. In the cement–bentonite system, the main hydration products—C–S–H, AFt, and calcite—showed strong peak under optimal fiber conditions. C–S–H appeared as broad peaks near 26.8° and 29.5° (2θ). Ettringite appeared as a sharp peak around 29.6°. Calcite appeared as distinct peaks at 26.7° and 29.7°. With the addition of polypropylene fiber, particularly at 2% content, the intensities of these peaks significantly increased, indicating that fiber promoted cement hydration by retaining internal moisture, facilitating ion transport, and providing nucleation sites. The most prominent hydration product peaks were observed in C-3 (150 kg/m3 cement + 2% fiber) and D-3 (200 kg/m3 cement + 2% fiber) samples, suggesting that this fiber dosage optimized the hydration environment. In contrast, a slight decrease in peak intensity at 3% fiber content might result from fiber agglomeration and disruption of the internal matrix continuity. The concurrent enhancement of C–S–H (providing strength), ettringite (early-stage filler), and calcite (secondary carbonation product) demonstrated that fiber not only improved early hydration but also supported secondary mineral formation, contributing to matrix densification and long-term structural stability.
Meanwhile, montmorillonite showed multiple broad peaks between 19.8° and 22.4°. As cement content increased, the intensity of these peaks decreased progressively from C-1 to C-4 and from D-1 to D-4. This decline was attributed to the ion exchange between Ca2⁺ released from cement hydration and Na⁺ in the montmorillonite interlayers, which led to partial transformation from Na-type to Ca-type montmorillonite and collapse of the layered structure. Fiber incorporation indirectly enhanced cement hydration and Ca2⁺ availability. The weakened montmorillonite peaks suggested a reduction in swelling capacity and crystalline integrity. The result indicated that mineralogical changes might favor the densification of the matrix by allowing hydration products to fill the disrupted structures. While this transformation enhanced the hydration environment and contributed to matrix densification, it may also indicate a reduction in the swelling capability of bentonite. As the self-sealing function of bentonite relies heavily on its swelling potential, this mineralogical change could compromise the long-term sealing performance of the barrier material. Therefore, although increased cement content and fiber addition improve hydration and strength, the associated loss in montmorillonite swelling capacity warrants further attention, particularly when long-term cut-off performance is considered.
In summary, XRD results demonstrated a clear dual mechanism: fiber addition at 2% not only enhanced the formation of hydration products but also modified the structure of bentonite. This synergy effectively improves both the strength and impermeability of cement–bentonite cut-off materials, particularly under conditions of reduced cement usage.

3.4.2. Scanning Electron Microscopy (SEM)

SEM results are shown in Figure 10. At the macroscopic scale, Group C (150 kg/m3 cement + 0~3% fiber) (especially Figure 10a) exhibited more visible pores, compared to Group D (200 kg/m3 cement + 0~3% fiber) (Figure 10e). The phenomenon was primarily attributed to the higher bentonite content in Group C and the noticeable shrinkage during freeze-drying. In contrast, Group D samples demonstrated fewer apparent pores, due to their higher cement and lower relative bentonite content, thus forming denser structures, as shown in Figure 10e,g.
Figure 10b,d,f,h show clear distinctions in microstructural composition and morphology. Group C (Figure 10b,d) contained abundant montmorillonite clay particles characterized by layered, sheet-like stacking structures. On the other hand, Group D (Figure 10f,h) presented numerous needle-like ettringite crystals and dense interwoven networks, which were significantly more abundant, compared to Group C.
It was observed that samples with fiber incorporation (Figure 10d,h) displayed a notably higher quantity of needle-like ettringite crystals. The result indicated that polypropylene fiber incorporation enhanced cement hydration, facilitating nucleation and growth of ettringite crystals, leading to a more uniform distribution within the material. Such microstructural changes effectively the filled internal void spaces and optimized the pore distribution, which enhanced the compactness and strength of the cut-off materials. These findings coincided with the permeability test results.

