Improving Sand–Bentonite Hydromechanical Performance With Shredded Facemask and Colloidal Nanosilica

Abstract

Sand–bentonite (SB) serves a crucial role as a landfill liner for waste containment and environmental protection. However, the challenge of insufficient strength in SB liners poses risks of failure and hazardous substance release. To address this, our study introduces an innovative approach by incorporating the shredded facemask (FM) and colloidal nanosilica (CN) to investigate their combined effects on enhancing the hydromechanical performance of SB mixtures. The research encompassed a wide range of tests, such as unconfined compressive strength (UCS), free swell, 1D consolidation, and 3D volumetric shrinkage tests. To explore the geochemical compositions and microstructures of both untreated and treated SB samples, microstructural analysis techniques were utilized, including X-ray fluorescence, X-ray diffraction, Fourier-transform infrared spectroscopy (FTIR), and scanning electron microscopy (SEM). Results show that the inclusion of FM improves both strength and dilative behavior, with an even more significant enhancement in strength observed upon the CN treatment. Optimal increases in UCS of 167% and 194% were achieved after treating with 0.5% FM and 15% CN in 10% and 15% bentonite contents for 28 days, respectively. Furthermore, notable improvements were also observed in swelling control, volumetric strains, and shrinkage limits. Additionally, the study met the prescribed benchmarks for hydraulic conductivity and adhered to the generally accepted minimum strength criteria for soil liners. Finally, FTIR and SEM analyses confirmed improved interlocking between SB particles and the viscous gels formation, leading to enhanced SB performance. The findings offer significant perspectives into the hydromechanical performance of FM-CN-treated SB and introduce an innovative approach featuring a robust design, contributing to more efficient and environmentally friendly practices in the treatment of SB.

This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

1. Introduction

Among the COVID-19 pandemic, a significant quantity of disposable facemask (FM) has been employed to safeguard individuals from potential infections. With the persistent presence of the pandemic in numerous global locations, donning FM is anticipated to become a regular practice in the coming days. The recycling and reusing of disposable FM pose substantial challenges to the global community [1, 2]. According to Kilmartin-Lynch et al. [3], the growing disposal of FM in landfills can result in a decomposition process that extends beyond 25 years. A prediction model of daily worldwide FM usage was applied by Nzediegwu and Chang [4], who estimated it at 6.88 billion masks (~206,000 tons). Traditional methods of handling disposable FM involve incineration and landfill, both of which are associated with drawbacks. The incineration process raises concerns about energy consumption and carbon emissions, conflicting with the carbon-neutral policies embraced by many nations. Simultaneously, the primary plastic component of FM, polypropylene (PP), has been found to persist for hundreds of years in landfills [5]. Currently, the primary approach to managing FM waste involves high-temperature incineration, utilizing the generated heat for steam and thermal power generation. However, this process results in the production of various toxic by-products, contributing to environmental pollution [6]. Consequently, there is an immediate and meaningful need to introduce an environmentally friendly and pragmatic approach to address the issue of FM waste.

In such situations, certain scholars have suggested utilizing disposable FM in civil engineering, offering a novel approach to address environmental concerns. Zhang et al. [7] found that incorporating disposable FM into well-graded limestone enhanced shear strength and reduced shear-induced volumetric dilation and stiffness. This innovative method for recycling disposable FM addresses energy consumption and land occupation issues post-COVID-19. Notably, the study focused on granular materials, leaving the impact of FM on cohesive soil mechanics unexplored. For instance, Saberian et al. [8] conducted experiments involving fractured FM mixed with recycled concrete aggregates. Through compaction tests, elastic modulus tests, and unconfined compressive strength (UCS) tests, they observed that FM inclusion significantly enhanced the strength, ductility, and stiffness of the specimens. The optimal content of FM fragments was determined to be 2%. Triaxial shear tests were also performed on sand reinforced with shredded FM reported by Ghadr et al. [9], revealing that FM could enhance the shear strength, with superior reinforcing effects being exhibited by longer fragments (sized at 2 cm length and 0.5 cm width). Furthermore, Xu et al. [10] found in their investigation that FM has the potential to improve the shear strength of clay, with the most effective reinforcement observed at a mask content of 0.5%.

Colloidal nanosilica (CN) distinguishes itself as an environmentally friendly stabilizer with zero carbon emissions, offering the potential to enhance the hydromechanical properties of soil [11, 12]. Moreover, recent soil stabilization with nanomaterials, such as CN, as an additive has been one of the popular developments [13–18]. Previous studies show that nanomaterials treat the soil’s hydromechanical properties due to their improvement mechanism [19, 20]. They proposed that the sample strength, swelling potential, and compressibility index increased. Conversely, the hydraulic conductivity, volumetric strains, and liquefaction potential decreased after treatment. As a result, an alternative approach involves repurposing FM by mixing it with CN in problematic soils, particularly in landfill liner buffer materials like sand–bentonite (SB), to improve its performance. Given the limited research in this area, it is crucial to investigate the potential of utilizing FM with this chemical eco-friendly stabilizer as an effective reinforcement material.

Extensive studies have been carried out to assess the performance and suitability of SB as a liner material across various applications. The appeal of SB stems from its favorable engineering properties and cost-effectiveness. In SB mixtures, the pores between the sand grains contain particles of bentonite that coat the sand grains with thin clay layers [21, 22]. Several studies have documented the utilization of SB mixtures with different bentonite contents, specifically 10% and 15% by dry soil weight [23–26]. In addition, according to Stewart et al. [27], the use of 10% sodium-type bentonite is adequate to achieve the desired reduction in sand permeability.

Nevertheless, insufficient strength in SB landfill liners can give rise to various issues. Primarily, when subjected to rapid loading or increased stress, the liners may fail, resulting in the release of hazardous substances into the surrounding environment. Hence, ensuring the sufficient strength through stabilization techniques is essential to preserve the integrity and effectiveness of SB liner systems. This interdisciplinary approach aims to break new ground in sustainable soil stabilization methods, contributing valuable insights to the field. By pioneering the amalgamation of CN stabilization and repurposed FM, our research not only addresses existing challenges but also propels the broader understanding of effective strategies for soil reinforcement into unexplored territory.

This study proposes an environmentally friendly and innovative approach to repurpose FM by incorporating it into SB mixtures, which are commonly used in landfill liners. Unlike conventional polymers, FM offers a sustainable method of recycling waste materials. By leveraging the combination of FM and CN, the study explores a reinforcement strategy aimed at addressing the limitations of SB, such as insufficient strength, while enhancing hydromechanical performance. Comprehensive experimental tests, including UCS, free swell, 1D consolidation, and 3D volumetric shrinkage tests, were conducted to evaluate the effectiveness of FM as a reinforcement material. The study builds upon existing research highlighting the engineering potential of SB while introducing FM-CN-treated mixtures as a method to improve its performance. Advanced microstructural analyses, including X-ray fluorescence (XRF), X-ray diffraction (XRD), scanning electron microscopy (SEM), and Fourier-transform infrared spectroscopy (FTIR), were utilized to investigate the material’s geochemical compositions and microstructural changes. By bridging a gap in current research, this work aims to provide valuable insights into sustainable soil stabilization methods and contribute to advancing knowledge in geotechnical engineering field.

2. Materials and Methods

2.1. Materials

2.1.1. Soils

In this study, sea sand was gathered naturally from a shallow open trench located in Anping district, Tainan City, Taiwan [9], aligning with the need for a suitable landfill management system in the southern region of Taiwan [28]. The XRF analysis indicated that the sea sand is predominantly siliceous (SiO₂ > 83% and Al₂O₃ > 15%). The physical and geometrical parameters of the testing sand are proposed as follows: D₁₀ = 0.135 mm, D₃₀ = 0.183 mm, D₅₀ = 0.223 mm, D₆₀ = 0.235 mm, Cu = 1.742, Cc = 1.056, Gs = 2.78, e_min = 0.646, and e_max = 1.038, where D₁₀, D₃₀, D₅₀, and D₆₀ represent particle diameters at 10%, 30%, 50%, and 60% finer, respectively; Cu and Cc are the uniformity and curvature coefficients; G_s is the specific gravity; and e_min and e_max denote the minimum and maximum void ratios [9]. According to grade curve characteristics, host sand is poorly graded. In this study, two types of SB with varying bentonite contents were utilized, namely SB10 and SB15, representing 10% and 15% bentonite content by dry sand weight, respectively. The bentonite is commercially available in Taiwan. In this study, the XRF analysis was carried out to identify the SB’s chemical composition. Analysis shows that the bentonite mainly contained SiO₂ = 72.23%, Al₂O₃ = 13.96%, Na₂O = 2.59%, K₂O = 0.58%, Fe₂O₃ = 1.49%, CaO = 1.83%, MgO = 1.7%, TiO₂ = 0.16%, and loss on ignition (LOI) = 5.46%. Figure 1 presents the SEM images of sea sand and bentonite. The XRD analysis was utilized to ascertain the mineralogical composition of both the sea sand and bentonite. According to the findings depicted in Figure 2, sea sand primarily consists of quartz. Additionally, the bentonite comprises primarily of montmorillonite.

[figure(s) omitted; refer to PDF]

2.1.2. CN

The CN used in this study was supplied by Golden Innovation Business Co., Ltd, with ~20 wt% suspension in water (hydrosol), density of 1.3 g/mL (at 20°C), viscosity of <80 cP (1 cP = 1 mPa-s) (at 25°C), and an average particle size of <30 nm. The pH of the CN suspension is ~7, indicating a neutral environment. The surface of CN particles is rich in silanol (Si–OH) groups, which are reactive and can form hydrogen bonds or ionic interactions with soil particles, thereby improving the bonding between the soil matrix and the treatment. This surface chemistry plays a critical role in enhancing the soil’s mechanical strength and stability when treated with CN.

2.1.3. FM

The viability of utilizing disposal FM as a reinforcing material was investigated in this study. The FM investigated consists of three layers: the top layer is made of nonwoven fabric, the middle layer is composed of melt-blown nonwoven fabric, and the bottom layer is made of PP composite fiber. The shredded FM pieces had dimensions of 20 mm in length (l_fm), 5 mm in equivalent diameter (D_fm), and 0.7 mm in thickness (t_h), as illustrated in Figure 3. Prior to shredding, the earloops and metal strips were removed from the FM. The shredded FM components can be considered a specialized fiber [29]. Previous studies on reinforcement systems have shown that the fiber aspect ratio (AR_fm = l_fm/D₅₀) ranges from 10 to 100 to facilitate effective interaction between the soil and reinforcement [9, 30, 31]. In this study, the AR_fm of the shredded FM is 89.69, which falls between the two extreme limits. Another key parameter in fiber-reinforced systems is the specimen aspect ratio, which is the ratio of the specimen’s equivalent diameter to the fiber’s length, AR_S = D_m/l_fm, where the test sample diameter (D_m) is 50 mm. Conventionally, this ratio varies between 0.15 and 10.2, and in this study, it is 2.5. These characteristics suggest that the geometry of the shredded FM is suitable for soil reinforcement and improving the mechanical properties of the treated mixture. In terms of the testing program, shredded FM was added to the SB mixtures at two different percentages of dry soil weight (0.3% and 0.5%). It is worth noting that, due to health concerns, only new FM was used in this research. This approach may also provide an additional advantage by minimizing the potential release of microplastics or harmful chemicals from material degradation, thereby avoiding any influence from pre-existing contamination. Study performed by Lee et al. [32] reported that the production of new FM contributes ~0.580 kg CO₂-eq per unit to climate change and generates 0.004 kg of waste. In this study, taking an example for one UCS sample, the carbon footprint for incorporating FM was calculated as 0.000696 kg CO₂-eq for 0.3% FM and 0.00116 kg CO₂-eq for 0.5% FM. Similarly, the waste generated was estimated at 0.0000048 kg for 0.3% FM and 0.000008 kg for 0.5% FM. These environmental contributions are relatively small compared to the significant improvements in hydromechanical performance, durability, and reduced maintenance needs, highlighting a favorable trade-off between the environmental impacts and the benefits in geotechnical applications.

