Bentonite Polymer Research Articles

Hydraulic conductivity of bentonite–polymer geosynthetic clay liners subject to wet–dry cycles used in landfill cover systems

This study evaluated the hydraulic conductivity of bentonite–polymer (B-P) geosynthetic clay liners (GCLs) subjected to wet-dry cycles that simulated seasonal conditions on a GCL installed in a landfill cover system. Hydraulic conductivity tests were conducted on B-P GCLs and NaB GCLs at 10 kPa in accordance with the ASTM D5084 standard. Two wet–dry methods were used to investigate the hydraulic conductivity of the GCLs under different field exposure conditions. The hydraulic conductivities of the NaB and B-P GCLs gradually increased during four wet–dry cycles. However, the hydraulic conductivity of the B-P GCL (K = 2.3 × 10-12 m/s) was lower than that of the NaB GCL (K = 6.3 × 10-11 m/s) by two orders of magnitude after four permeation–drying cycles. In addition, the hydraulic conductivity of the B-P GCL was lower (K = 5.8 × 10-12 m/s) than that of the NaB GCL (K = 7.8 × 10-10 m/s) by two orders of magnitude after four saturation–drying cycles. The results suggest that B-P GCLs can enhance the effectiveness of landfill cover systems compared to Na-B GCLs in managing conditions during wet and dry seasons.

Self-healing capacity of GCLs under simulated field conditions

The self-healing capacities of a GCL with natural bentonite (NB) as core (NB-GCL) and a GCL with a polymerized bentonite (PB) as core (PB-GCL) were investigated under simulated field conditions, i.e. geomembrane-GCL-a clayey subsoil layer composite liner system with a damage hole on the geomembrane and the GCL. The clayey subsoils tested had initial water contents of 22.2% and 26.8%. The liquids used were deionized (DI) water and 0.3 M NaCl solution. The test results indicate that for both the PB-GCL and NB-GCL only hydrated on the subsoils for 1–3 months, a damage hole of 15 mm in diameter was almost not self-healed. For cases of applying a constant liquid head on the top of the geomembrane of 100 mm, the sizes of damage holes were self-healed in term of diameter is about half of the values reported in the literature for tests with plenty liquid supply to the damaged GCLs. Further, for the conditions tested and for the cases self-healed, DI water leaked into the subsoil was less than about 60 g, and 0.3 M NaCl solution leaked was less than 150 g.

Effect of wet and dry cycles on lighter bentonite-polymer geosynthetic clay liners

This study evaluated the hydraulic conductivity (K) of lighter bentonitepolymer (B-P) geosynthetic clay liners (GCLs) subjected to wet-dry cycles which simulated seasonal conditions on a GCL installed in a landfill cover system. Lighter bentonite-polymer composite (B-P) GCLs are manufactured with a lower mass per unit area of sodium bentonite (Na-B) (approximately 2.9 kg/m2) than conventional Na-B GCLs (approximately ≥ 3.6 kg/m2) which can reduce the transportation and installation costs compared to conventional Na-B GCLs. Hydraulic conductivity tests on lighter B-P GCL and Na-B GCL were conducted with a synthetic soil porewater leachate to represent wet season conditions on a landfill cover. Between hydraulic conductivity tests, GCL samples were placed in sealed chambers with a relative humidity of 75% until the water content of the specimens decreased to 55%, which represented dry season conditions of a GCL under a geomembrane in a cover system. Slight increases of the hydraulic conductivity were measured for both the Na-B and lighter B-P GCLs. After 5 wet-dry cycles K of the Na-B GCL increased from 3.1 x 10-12 m/s to 8.3 x 10-12 m/s, and K of the lighter B-P GCL increased from 1.6 x 10-12 m/s to 8.3 x 10-12 m/s.

Pore Scale Simulation of Rheology Properties on Residence Time of Polymer Hydrogel and Hydraulic Conductivity of Bentonite Polymer Composite Geosynthetic Clay Liners

Flow in an idealized bentonite polymer composite geosynthetic clay liner (BPC-GCL) containing bentonite comprising two idealized circular granules was simulated using a COMSOL hydrodynamic model. The effect of the polymer rheology properties, including viscosity, surface tension, and contact angle, on the hydraulic conductivity of BPC-GCLs was investigated. The results showed that the hydraulic conductivity of BPC-GCLs significantly decreased by 2–4 orders of magnitude with polymer loadings of 3.3%, 6.5%, and 9.8% compared to conventional geosynthetic clay liners (GCLs). The polymer rheology properties are critical to the residence time and the hydraulic conductivity of BPC-GCLs. The residence time increases with the viscosity, surface tension, and contact angle of polymer hydrogel. In the overall study, the hydraulic conductivities increased significantly from 2.80 × 10−9 m/s to 1.40 × 10−7 m/s when the residence time was insufficient. When the viscosity of the polymer hydrogel is 5000 Pa∙s, 1 × 104 Pa∙s, and 1 × 105 Pa∙s, the residence time of the polymer hydrogel in the domain of BPC-GCLs is 14 min, 23 min, and 169 min, respectively. When the surface tension of the polymer hydrogel is 0 N/m, 0.01 N/m, and 0.02 N/m, the residence time of the polymer hydrogel in the domain of BPC-GCLs is 9 min, 17 min, and 23 min, respectively. When the contact angle between the polymer hydrogel and the NaB granules is 30° to 60°, the residence time of the polymer hydrogel in the domain of BPC-GCLs is 9 min and 33 min. These few minutes can approximate the actual passage of several days in physical time. When the viscosity, the surface tension, and the contact angle are higher than 1 × 106 Pa∙s, 0.03 N/m, and 60°, the residence time of the polymer hydrogel in the domain of BPC-GCLs tends to be very long, which means that a very low hydraulic conductivity of BPC-GCLs can be maintained in the very long term. This research unveils a nuanced and profound correlation between the rheological properties of the polymer hydrogel and the resulting hydraulic conductivity. This discovery enhances the understanding of the potential to tailor hydrogel characteristics for BPC-GCLs. The advanced model developed in this study also lays the groundwork for constructing a more realistic model that considers irregular geometries, interconnected pores, and diverse polymer distributions within the pore spaces.

Orthogonal Design and Microstructure Mechanism Analysis of Novel Bentonite Polymer Slurry in Pipe Jacking

The selection of an appropriate slurry ingredient and its percentage ratio is a vital and necessary task for engineers in slurry pipe jacking operations. However, traditional bentonite grouting materials are difficult to degrade because of their single and non-biodegradable composition. Nowadays crosslinked polymers have been widely considered due to their excellent performance and application in engineering practices, which enlighten novel polymer slurry in pipe jacking. This study innovatively proposed using boric acid crosslinked polymers added into polyacrylamide bentonite slurry, which not only solves the shortcomings of traditional grouting materials but also meets the general working performance requirements. The new slurry's funnel viscosity, filter loss, water dissociation ratio and dynamic shear were tested according to an orthogonal experiment. