Geosynthetic clay liners (GCLs) are mostly used as flow barriers in landfills and waste containments due to their low hydraulic conductivity to prevent the leachate from reaching the environment. The self-healing and swell-shrink properties of soft clays (expansive soils) such as bentonite enable them as promising materials for the GCL core layers. However, it is important to modify their physico-chemical properties in order to overcome the functional limitations of GCL under different hydraulic conditions. In the present study, locally available black cotton soil (BCS) is introduced in the presence of an anionic polymer named carboxymethyl cellulose (CMC) as an alternative to bentonite to enhance the hydraulic properties of GCL under different compositions. The modified GCL is prepared by stitching the liner with an optimum percentage of CMC along with various percentages of BCS mixed with bentonite. Hydraulic conductivity tests were performed on the modified GCL using the flexi-wall permeameter. The results suggest that the lowest hydraulic conductivity of 4.58 × 10−10 m/s is obtained when 25% of BCS is blended with bentonite and an optimum 8% CMC and further addition of BCS results in the reduction of the hydraulic conductivity.

  • Modification of bentonite with locally available black cotton soil and caboxymethyl cellulose polymer.

  • Enhancement of hydraulic performance of modified geosynthetic clay liner.

  • Sodium content is low in the modified bentonite beyond the optimum percentage of black cotton soil blended with carboxymethyl cellulose polymer.

The expansive soils are widely present in all continents in different forms of soft clay minerals, such as smectite, bentonite, kaolinite, montmorillonite, beidellite, vermiculite, attapulgite, nontronite, and chlorite. In India, the black cotton soil (BCS) or vertisol (a type of montmorillonite) is available in plenty which exhibits high swell-shrink properties when exposed to moisture conditions (Oza & Gundaliya 2013; Ajit et al. 2022; Woldesenbet 2023). The versatility of expansive soils poses critical challenges to the infrastructural development activities happening on the ground and underground soils. Due to their inability to withstand the changes in environmental conditions such as an external pressure or infiltrating moisture, they tend to exhibit significant changes in their volume (swelling and shrinking) thereby causing the supporting infrastructure to lose strength and ultimately fail. Apart from the structural stability concerns, one key aspect of concern limiting the utility of these soft soils in the engineering context is in the form of geosynthetic linings which are provided for limiting the leachate from a waste containment facility constructed on the ground or underground locations (Scalia & Benson 2010). The failures of the liner system owing to the panel separation due to shrinkage, punctures, wrinkles, installation errors, liner design errors, etc. cause critical challenges both to the manufacturers as well as the operators (Rowe 2014; Rowe et al. 2017). Due to such conspicuous nature of the very physical structure, they require special attention to improve their physical and chemical behaviour when subjected to critical environmental conditions. Interestingly, this is not a new problem and the geotechnical engineers have attempted to address it only superficially, or at least tactfully for the sake of prevailing load and stability conditions.

Recent developments in this field have contributed a variety of innovative clay liners, geomembranes, and geosynthetic composites which are commercially available in various forms and textures (Garcia et al. 2007; Cheah et al. 2016; Jayalath et al. 2018; Gallage & Jayalath 2019; Weerasinghe et al. 2019, 2020). The composite geosynthetics are commonly available as geosynthetic clay liners (GCL), which consist of a layer of geotextiles or adhesive attached to a geomembrane and a core of low-permeability sodium bentonite clay with a woven and non-woven structure. The GCLs made of soft clays are found to offer resistance against the seeping moisture by virtue of internal expansion within the pore spaces resulting in a reduction in permeability (Pitanga et al. 2011; Viswanadham et al. 2012; Al-Taie & Pusch 2014; Rajesh & Viswanadham 2015). The bentonite clay used for the purpose of landfill lining has to have critical hydraulic properties to arrest the seepage within the landfill, in addition to their swell shrink and thermal features. Compared to calcium bentonite clay, sodium bentonite needs only minimum thickness (5–10 mm) and is less permeable (∼10−11 m/s) (Acikel et al. 2018). The geomembranes made of high-density polyethylene or reinforced polypropylene are found to exhibit much lesser permeability (∼10−15 m/s) compared to the clay liners and are commonly available as flexible sheets for direct field implementation (Weerasinghe et al. 2021).