[figure(s) omitted; refer to PDF]

2.2. Methods

In this research, four distinct mixtures were defined, each composed of different combinations, as follows:

• SB.

• SB-FM.

• SB-CN.

• SB-FM-CN.

Table 1 presents the sample ID, compositions, and test lists. The influence of FM, CN, and FM-CN on the hydromechanical performances of both SB10 and SB15 was examined in this research.

Table 1

Experimental tests and associated samples.

Tests	Group	Sample ID	FM (%)	CN (%)	Curing period (days)
	SB10	S1	—	—	—
	—	S1F1	0.3	—	—
Compaction	—	S1F2	0.5	—	—
	SB15	S2	—	—	—
	—	S2F1	0.3	—	—
	—	S2F2	0.5	—	—
	SB10	S1	—	—	—
	—	S1F1	0.3	—	—
	—	S1F2	0.5	—	—
	—	S1C1	—	5	7, 28
	—	S1C2	—	15	7, 28
	—	S1F1C1	0.3	5	7, 28
	—	S1F1C2	0.3	15	7, 28
	—	S1F2C1	0.5	5	7, 28
	—	S1F2C2	0.5	15	7, 28
UCS	SB15	S2	—	—	—
	—	S2F1	5	—	—
	—	S2F2	15	—	—
	—	S2C1	—	0.3	7, 28
	—	S2C2	—	0.5	7, 28
	—	S2F1C1	0.3	5	7, 28
	—	S2F1C2	0.3	15	7, 28
	—	S2F2C1	0.5	5	7, 28
	—	S2F2C2	0.5	15	7, 28
Free swell; 1D consolidation; 3D volumetric shrinkage	SB10	S1	—	—	14
		S1F2	0.5	—	14
		S1C2	—	15	14
		S1F2C2	0.5	15	14
	SB15	S2	—	—	14
		S2F2	0.5	—	14
		S2C2	—	15	14
		S2F2C2	0.5	15	14

2.2.1. Sample Preparation

Sea sand and bentonite were dried in an oven at 50°C for a minimum of 48 h, after which they were mixed under various contents of FM and CN based on the dry soil weight. SB mixture was produced by two different contents of bentonite (10% and 15% by dry sand weight). Two FM contents (0.3% and 0.5% by dry SB weight) and two CN contents (5% and 15% by dry SB weight) were added to SB mixtures, respectively. Sodium chloride salt (NaCl) with 0.003 N ionic concentration was added to CN treated sample for fastening the gelation procedure [33, 34]. SB and additives underwent thorough mixing using a standard mixing apparatus. Each combination was adjusted with additional water as necessary to achieve the optimum moisture content (OMC). For each condition, at least two samples were prepared. The samples were then wrapped and subjected to room temperature (22–24°C) in an environmental chamber for varying curing periods (7, 14, and 28 days, respectively).

2.2.2. Compaction Test

The maximum dry unit weight (MDU) and OMC of the SB mixtures with varying contents of CN and FM were determined via standard Proctor compaction tests based on ASTM D698–07 [35]. The samples were compacted in three layers with varying water content using a mold measuring 116.44 mm in height and 101.66 mm in diameter.

2.2.3. UCS Test

According to ASTM D2166–00 [36], a series of UCS tests was performed on untreated and treated SB mixtures. The sample’s height and diameter were 100 and 50 mm, respectively. All tests were done under the strain control condition with a constant speed of 1 mm/min. The study utilized a standard UCS test loading frame (HM-5030, Humboldt Mfg. Co.) equipped with a load cell (HM-2300.020) providing an accuracy of ±0.001 kN, along with a linear strain conversion transducer (LSCT) (HM-2310.20) offering an accuracy of ±0.001 mm. The characteristics of the samples were discerned through the analysis of stress–strain curves. The repeat assessment was performed prior to testing the mixtures. The results as depicted in Figure 4 indicate a high level of comparability in which the difference was 2.11% for SB10 reinforced with 0.5% FM, thus providing evidence for the adequacy and reproducibility of the test data. For all tests conducted, each data point was obtained using at least two distinct samples that were subjected to similar conditions.

[figure(s) omitted; refer to PDF]

2.2.4. Free Swell Test

ASTM D4546–08 [37] was employed to conduct the free swell tests on untreated and treated SB mixtures. Samples were compacted at their OMC and MDU in rings measuring 63.5 mm in diameter and 14 mm in height, after which they were placed in an oedometer apparatus. In the following, a 1 kPa load was fixed in the oedometer apparatus and the cell was filled with water. Samples underwent swelling, with vertical displacement being subsequently measured by an LSCT.

2.2.5. 1D Consolidation Test

1D consolidation test was done according to ASTM D2435–03 [38]. Samples, compacted at their target OMC and MDU in rings with 63.5 mm diameter and 14 mm height, were then saturated with distilled water under 7 kPa loading in an oedometer apparatus to facilitate swelling. Once full saturation was achieved, loading and unloading steps were initiated, ranging from 30.97 to 991.04 kPa. The compression changes were recorded by sample vertical deformation which was measured by an LSCT.

2.2.6. 3D Volumetric Shrinkage Test

Samples were prepared by compacting the mixtures in their OMC and MDU in oedometer rings with 63.5 mm diameter and 14 mm height. Then, samples were left to saturate with distilled water. Once saturated, oedometer ring was initially extracted. Average heights, average diameters, and weights of untreated and treated SB mixtures were periodically monitored at a constant room temperature (22–24°C) and recorded until no changes were detected. The samples’ moisture content was assessed through complete drying in an oven set to 110°C.

2.2.7. Microstructural Analysis

The microstructure of untreated and treated samples underwent analysis using FTIR and SEM tests. FTIR tests were performed by Nicolet 6700 FTIR spectrometer (Thermo Scientific), and SEM tests were done by a Hitachi SU8010 device. Following curing and compaction tests to simulate site conditions, 1-month air-dried treated samples underwent microstructural analysis. This step was taken to ensure the complete development of any chemical reactions that may affect the strength and performance of SB.

3. Results and Discussion

3.1. Compaction

The OMC and MDU of SB unreinforced and reinforced with FM are shown in Figure 5. It is noteworthy that SB treated with CN exhibited no noticeable changes in MDU and OMC. As a result, only the results for SB reinforced with FM are presented. The MDU of untreated SB sample increased with higher bentonite content, while the OMC decreased. This increase in MDU can be attributed to the clay particles acting as lubricants between the sand particles [39]. Moreover, the decrease of OMC with increased bentonite content is likely attributed to the clay agglomeration within the macropore phase. This underscores the significance of varying clay content in readjusting the relative proportion of macro- and micro-pores. In reinforced samples with FM addition, MDU was decreased, and OMC showed reverse behavior. The MDU was decreased due to the exchange of the soil particle with FM particles, leading to changes in particle packing within the mixture. Additionally, the increase in OMC results from FM particles absorbing water, diminishing the lubrication between soil particles, and consequently reducing the compressibility.

[figure(s) omitted; refer to PDF]

3.2. UCS Test

Figure 6 and Table 2 present the UCS for untreated SB10 and SB15, as well as SB10 and SB15 treated with FM, CN, and FM-CN. The results show that SB15 exhibits a 39% increase in strength compared to SB10, indicating that the addition of bentonite content contributes to the improvement in UCS. Furthermore, SB15 demonstrates a slightly more brittle behavior in comparison to SB10. Note that, both untreated SB samples depicted in Figure 7 show insufficient performance that falls below the required standard. The subsequent section elucidates the impact of FM, CN, and FM-CN on UCS behavior.

[figure(s) omitted; refer to PDF]

Table 2

Results of UCS test for SB with different treatments.

Sample ID	$\epsilon$fa	qfb (kPa)	Ec (kPa)
S1	0.036	118.71	3323.34
S1F1	0.038	153.80	4041.18
S1F2	0.053	175.71	3346.8
S1C17	0.036	154.83	4331.22
S1C27	0.035	176.22	5063.96
S1C128	0.034	181.82	5393.78
S1C228	0.028	203.72	7158.03
S1F1C17	0.014	185.71	13,032.58
S1F1C27	0.016	203.57	12,835.53
S1F2C17	0.021	259.23	12,248.52
S1F2C27	0.017	260.25	15,571.15
S1F1C128	0.013	209.69	16,616
S1F1C228	0.015	220.92	14,582.07
S1F2C128	0.017	271.96	15,853.4
S1F2C228	0.015	317.29	21,633.5
S2	0.028	165.52	5797.59
S2F1	0.025	230.20	9117.32
S2F2	0.032	235.29	7297.29
S2C17	0.012	330.02	28,459.53
S2C27	0.012	349.38	30,349.63
S2C128	0.011	367.20	33,141.56
S2C228	0.009	389.61	40,759.92
S2F1C17	0.015	307.61	20,960.31
S2F1C27	0.012	390.12	31,499.5
S2F2C17	0.017	358.54	20,888.93
S2F2C27	0.016	403.36	25,449.1
S2F1C128	0.016	355.49	22,508.63
S2F1C228	0.014	394.19	27,839.37
S2F2C128	0.019	411.00	22,017.95
S2F2C228	0.018	486.89	27,537.68

^aStrain at failure (peak of UCS).

^bPeak of UCS.

^cSecant elastic modulus.

3.2.1. FM Reinforcement

Figure 6a,e presents the UCS curves of both SB reinforced with FM. In SB10, the UCS demonstrated an increase of 30% and 48% when reinforced with 0.3% and 0.5% FM, respectively. The increase in UCS for SB15 was 39% and 42% for the corresponding FM contents used. In general, the results suggest that the inclusion of FM enhances the strength of the samples. However, the dilative behavior due to FM inclusion is more noticeable in SB10. Similarly, the study performed by Kang, Cao, and Bate [40] investigated the effects of polymers, such as naturally occurring biopolymers (i.e., chitosan, xanthan gum) and synthetic polymer (i.e., polyethylene oxide, PEO), on the strength and dilative behavior of polymer-modified kaolinite (KA) and fly ash kaolinite (FAKA) mixtures. Their findings demonstrated strength improvement and dilative behavior induced by polymer inclusion, comparable to the enhancements observed in the present study, where FM incorporation similarly improved strength and promoted noticeable dilative behavior, particularly in SB10. It is worth mentioning that the MDU of the specimens decreases after the FM inclusion, suggesting the production of lightweight material, despite the concurrent increase in strength. Furthermore, as depicted in Figure 7, only FM-SB15 samples marginally meet the minimum strength for landfill liners [41, 42].