3.5. Economic and Environmental Evaluation

To comprehensively evaluate the economic feasibility and carbon reduction potential of PPF reinforcement, a comparison was conducted between two representative samples: C-3 (150 kg/m3 cement + 6 g PPF) and D-1 (200 kg/m3 cement without fiber). The results showed that C-3 exhibited higher unconfined compressive strength, comparable shear strength, and lower hydraulic conductivity than D-1, indicating that enhanced mechanical and impermeability performance could be achieved with reduced cement content and PPF addition.
Material costs were calculated and estimated based on experimental mix ratios and market prices, where the price of cement, bentonite, and polypropylene fiber were 0.45 CNY/kg, 0.60 CNY/kg, and 15.00 CNY/kg, respectively. Under these assumptions, C-3 had a total material cost of 0.1575 CNY per sample, which was slightly lower than D-1 (0.18 CNY/sample). When scaled up to one cubic meter of cut-off wall material (based on UCS specimen volume), the cost of C-3 was approximately 157.5 CNY/m3, which was lower than the cost of D-1 at 180 CNY/m3.
In terms of environmental performance, cement production accounts for approximately 7–8% of global CO2 emissions. Based on emission factors of 0.89 kg CO2/kg for cement (IPCC 2019 Refinement), 1.66 kg CO2/kg for polypropylene fiber (PlasticsEurope 2019; Ecoinvent v3.5), and 0.06 kg CO2/kg for bentonite (Ecoinvent v3.5), the embedded carbon footprint of C-3 was estimated to be 139.8 kg CO2/m3, which was lower than that of D-1 (180 kg CO2/m3) [57,58,59]. The result was attributed to the 25% reduction in cement content and minimal additional impact from the low dosage of fiber.
These results demonstrate that incorporating PPF into cement–bentonite slurry systems not only enhance the performance of cut-off walls but also reduce both cost and carbon emissions, offering a dual advantage for sustainable and economically viable cut-off wall construction.

4. Conclusions

This study investigated the effects of PPF incorporation on the mechanical properties, impermeability, and environmental–economic performance of cement–bentonite cut-off wall materials under varying cement contents. Laboratory tests including UCS, direct shear, and falling head permeability tests were conducted after 7, 14, and 28 days, combined with XRD and SEM analyses.
The results demonstrated that PPF significantly enhanced the UCS, cohesion, and impermeability of the cement–bentonite matrix, especially at low-to-moderate cement contents. The optimal performance was observed in specimen C-3 (150 kg/m3 cement + 2% fiber), which was 31% higher than that of D-1 (200 kg/m3 cement, no fiber) and reduced the permeability to 4.17 × 10⁻⁶ cm/s. XRD and SEM analyses confirmed that PPF addition promoted the hydration reaction and formation of dense microstructures, with more needle-like ettringite and better retention of swelling bentonite.
Economic and environmental evaluations further revealed that PPF use allowed for a 25% reduction in cement content, achieving better mechanical and hydraulic performance. Compared with D-1, the cost and carbon emissions per cubic meter of C-3 decreased by approximately 12.5% and 12.6%, respectively. These results were attributed to both low cement and PPF dosage. The results suggested that incorporating PPF into cement–bentonite slurry was a cost-effective and low-carbon strategy to improve the performance of cut-off wall materials, offering practical guidance for sustainable contaminated site management and underground construction.

Author Contributions

Conceptualization, Z.Y. and R.F.; Methodology, Z.Y.; Validation, Y.Z. (Yajun Zhang), Y.Z. (Yuhuan Zhu) and R.F.; Formal analysis and data curation, Z.Y.; Investigation, Z.Y. and Y.Z.; Resources, R.F.; Writing—original draft preparation, Z.Y. and Y.Z. (Yuhuan Zhu); Writing—review and editing, Y.Z. (Yajun Zhang), Y.L., Y.Z. (Yuhuan Zhu). and R.F.; Supervision, R.F.; Project administration, R.F.; Funding acquisition, R.F. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by National Natural Science Foundation of China [2023YFC3707704, 2023YFC3707700].

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The raw data supporting the conclusions of this article will be made available by the authors on request.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. The XRD result of bentonite.
Figure 1. The XRD result of bentonite.
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Figure 2. The cumulative volume of particle size of bentonite and cement.
Figure 2. The cumulative volume of particle size of bentonite and cement.
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Figure 3. The preparation process of the cement–bentonite samples.
Figure 3. The preparation process of the cement–bentonite samples.
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Figure 4. Unconfined compressive strength (UCS) test results for different sample groups after 7, 14, and 28 days. (a) Group A; (b) Group B; (c) Group C; (d) Group D.
Figure 4. Unconfined compressive strength (UCS) test results for different sample groups after 7, 14, and 28 days. (a) Group A; (b) Group B; (c) Group C; (d) Group D.
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Figure 5. Percentage increase in UCS (%).
Figure 5. Percentage increase in UCS (%).
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Figure 6. Variation of internal friction angle (φ) and cohesive force (c) with fiber content and cement content.
Figure 6. Variation of internal friction angle (φ) and cohesive force (c) with fiber content and cement content.
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Figure 7. Variation of hydraulic conductivity (k) with fiber content and curing time in Groups C and D.
Figure 7. Variation of hydraulic conductivity (k) with fiber content and curing time in Groups C and D.
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Figure 8. XRD pattern of specimen C-1 and standard diffraction peaks of key minerals.
Figure 8. XRD pattern of specimen C-1 and standard diffraction peaks of key minerals.
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Figure 9. XRD patterns of specimens in Group C (a) and Group D (b), with magnified views highlighting key hydration and clay mineral peaks.
Figure 9. XRD patterns of specimens in Group C (a) and Group D (b), with magnified views highlighting key hydration and clay mineral peaks.
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Figure 10. SEM images of samples with varying cement and polypropylene fiber contents: (a) C-1 100×; (b) C-1 20,000×; (c) C-3 100×; (d) C-3 20,000×; (e) D-1 100×; (f) D-1 20,000×; (g) D-3 100×; (h) D-3 20,000×.
Figure 10. SEM images of samples with varying cement and polypropylene fiber contents: (a) C-1 100×; (b) C-1 20,000×; (c) C-3 100×; (d) C-3 20,000×; (e) D-1 100×; (f) D-1 20,000×; (g) D-3 100×; (h) D-3 20,000×.
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Table 1. Chemical compositions of bentonite and cement by XRF.
Table 1. Chemical compositions of bentonite and cement by XRF.
Chemical ComponentsCementBentonite
%%
Na2O0.533.424
MgO4.843.174
Al2O310.0415.203
SiO223.95565.746
P2O50.102—
SO35.508—
Cl0.1870.091
K2O1.1112.973
CaO46.3226.778
TiO20.8060.202
MnO0.3440.097
Fe2O35.9972.023
ZnO0.0410.045
SrO0.2170.125
BaO—0.117
Table 2. Physical properties of polypropylene fibers.
Table 2. Physical properties of polypropylene fibers.
LengthRelative DensityTensile StrengthElastic ModulusDiameterTensile Limit
6 mm0.91>486 Mpa>4.8 GPa18–48 μm>15%
Table 3. Mix proportions for specimens.
Table 3. Mix proportions for specimens.
Specimen IDRaw Materials (kg/m3) 1Mix Proportion 2
CementBentoniteWaterPPF
A-15010010000%
A-25010010001%
A-35010010002%
A-45010010003%
B-110010010000%
B-210010010001%
B-310010010002%
B-410010010003%
C-115010010000%
C-215010010001%
C-315010010002%
C-415010010003%
D-120010010000%
D-220010010001%
D-320010010002%
D-420010010003%
1 The listed values represent the original masses of raw materials required to prepare one cubic meter of cement–bentonite slurry, based on typical field application proportions. 2 The mix proportion is calculated as the mass ratio of PPF to the combined mass of cement and bentonite.
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Yang, Z.; Zhang, Y.; Zhu, Y.; Li, Y.; Fu, R. The Strength, Permeability, and Microstructure of Cement–Bentonite Cut-Off Walls Enhanced by Polypropylene Fiber. Sustainability 2025, 17, 3656. https://doi.org/10.3390/su17083656

AMA Style

Yang Z, Zhang Y, Zhu Y, Li Y, Fu R. The Strength, Permeability, and Microstructure of Cement–Bentonite Cut-Off Walls Enhanced by Polypropylene Fiber. Sustainability. 2025; 17(8):3656. https://doi.org/10.3390/su17083656

Chicago/Turabian Style

Yang, Zonghan, Yajun Zhang, Yuhuan Zhu, Yuxin Li, and Rongbing Fu. 2025. "The Strength, Permeability, and Microstructure of Cement–Bentonite Cut-Off Walls Enhanced by Polypropylene Fiber" Sustainability 17, no. 8: 3656. https://doi.org/10.3390/su17083656

APA Style

Yang, Z., Zhang, Y., Zhu, Y., Li, Y., & Fu, R. (2025). The Strength, Permeability, and Microstructure of Cement–Bentonite Cut-Off Walls Enhanced by Polypropylene Fiber. Sustainability, 17(8), 3656. https://doi.org/10.3390/su17083656

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