In addition to the structural support for the confining layers, the GCLs help control seepage and release of potentially harmful materials from the tailing ponds and mining containment areas into the environment (Xiong et al. 2009; Egloffstein et al. 2012; Mazzieri & Di Emidio 2015). However, the leachate characteristics from the landfills can significantly affect the hydraulic conductivity of various soils depending on their structure and qualities (Han et al. 2009; Xu et al. 2011; Ozcoban et al. 2013; Aldaeef & Rayhani 2014; Bradshaw & Benson 2014). The leachate can also impact the physico-chemical structure of the clay minerals present in the GCL causing changes in the hydraulic conductivity (Ozcoban et al. 2022). According to Han et al. (2009), the hydraulic conductivity of GCL for distilled water decreased from 3.5 × 10−7 to 5.35 × 10−9 cm/s. Ozcoban et al. (2013) reported that the hydraulic conductivity of the compressed clay dropped from 5.2 × 10−8 to 6.45 × 10−8 m/s when leachate water was used compared to distilled water. In a similar study, Ozcoban & Acarer (2022) supplemented three different nanomaterials, namely, iron oxide, aluminium oxide, and Oltu clay to kaolin at two different rates (1 and 5%) to study the permeability of the clay. The removal rates of nanomaterial-added kaolin samples were found to be higher than that of kaolin without nanomaterials.

It is understood that the hydration and self-healing characteristics of the GCL mainly depend on the initial placement condition and the ability to absorb moisture from the adjacent soil (Acikel et al. 2018). During the operations, the GCLs may or may not be sufficiently saturated to achieve the designed hydraulic conductivity of the cap barrier depending on the temperature and moisture conditions of the adjacent soil medium. This indicates that the water retention behaviour and unsaturated hydraulic conductivity of GCLs may need to be established for the proper understanding of hydraulic flow through GCLs (Rowe 2014; Rajesh & Khan 2018; Ghavam-Nasiri et al. 2019). In other words, it is imperative to ascertain whether the hydraulic performance of GCL can be enhanced by maintaining its optimum capacity throughout the barrier application (Daniel et al. 1997; Rowe et al. 1997; Xiong et al. 2009; Egloffstein et al. 2012; Mazzieri & Di Emidio 2015). The major factors affecting the hydraulic conductivity of the GCL are found to be overburden confining stress, shrinkage effect from temperature variation, chemical compatibility of bentonite to limit different permeants, wrinkle effect from the overlying geomembrane, and side slope shear effects (Fox et al. 1998; Weerasinghe et al. 2019). As evident from the recent research on the design of geosynthetics, the hydraulic performance of GCL has increased from about 1 × 10−7 to 1 × 10−12 m/s by enhancing the structural and textural characteristics of the core clay layer (Petrov et al. 1997; Kendall & Buckley 2014; Weerasinghe et al. 2020). As stated before, even a marginal reduction in the hydraulic conductivity is considered an improvement in the hydraulic performance of the liner as it reduces the opportunity for dissolved chemicals to pass through the hydraulic barrier.

Based on the literature, it is inferred that the soft soils can be effectively modified to improve their engineering properties so as to make them less-problematic, and, in a larger sense, more-adaptive for the challenging geotechnical applications (Benson et al. 2005). The ionic nature of the hydrating (permeating) fluids is found to be influential in achieving the self-healing capacity and reduction in hydraulic conductivity in order to ensure the service life of the liners (Bouazza et al. 2006). For this reason, chemical stabilization has been practiced as an effective strategy for improving the engineering properties of expansive soils for the application of soil stabilization. However, the conventional approaches in employing cement and lime are becoming more objectionable based on environmental concerns. The use of geopolymers is found to be promising in this aspect as they utilize alkali solutions for making cementitious binders and they encourage the addition of industrial wastes rich in aluminosilicate compounds. Chemical polymers such as propylene carbonate (PC), sodium carbonate (SC), carboxymethyl cellulose (CMC), benzyl tri-ethyl-ammonium bromide (BTEAB), and hexa-decyl-tri-methyl-ammonium chloride (HDTMAC) are preferred as they can target on improving the performance of the bentonite core without affecting adjacent layers (Malusis & Di Emidio 2014; Scalia et al. 2014; Fehervari et al. 2016; Mazzieri et al. 2017). It is observed that the absorption with cationic polymers is very quick but irreversible and the hydraulic conductivity of the clay liner could not significantly improve as a result (Ashmawy et al. 2002; McRory & Ashmawy 2005). The PC-treated bentonite possesses strong chemical resistance but is found to be less effective with insufficient swelling when the permeating fluid is an active electrolyte leading to substantially increased hydraulic conductivity (Onikata et al. 1996; Mazzieri & Pasqualini 2008).

Notably, bentonite has demonstrated superior hydraulic performance as GCL material in a wide range of extreme solutions when treated with an anionic polymer due to the presence of carboxylic acids (COOH) or sulphonic acids (SO3H) (De Camillis et al. 2017). The modification of the bentonite with sodium-based CMC revealed that the wet-dry cycle has a major impact on the self-healing and hydraulic characteristics of the GCL (De Camillis et al. 2017). In a similar study, Qiu & Yu (2007) reported that CMC-added natural bentonite has better water potential for both the wetting and drying paths, but the efficacy of the treatment is remarkably dose-dependent, hence requiring further optimization. From a recent study, Rajesh & Jain (2022) observed that the self-healing property of the desiccated-rehydrated specimens of the CMC-amended bentonite-based GCLs was enhanced under varying apparent degrees of saturation and suction pressure, indicating the influence of airflow characteristics on the GCL performance. Based on the literature reports on the experimental investigations on the performance of GCL at the laboratory scale, it is inferred that controlling the hydraulic properties of the clay liners without compromising the mechanical strength is very challenging. Though several modifications to the basic liner systems are attempted, the application of CMC-based GCL with typical Indian soil such as BCS and bentonite clay is not sufficiently attempted. Based on these aspects, the present study is formulated to analyse the hydraulic performance of bentonite-based GCL which is blended with CMC at different percentages of locally available BCS.

Collection of soft clay minerals from the study area

The raw materials for the preparation of the proposed GCL composite were identified and procured from the locality of the study area which is located in Udumalpet, Tamil Nadu, India (10.61943 N, 77.15159 E), where the soft clay deposits are abundantly present. The black cotton soils (BCS) present in the topsoil horizon were scraped with the help of a hand rake and collected in polyethylene bags before taking to the laboratory for preliminary analysis. The bentonite clay mineral (BCM) used in this study was obtained from M/s Swell Well Minechem Pvt Ltd, India, which is having 85% of montmorillonite mineral. The properties of the collected soft soil samples as per the standard testing procedures followed in the laboratory are listed in Table 1.

Table 1

Properties of the soft soil samples collected from the study area

ParameterBlack cotton soil (BCS)Bentonite clay mineral (BCM)
Liquid limit 126% 285% 
Plastic limit 70.76% 55.2% 
Plasticity index 55 (–) 230 (–) 
Optimum moisture content 18% 14% 
Maximum dry density 1.91 g/cm3 1.4 g/cm3 
Swell index 18 mL/2 g 24 mL/2 g 
Colour Dark grey Light yellow 
Specific gravity 2.30 (–) 2.67 (–) 
Ash content 83.2% – 
Organic content 0.53% <0.25% 
pH 8.2 (–) 8.7 (–) 
ParameterBlack cotton soil (BCS)Bentonite clay mineral (BCM)
Liquid limit 126% 285% 
Plastic limit 70.76% 55.2% 
Plasticity index 55 (–) 230 (–) 
Optimum moisture content 18% 14% 
Maximum dry density 1.91 g/cm3 1.4 g/cm3 
Swell index 18 mL/2 g 24 mL/2 g 
Colour Dark grey Light yellow 
Specific gravity 2.30 (–) 2.67 (–) 
Ash content 83.2% – 
Organic content 0.53% <0.25% 
pH 8.2 (–) 8.7 (–) 

Selection of additives for the composite liner design

The identification and optimization of suitable ingredients are very crucial for preparing composite liners as the diversity in their individual and combinational applications makes it challenging to design them for a specific application. The CMC polymer is used in this study to improve the swelling characteristics of GCL in order to control the hydraulic properties. It is derived from cellulose, which is a natural polysaccharide found in plant cell walls and is produced by chemically modifying cellulose through the introduction of carboxymethyl groups (–CH2–COOH) onto the cellulose backbone. This modification enhances the water-solubility and functional properties of the cellulose. We have considered an optimal dosage of 8% of CMC for the amended bentonite based on the limitations on plasticity index and swell pressure as reported in the literature (Jain 2017).

The laboratory-scale experiments for the GCL preparation involve a sequence of steps starting with the individual components and then mechanically joining them with sufficient endurance as well as flexibility. For the preparation of modified GCL specimens, a non-woven type of geotextile is selected for its low mass per unit area characteristics. The polypropylene non-woven geotextile having a thickness of 1.5 mm with a mass per unit area of 100 g/m2 and a tensile strength of 8 kN/m is used as the cover for the geotextiles in the preparation of the modified GCL specimens.

Preparation of composite liner specimens

The laboratory procedure followed for the fabrication of BCS-amended GCL consists of sequential synthesis of composite layers followed by a mechanical stitching process to get a homogenous combination of layers. As the initial step, a pre-fixed quantity of BCM and BCS powders were thoroughly mixed until a uniform colour was attained. The addition of BCS was increasingly varied from 5 to 35% for different compositions. Meanwhile, the CMC solution was prepared using the chemical modification of plant leaf cell-based cellulose with carboxymethyl groups. The prepared CMC was incorporated into the BCM–BCS mixture with an intermittent amalgamation up to a maximum contribution of 8%. The entire mixture was destabilized thoroughly using a mechanical stirrer to avoid any lumps and mixed further until it was completely spread evenly. Then, the composite specimen was kept in the oven for 24 h at 100° ± 6 °C to make it a dry soil sample. The same process was repeated for various percentages of BCS–BCM mixture along with 8% of CMC. The pictorial representation of the constituents, namely, bentonite clay, CMC, BCS, and the mixing as well as the stitching process is provided in Figure 1.
Figure 1

Pictorial representation of the soil samples and GCL preparation procedure.

Figure 1

Pictorial representation of the soil samples and GCL preparation procedure.

Close modal

Physico-chemical testing of GCL specimens

The prepared GCL specimens were tested for their physical, microstructural, and hydraulic characteristics in order to assess the improvements in the swell-shrink properties due to the composite effects. The Swell Index test was conducted in accordance with ASTM D5890 (2019) on the raw bentonite, BCS, and BCM-BCS composite having 8% CMC (ASTM D5890). For this, the powder samples were taken after passing through the No. 200 sieve and oven drying at 105 °C for 24 h. The soil particles were added incrementally at the rate of 0.1 g until an accurate weight of 2 g and were added to a 100 mL graduated cylinder. The remaining portion of the cylinder was filled with deionized water and kept idle for 24 h for undisturbed swelling to occur. The free swell index value of each soil sample was determined using the procedure reported by Yang & Reddy (2018). The same procedure can be extended to test the specimens having polymers because of their lower density than the raw BCM samples (Billmeyer 2012).

As mentioned earlier, to test the effect of the permeating water quality on the hydraulic performance of the GCL, the pH of the solution was regularly monitored as per the standard procedure (Wilkinson et al. 2016). To measure the pH, an aqueous extract of the GCL specimens was prepared by adding 30 mg of the powder in 1 L of water, dispersing for 30 min at 200 rpm in a flocculator, followed by separating the top 10 mL from the mixture for the pH measurement using a table-top electronic pH meter (M/s U-Tech Pvt Ltd, India). In order to assess the microstructural features of the synthesized GCL specimens, non-destructive characterization tests such as X-ray diffraction analysis (XRD) and scanning electron microscopy (SEM) were performed. The sample preparation and administration for the characterization tests were carried out as per the procedure reported by Fan et al. (2017).

As the most important hydraulic parameter for GCL composites, the hydraulic conductivity of the specimens was determined with the help of a flexi-wall permeameter setup. The standard procedure is given by ASTM D6766 based on a constant head approach and is recommended by a few studies in the past (Jo et al. 2005; Scalia et al. 2014). The experiment is conducted using deionized water as the hydration-permeation fluid due to its low ionic strength and hence free from the swell characteristics of BCM. The apparatus consists of a transparent cylindrical cell made of acrylic material whose walls are capable of exerting uniform pressure on the soil sample to ensure proper contact and minimize leakage. Saturated flow tests were conducted on a 100 mm sized cylinder which is kept as the flow cell chamber having been sealed with silica gel (Figure 2). Desiccators were provided for 5 days to keep the specimen so as to maintain uniform moisture distribution. The modified GCL specimens were hydrated and saturated before placing in the flexi-wall permeameter, which is sandwiched between two porous stones and secured with rubber membranes (ASTM D 6766 -12). The sample was constantly pressurized by keeping the initial pressure of 2.0 kg/cm2 and back pressure of 1.0 kg/cm2 to maintain the required flow conditions. The hydraulic conductivity values of the GCL composite specimens kept in the flow cell were estimated for every 24 h of fluid flow by measuring the differences in the hydraulic heads corresponding to the time of observations.
Figure 2

Schematic diagram of flexi-wall permeameter.

Figure 2

Schematic diagram of flexi-wall permeameter.

Close modal
The fundamental concept related to the application of this procedure to determine the hydraulic conductivity of the GCL is based on Darcy's law as given by the equation (ASTM D6766):
formula
(1)
where Q is the flow quantity (m3), L is the length of the sample along the flow path (m), A is the cross-sectional area of the sample (m2), t is the time interval (s), and h is the difference between hydraulic head across the specimen (m).

It is to be understood that the flow cell was uniformly filled up to saturation and sufficient care was exercised to avoid the possibility of air entrainment throughout the flow paths during the testing. The flow was initiated by using a mechanical compressor and the GCL kept in the flow cell was subjected to a uniform pressure which allows the flexible membrane to adjust its plasticity effect. The application of pressure was regulated through a valve-controlled mechanism based on the readings on the permeameter columns. The effluent was collected from the flow cell by virtue of the porous disk kept at the bottom and withdrawn with a separate flow line for further chemical analysis.

Compositional heterogeneity as a characteristic cause of preferential swelling

One common feature of all clay minerals is their intrinsic behaviour of swelling attributed to the volumetric transformations due to the absorption of water molecules in their interstitial micro-pores. Nonetheless, the type of minerals that are predominant in a typical clay tends to influence the extent of swelling to a great extent. Based on the experimental results for different combinations of BCS and BCM in the presence of a fixed addition of CMC, it is observed that the temporal variations in the swelling index for different mixtures (based on the amount of BCS present) have shown a diverging trend concerning exposure time, while finally stabilized to a consistent value within a period of 24 h (1,440 min) without any confining stress (Figure 3). The highest value of swell index (38 mL/ 2 g) was observed for the GCL composition with a 25% BCS which has declined for a further increase in BCS. This is about 58% higher than the individual swell index of BCM as well as 111% higher than that of BCS obtained from independent studies. The overall trend is a linear-type of increase for the swell index with increasing BCS contributions. This is corroborative by Chapuis (1990) and Sun et al. (2015) who reported that free swell was significant in clay soils due to the hydration of bentonite.
Figure 3

Variations in swell index observed for different ratios of BCS in the BCM-blended GCL.

Figure 3

Variations in swell index observed for different ratios of BCS in the BCM-blended GCL.

Close modal

The presence of CMC is proclaimed to have benefited the characteristic swelling behaviour of the prepared GCL composite in at least two ways: firstly, it enabled a uniform and effective binding environment for the two types of clay minerals, and secondly, it demonstrated the direct impact of water absorption by the composite material. When the soil was fully saturated, the CMC was partially separated from the BCM composite specimens indicating that bentonite plays a major role in the swell behaviour. This is similar to that observation by Tian & Benson 2019, where the authors concluded that the swelling behaviour of the polymer-modified soil is controlled by the presence of montmorillonite-type minerals present in bentonite clay. It is also noted that the swell index started declining for higher addition of BCS (above 25%) which can be attributed to the fact that a substantial reduction in the available BCM content has limited the availability of pore space for efficient hydration to occur. The typical swelling behaviour exhibited by the GCL composite resembles the postulates of a strong parent material influence (PMI) which is generally anticipated when a complex composite is exposed to different environmental conditions (Balaganesh et al. 2022, 2023). Considering these views, the results from the present study can be taken as affirmative propositions towards developing the BCM-blended BCS-based GCL for geotechnical applications.

Microstructural modifications in polymer-amended clay composites

As observed in the previous section, the incremental additions of BCS have improved the swelling behaviour of CMC-amended GCL specimens to make them suitable for geotechnical applications. The presence of CMC has benefited the bonding of clay minerals in the composite to stay strongly against shear effects while experiencing preferential swelling. An investigation of the microstructural modifications can provide more details about the interactions between the polymer chains and the clay particles as they get reinforced in the composite structure. Results from scanning electron microscope (SEM) images of two magnifications (500× and 10k×) are presented here for the GCL composite specimens for varying BCS proportions (Figure 4). It is evident from the images that the gradual development of cross-linking features is distinctively visible in the microstructure with an increase in BCS content. Based on the images with 500× magnification, the variations in the pore voids in the bentonite structure can be visibly differentiated for increasing BCS content with a corresponding increase in the granular distribution. This also indicates the possibility of a uniform pore-filling mechanism to generate a homogeneous grain distribution for each of the BCM additions.
Figure 4

Microstructural modifications observed on the surface of BCS-modified BCM-based GCL-CMC composites.

Figure 4

Microstructural modifications observed on the surface of BCS-modified BCM-based GCL-CMC composites.

Close modal

Further, closer observations at 10k× magnification revealed evidence of a progressive deformation in the crystalline structure of the clay minerals with many flocculated and curly features due to the increased interfacial adhesion between the CMC polymer and the clay phases. The presence of CMC has caused the necessary linkages between the clay particles thus contributing to the toughening mechanisms by absorbing and dissipating energy during physical deformation processes such as swelling. The addition of BCS has effectively contributed towards an increase in the specific surface area, which is evident from the edge-to-edge and face-to-face association of the clay particles. However, a higher amount of BCS (greater than 25%, which is found to be the optimum) has resulted in distorted microstructural features in the images at both magnifications. It is attributed to the fact that the CMC polymers are responsible for achieving an exfoliation effect in the GCL nanocomposites (where individual clay layers are dispersing in the polymer) compared to the intercalation effect (where the clay layers are separated by the polymer chains). It is conclusively evident that an optimum combination of blended clay layers in the polymer matrix is suitable for enhancing the barrier properties of the GCL composite material and preventing their early-stage degradation for effective field applications.

Hydraulic performance of polymer-amended clay composites

The increased swelling behaviour as well as the modifications in the interstitial porous structure of the polymer-amended geosynthetic composite strongly emphasize the possibility of pertinent changes in hydraulic performance when subjected to a fluid flow, by virtue of the alterations in porosity (in terms of water retention) and permeability properties. Based on the experimental results obtained from the permeameter apparatus, the hydraulic conductivity is estimated for different specimens of modified GCL composites concerning the permeation time. It is observed that the hydraulic conductivity for different specimens (used in the flow cell) with different BCS proportions showed variations from 1.21 × 10−7 to 3.7 × 10−8 m/s during the first 24 h of the experiment in the following order of BCS additions: BCS15 > BCS10 > BCS30 > BCS5 > BCS20 > BCS25 (Figure 5). This may be attributed to the differences in the initial pore-scale flow development patterns which is expected for any continuum-based or Darcy-scaled flow system. The hydraulic conductivity is assumed to be minimally influenced by the polymeric action at this stage as the pore-space reduction due to swelling could be just manifested within this period.
Figure 5

Temporal changes in hydraulic conductivity for different proportions of BCS present in CMC-amended BCM-based GCL composites.

Figure 5

Temporal changes in hydraulic conductivity for different proportions of BCS present in CMC-amended BCM-based GCL composites.

Close modal
As time progresses, a significant reduction in the hydraulic conductivity is demonstrated by the composite samples with higher BCS contribution (BCS20, BCS25, and BCS30) while the reduction was only marginal for the samples with lower BCS proportion (BCS5, BCS10, and BCS15). The lowest values of hydraulic conductivity were obtained for BCS25 specimens at every stage of the experiment, collaborating with our earlier findings concerning the swell index and microstructural modifications. It is to be understood that the presence of CMC polymers in the clay matrix has substantially decreased the hydraulic property of the GCL making it compatible for application in flow-resistant systems and water-related environments. Another plausible reason is that any increase in the osmotic swelling of montmorillonite mineral may cause a corresponding enhancement in the hydraulic conductivity (Petrov et al. 1997; Jo et al. 2001; Jo et al. 2004; Katsumi 2010; Shackelford et al. 2010; Kong et al. 2017). This is found to be more relevant as there is very little possibility of change in osmotic pressure as we used the deionized water with very low ionic strength for the experiments. A closer investigation based on the X-ray diffraction analysis (XRD) results also confirmed the fact that there are no significant ionic peaks observed in the spectral image (Figure 6). The figure shows that the Na content is very low when compared to the mineral composition of the composite material indicating less influence of the chemistry of GCL (in terms of the ionic species) compared to the physics of GCL (swelling behaviour) in changing the hydraulic conductivity. The XRD results for the GCL composites also reveal the chemical composition primarily of oxides of Si, Al, Ca, and Fe along with Na and Mg.
Figure 6

Elemental composition of bentonite as revealed by XRD.

Figure 6

Elemental composition of bentonite as revealed by XRD.

Close modal

Chemical stability of polymer-amended clay composites

Apart from the functional and operational features, the polymer-amended clay composites are often challenged on their chemical stability which affects their multifunctional applications. This is critical when polymers and clay minerals interact themselves as well as get exposed to different environmental conditions, thereby affecting their long-term stability and durability. As the clay particles possess some negative charges even in the presence of polymeric complexes, their exposure to a hydroxylated or protonated aqueous environment can certainly induce thermodynamically feasible ion exchange reactions causing them to disintegrate over some time. In essence, the most critical chemical parameter to test the chemical stability of the clay composite is pH as it can reveal the net charge on the substance. In this regard, the pH value of the GCL composite was tested for different BCS combinations (Figure 7).
Figure 7

Variations in pH values for different BCS proportions in CMC-amended GCL composite.

Figure 7

Variations in pH values for different BCS proportions in CMC-amended GCL composite.

Close modal

It was observed that BCS20 and BCS25 specimens have the lowest (or near-neutral) pH values corresponding to the fixed amount of CMC addition (8%). One specific reason for the higher range of pH is the presence of typical clay minerals in the composites making it as a net-negative (alkaline) material towards chemical equilibrium. Another aspect is to realize that the anionic polymer concentration is optimum for all the modified bentonite samples considered in the study. As per Tekin et al. (2010), cationic polymers have more adsorption capacity when the pH value is high due to the attraction of positive cations present in the polymer towards the negative charges present in the clay samples. However, since anionic polymer has been used in this study, the negative charges in the clay sample are not being attracted to the positive charges, causing the pH value to decrease. The influence of acids and bases on hydraulic conductivity generally occurs through the breakdown of clays as per Ozcoban et al. (2022). The test results show that the addition of BCS can decrease the adsorption capacity of the CMC-amended GCL composite thereby making it more resilient to the adverse chemical environment present in the soil systems. Wilkinson et al. (2016) and Linggi et al. (2021) also support the finding that soils with low pH values normally require anionic polymeric solutions to neutralize the pH of the soil. A similar result can be obtained when the polymeric composites are used for stabilizing the expansive soils for various geotechnical applications.

In this study, the hydraulic performance of bentonite blended with CMC and different percentages of locally available BCS was analysed. The variation of swell index, pH, and microstructural characteristics of the modified bentonite was studied. The swell index of modified bentonite increases with an increase in the percentage of BCS and attains a maximum for 25% addition of BCS. Further inclusion of BCS reduces the swell index since the bentonite content which plays a major role in the swelling behaviour is reduced. pH value decreases with the addition of BCS and attains a minimum for 20% addition of BCS. The stability of the bentonite decreases due to the presence of anionic CMC polymer solution and thus results in decrement of the pH. The microstructural properties from the SEM images denote that the pore void space decreases with the addition of BCS up to 25%, and thereafter the pore voids increase. The hydraulic conductivity of the modified GCL decreases up to 25% addition of BCS and increases with further addition of BCS. Based on the XRD, the Na content is found to be low in the modified bentonite for the addition of BCS beyond 25%. Consequently, this has increased the hydraulic conductivity of the modified GCL. From this study, it can be concluded that the optimum percentage of BCS and CMC for better hydraulic performance of GCL is 25 and 8%. Although the hydraulic conductivity of the modified GCL is higher compared to the stand-alone GCL with bentonite, nevertheless, this study has far-reaching applications in the disposal of solid wastes in landfills.

Funding received from Dr Mahalingam College of Engineering and Technology, Pollachi, Tamil Nadu, India under the in-house R&D scheme (Ref. No: In-House R&D/2022-23/01) is gratefully acknowledged.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.

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