3.2.2. CN Treatment

According to Figure 6b,f, UCS of both SB samples increased with CN treatment. Figure 6b shows that UCS of SB10 treated with 5% and 15% CN increased by 30% and 49% in 7 days and 53% and 72% in 28 days in comparison with untreated SB10, respectively. In Figure 6f, the UCS of SB15 treated with 5% and 15% CN enhanced by 99% and 111% in 7 days and 122% and 135% in 28 days compared to untreated SB15, respectively. Therefore, curing time and bentonite content had a positive relationship with the UCS of samples. Also, the results indicate that the increased strength following the CN treatment can be attributed to the pore-filling effect of the CN particles and viscous gels within the soil. The presence of these viscous gels enhances the friction between SB particles and reduces the spacing between soil particles, leading to an increase in interfacial bond strength. Moreover, it can be inferred that CN treatment induces physical alterations such as improved packing, likely due to the clustering effect of CN particles. Additionally, the observed behavior of CN-treated SB samples shifted from dilative to brittle, which is likely due to enhanced interparticle bonding, stress concentrations from viscous gel formation, and altered moisture retention properties caused by the microscopic interactions between CN and the soil matrix. As depicted in Figure 7, it was found that only the CN-treated SB15 samples met the acceptable value for use as landfill liners [41, 42], whereas CN-treated SB10 samples remained below the minimum requirement.

3.2.3. The Combined Effects of FM-CN Treatment

Figure 6c–d,g–f presents the results of UCS for SB10 and SB15 treated with FM-CN. In general, the samples exhibit a similar trend, whereby the UCS increases with higher FM and CN content in 7 and 28 days. Specifically, in contrast to untreated SB10, the UCS of 0.3% FM-SB10 treated with 5% and 15% CN increased by 56% and 71% in 7 days and 77% and 86% in 28 days, respectively. With an increase in the FM content, the UCS of the 0.5% FM-SB10 treated with 5% and 15% CN samples increased by 118% and 119% in 7 days and 129% and 167% in 28 days compared to untreated SB10, respectively.

Further, for FM-CN-treated SB15 samples, the UCS of 0.3% FM-SB15 treated with 5% and 15% CN samples increased by 86% and 136% in 7 days and 115% and 138% in 28 days, respectively, compared to untreated SB15. Similarly, after the increase in the FM content from 0.3% to 0.5%, the UCS of samples after 0.5% FM-SB15 treated with 5% and 15% CN increased by 117% and 144% in 7 days and 148% and 194% in 28 days, respectively. Furthermore, as depicted in Figure 7, all samples of SB15 that underwent treatment with FM-CN can meet the commonly accepted minimum strength for soil liners. According to Daniel and Wu [41] and USEPA [42] guideline, the recommended minimum UCS for soil liners is 200 kPa, which serves as a benchmark for adequate performance in waste containment systems. The findings indicate that the treated SB15 exceeded this threshold, thereby affirming its suitability as a landfill liner material. Similarly, the study performed by Buragohain et al. [43] explored the use of FA–bentonite mixtures for landfill liners and they reported that a FA content of 70% or lower can meet the standard strength requirement of 200 kPa for soil liners. Hence, this comparison further solidifies the relevance and strength of the present study in advancing the application of innovative materials. The UCS results for the treated SB mixtures not only surpass the minimum strength criteria but also align with existing literature, thereby supporting the potential of FM-CN-treated SB as a viable material for landfill liner applications.

Figure 8 depicts the illustration of microscale mechanism underlying the performance enhancement of the SB mixture treated with FM and CN. As outlined in the research reported by Liu and Hung [44], incorporating a combination of FM and CN into SB mixture led to a noteworthy enhancement in UCS. In addition, the combined effects of FM and CN result from complementary but distinct mechanisms at the microstructural level. Specifically, FM fibers act as a reinforcing network within the SB matrix, bridging gaps between soil particles and enhancing particle-to-particle interlocking. This bridging effect creates a three-dimensional framework that increases shear resistance and contributes to the load transfer across the matrix, leading to an increase in strength. CN, on the other hand, operates as a pore-filling agent, reducing the porosity of the SB matrix by occupying voids between particles. At the microscale, CN also facilitates the formation of viscous gels by binding with fine bentonite particles and adhering to FM fibers. These gels create a denser and more bonded structure, promoting particle aggregation and reducing water permeability. The interaction between FM and CN is particularly significant at the interface. CN gels coat FM fibers, enhancing the adhesion between the fibers and the surrounding soil particles. This adhesion strengthens the composite network, improving both the mechanical interlocking and the matrix’s overall hydromechanical performance.

[figure(s) omitted; refer to PDF]

The secant elastic modulus (E) of the SB samples is listed in Table 2. Results show that the FM-CN treatment led to an increase in E. The E values were observed to further improve with higher CN content and longer curing time. Figures 9 and 10 compare the results of minimum and maximum treatments on SB10 and SB15, respectively. In particular, the minimum treatment includes samples treated with 0.3% FM, 5% CN, and 0.3% FM-5% CN in 7 days, whereas the maximum treatment includes samples treated with 0.5% FM, 15% CN, and 0.5% FM-15% CN in 28 days for both SB samples. As seen, the improvement is more pronounced in the SB15 samples.

[figure(s) omitted; refer to PDF]

3.3. Free Swell Test

Figure 11 shows the swelling strains of untreated and FM-CN-treated SB samples. The free swell curve is categorized into three distinct phases, namely a small initial swell, a large primary swell, and a small secondary swell [45]. The initial swell pertains at the macrostructural level, whereas at the microstructural level, the primary and secondary swells pertain. Based on the results, the final swelling strains of untreated SB10 and SB15 were 12% and 29%, respectively. The higher content of montmorillonite in SB15 was the reason for the higher swelling strain.

[figure(s) omitted; refer to PDF]

The final swelling strains of 0.5% FM-SB10 and 0.5% FM-SB15 increased by 11% and 8%, respectively. This increment happened due to FM’s high water absorption capacity and void ratio which made the condition have a larger swelling strain. In addition, the final swelling strains of 15% CN-treated SB10 and SB15 decreased by 51% and 13%, respectively. It seems that after the CN treatment, the water absorption capability of bentonite decreased. This phenomenon happens due to the filling effect and the viscous gels of CN, which increase the friction between the SB particles. Later with 0.5% FM-15% CN treatment, the final swelling strains of treated SB10 and SB15 decreased by 45% and 11%, respectively. The impact of CN treatment appears to be more pronounced in SB10 compared to SB15. This could be attributed to the presence of larger pore spaces between the particles in SB10, which allows for greater infiltration of CN particles and the formation of viscous gels.

3.4. 1D Consolidation Test

3.4.1. FM and CN Effects on Compression and Recompression Indices

The effects of FM, CN, and FM-CN on the compressibility of SB samples were examined through 1D consolidation tests. Figure 12 represents the compression curves of untreated and treated SB samples under 14 days of curing. Also, compression (C_c) and recompression (C_r) indices are listed in Table 3. C_c and C_r, in this study, were determined by selecting specific loading points with a linear gradient, specifically the terminal points observed at high effective stress levels. This intentional restriction ensured the accuracy of the computation of these indices.

[figure(s) omitted; refer to PDF]

Table 3

Results of the compression and recompression indices.

Sample ID	Cc	Cr
S1	0.076	0.015
S1F2	0.099	0.021
S1C2	0.067	0.013
S1F2C2	0.085	0.014
S2	0.109	0.017
S2F2	0.127	0.019
S2C2	0.102	0.015
S2F2C2	0.114	0.017

The results in Table 3 show that FM reinforcement to the SB increased both C_c and C_r and showed similar behavior to fiber-based material. Specifically, C_c of 0.5% FM-SB10 increased by 30%, whereas 0.5% FM-SB15 increased by 17%. This phenomenon occurred due to the higher compressibility of FM filaments. Qu and Sun [46] and Gumanta et al. [47] reported similar behavior in the compressibility of fiber-reinforced soils. The increase in compressibility observed due to FM appeared to detrimentally affect the soil performance. However, it is worth emphasizing that this effect can be regarded as insignificant, given that the increment falls within reasonably acceptable ranges and diminishes with the application of CN treatment. This is evident in the FM-CN-treated SB samples, where the C_c decreased by 14% and 10% in 0.5% FM-15% CN-treated SB10 and SB15, respectively, in comparison to the 0.5% FM-SB10 and 0.5% FM-SB15 samples. In addition, CN addition to the SB reduced both C_c and C_r values. Upon comparison of CN-treated SB to untreated SB samples, a reduction of 12% and 6% in the Cc was observed for both 15% CN-treated SB10 and SB15, respectively. This phenomenon occurred due to the filling effect and formation of viscous gels of CN, which were contributed to an increase in friction between the SB particles. Changizi and Haddad [48] and Ghadr et al. [49] proposed the same result when they treated soils with nanosilica-based material.

3.4.2. Saturated Hydraulic Conductivity

The adequacy of soil as landfill liners relies significantly on its saturated hydraulic conductivity (k) [50]. In this research, the determination of k was approached indirectly through the analysis of consolidation test results, as expressed in Equation (1) [51, 52]:$$\begin{array}{}\text{(1)}& k=\frac{C_v\times a_v\times {\gamma }_w}{1+e_0},\end{array}$$where the coefficient of consolidation ($C_v$) is derived through Taylor’s square root time method for each loading state (Equation 2), while the compression coefficient ($a_v$) within a specific loading stress range is defined by Equation (3). Other variables include the unit weight of water (${\gamma }_w$) and the initial void ratio (${\mathrm{e}}_0$):$$\begin{array}{}\text{(2)}& C_v=\frac{{\mathrm{H}}_{dr}^2{T}_v}{t_{90}},\end{array}$$where the variable $t_{90}$ represents the time required for a 90% degree of saturation, $T_v$ denotes the time factor (0.848 for 90% degree of consolidation), and ${\mathrm{H}}_{dr}$ is the length of the maximum drainage path.$$\begin{array}{}\text{(3)}& a_v=\frac{e_i-e_{i+1}}{{\sigma }_{\mathrm{i}+1}^{{}^{\prime }}-{\sigma }_{\mathrm{i}}^{{}^{\prime }}},\end{array}$$where the void ratios $e_i$ and $e_{i+1}$ correspond to the effective stresses ${\sigma }_i^{\prime }$ and ${\sigma }_{i+1}^{\prime }$ for a specific level and the subsequent level, respectively.

As outlined in the DEP MA Landfill Technical Guidance Manual, landfill liners are recommended to have k lower than 1 × 10⁻⁷ cm/s [53]. Figure 13 presents the correlation between k and effective stress for SB subjected to various treatments. The findings suggest that the k of SBs declined as the bentonite content increased, which can be attributed to the way bentonite fills the gaps within sand particles. It swells and effectively occupies the voids when bentonite undergoes complete hydration, causing an overall decrease in k. Additionally, the anticipated pattern of decreasing k in the case of SBs with increasing the effective stress is evident. This decrease in k aligns with the results of consolidation tests (Figure 12), where an increase in effective stress corresponds to a decrease in void ratio. The reduction in available void space impedes the flow of free water, resulting in a lowered k.

[figure(s) omitted; refer to PDF]

Moreover, the inclusion of FM has led to an increase in k for both types of SB. This can be attributed due to the adoption of a laminar structure by FM, which could create preferential flow paths for water, resulting in an increase in k. A similar trend was also observed in the study performed by Li, He, and Kang [54] where they investigated the impact of polymer inclusion on k of FA–clay mixtures. The addition of different polymers (chitosan, PEO, and xanthan gum) influenced k value, with PEO and xanthan gum increasing k in certain conditions. The increase in k can be attributed to changes in material structure or flow paths—where the polymers affected the soil particle interaction. In the context of CN, a decrease in the parameter k may occur because the viscous gels bind soil particles together through microprocesses, generating smaller voids within the samples. This, in turn, influences the overall porosity and permeability of the SB mixture, leading to lower values of k. Additionally, study performed by Taha and Taha [55] also exhibited a similar trend where the decrease in k was found in SB samples treated with nanomaterial compared to untreated SB, with a more notable decrease observed as the nanomaterial content increased. Additionally, a smaller k was found in SB samples with higher bentonite content.

Nevertheless, it is essential to underscore that despite the observed increase in k in the FM-CN-treated SB, all mixtures remain compliant with the criteria for landfill liner applications [53], particularly when the effective stress exceeds around 500 kPa. This suggests that the liners maintain their structural integrity and performance, even with the increased k values. While the combination of FM and CN affects k, it does not undermine the overall suitability of the SB mixtures as effective materials for landfill liners.

3.5. 3D Volumetric Shrinkage Test

The volumetric shrinkage strains of untreated and FM-CN-treated SB10 and SB15 are depicted in Figure 14. According to the results, volumetric shrinkage strains of SB increased with bentonite (clay) content. Clay contents can absorb more water than sand particles then higher desiccation will occur in SB15. In addition, the results show that samples reinforcement with FM increased volumetric shrinkage strain. This phenomenon can be directly attributed to the fact that FM possesses a higher water-retaining capacity. Conversely, all samples treated with CN additives have lower volumetric shrinkage strain. It seems that CN particles and their viscous gels reduced the SB water absorption by blocking the sample’s voids.

[figure(s) omitted; refer to PDF]

Figure 15 shows the dry path of the untreated and FM-CN-treated SB10 and SB15 during the procedure of volume change. As observed, water content progressively reduced along the drying/desiccation path, causing the shrinkage curve to intersect the contour lines representing saturation levels ranging from 100% to 20%. The Soil Vision software was used to plot the hyperbolic shrinkage curves [56], according to the model proposed by Fredlund [57] and Fredlund, Wilson, and Fredlund [58]. Curve fitting was performed using the following equation:

[figure(s) omitted; refer to PDF]

$$\begin{array}{}\text{(4)}& e_w=a_{sh}{\frac{w^{c_{sh}}}{{b_{sh}}^{c_{sh}}}+1}^{\frac{1}{c_{sh}}},\end{array}$$where $e_w$ is total void ratio, a_sh is the minimum void ratio, w is the gravimetric water content, b_sh is slope of the tangent line, and c_sh is the curvature of the curve.

Figure 16 and Table 4 present the minimum void ratio (a_sh) and shrinkage limit (SL) of both untreated and FM-CN-treated SB samples. As shown, the a_sh and SL in SB15 increased by 28% and 23%, respectively, compared to SB10. The reason for that is the addition of bentonite produces particle aggregation (higher dry density detected in compaction test) and an increase in the water retention capacity of the mixture due to bentonite’s high water absorption behavior. Also, FM addition to untreated and CN-treated SB samples increased both a_sh and SL. The main reason is that FM adopts a laminar structure and clay particles surrounding them which lets them absorb more water due to a high void ratio (lower dry density) which can contain more water inside them. Therefore, higher volume change (3D shrink) will happen in FM-reinforced samples. Conversely, it was found that a_sh and SL of the samples decreased with CN treatment. In particular, the a_sh and SL decreased by 10% and 5% in 15% CN-treated SB10, and 5% and 4% in 15% CN-treated SB15, respectively. These findings hint at the possibility that the treatment with CN may have contributed to a decline in the water absorption rate of the SB samples, subsequently leading to lower a_sh and SL.

[figure(s) omitted; refer to PDF]

Table 4

Results of the minimum void ratio and shrinkage limit of SB with different treatments.

Sample ID	$a_{sh}$ (minimum void ratio)	Shrinkage limit (SL)
S1	0.483	0.2029
S1F2	0.6425	0.2712
S1C2	0.436	0.1912
S1F2C2	0.4608	0.2039
S2	0.6159	0.2504
S2F2	0.7414	0.3026
S2C2	0.5827	0.2437
S2F2C2	0.6119	0.2592

3.6. Microstructural Analysis

Figure 17 displays the FTIR patterns of both untreated and CN-treated SB15 samples, revealing the presence of silicon-carbon (Si─C) bonds at wavelengths of 694 and 796 cm⁻¹, as well as silicon dioxide (SiO₂) peaks at 1035 cm⁻¹, consistent with findings from previous studies [59, 60]. The CN-treated SB15 sample showed significantly higher levels of Si─C and SiO₂ compared to the untreated SB15, supporting the UCS results shown in Figure 6 and suggesting that the incorporation of CN enhances the material properties of the SB mixtures. The increased Si─C and SiO₂ in the CN-treated samples indicates that CN promotes the formation of viscous gels within the soil matrix, which play a vital role in binding soil particles together. This binding action contributes to the observed improvements in compressive strength and shrinkage properties of the SB, enhancing the structural integrity and overall performance. Additionally, the FTIR spectra indicate signs of hydration, suggesting that the extra cementitious gels formed during the process consumed pore water for hydration, thus further supporting the beneficial effects of CN in geotechnical applications.

[figure(s) omitted; refer to PDF]

Figure 18 presents SEM images that provide critical insights into the structural and packing arrangement of SB15 particles, allowing for the detection of both macroscopic and microscopic changes induced by stabilization treatments. Figure 18a displays the SEM image of untreated SB15, showing loosely packed particles with no visible cementing agents. In contrast, Figure 18b,c exhibits SEM images of CN-treated SB15, revealing the distinct presence of silica crystals characterized by a bright, fuzzy surface texture. These silica crystals appear to form on particle surfaces and at the contact points between SB15 platelets, promoting structural cohesion. The CN treatment is observed to coat the flocculated clusters of bentonite particles, reinforcing their structure and effectively filling voids within the matrix. This void-filling action contributes to the formation of a denser matrix with a reduced void ratio, which is strongly correlated with the observed improvements in compressive strength.

[figure(s) omitted; refer to PDF]

Figure 18d depicts SEM evidence of FM-CN-treated SB15, highlighting the formation of a robust bridging structure facilitated by FM fibers. These fibers act as reinforcements within the matrix, creating interparticle connections and enhancing the stability of the SB mixture. Although the SEM images suggest limited direct bonding between FM fibers and adjacent soil particles, the fibers establish a physical framework that restricts particle mobility under applied loads. This framework resists deformation, improving the internal stability and mechanical strength of the SB mixture. The observed microstructural changes align with the UCS results (Figure 6), where samples treated with FM showed a significant increase in strength. The bridging mechanism formed by FM fibers, combined with the cohesive and densifying effects of CN treatment, effectively binds particles together, enhancing both the strength and compressibility characteristics of the stabilized samples.

4. Concluding Remarks

The hydromechanical performance of SB was investigated in this research through a set of experimental tests that examined the effects of FM, CN, and FM-CN combinations. Additionally, microstructural analysis was conducted to examine the composition of geochemical and microstructure of both the untreated and treated SB.

The study’s experimental results lead to the following conclusions:

1. In general, the incorporation of FM was noted to enhance the strength and dilative behavior of both SB samples. Additionally, samples treated with FM-CN displayed a more substantial increase in strength with higher CN content. Furthermore, the addition of CN induces the samples to exhibit brittle behavior.

2. FM-CN-treated SB with higher bentonite content exhibited UCS that surpassed the typically accepted minimum strength of 200 kPa for landfill liners.

3. The final swelling strain of both SB decreased after CN treatment due to its filling effect and viscous gels, while it increased after FM treatment due to its water absorption capacity and void ratio.

4. FM-CN-treated SB samples generally adhered to the minimum standards for hydraulic conductivity (k < 10⁻⁷ cm/s) under relatively higher effective stresses.

5. The SB sample with higher bentonite content exhibited an increase in a_sh and SL due to the particle aggregation and increased water retention capacity resulting from the increment of bentonite content. Likewise, incorporating FM resulted in higher a_sh and SL, whereas conversely, the CN treatment resulted in a decrease for both SB samples.

6. Microstructural analysis revealed that CN particles were observed to fill the voids, forming a dense matrix, while also increasing surface roughness and viscous gels through precipitation in floc-like structures on SB. This improved interlocking among soil particles, leading to enhanced compressive strength of samples. Additionally, the presence of FM fibers generated robust bridges, thereby enhancing the samples’ strength.

This study revealed the effectiveness of adding CN, FM, and CN-FM to enhance SB’s hydromechanical performance. Despite some adverse effects when adding FM to SB, the repurposed FM can reduce waste production, improves SB strength, exhibits dilative behavior, and meets the minimum requirements for landfill liners. Moreover, combining with CN has the potential to mitigate the adverse effects associated with the introduction of FM, contributing to more efficient and environmentally friendly SB treatment practices.

Author Contributions

Ching Hung: conceptualization, investigation, supervision, visualization, writing–original draft, writing–review and editing, funding acquisition. Febi Satria Gumanta: data curation, methodology, visualization, writing–original draft, writing–review and editing. Chih-Hsuan Liu: investigation, methodology, writing–original draft, writing–review and editing. Bo-Ren Lin: data curation, methodology. Soheil Ghadr: conceptualization, data curation, investigation, writing–original draft.

Acknowledgments

The authors appreciate the support from the National Science and Technology Council (NSTC), Taiwan. The research was, in part, supported by the Higher Education Sprout Project, Ministry of Education, Taiwan, Headquarters of University Advancement to the National Cheng Kung University (NCKU). The authors would like to extend their gratitude to Prof. Sheng-Sheng Yu from the Department of Chemical Engineering, NCKU, for their friendly support in microstructural analysis.

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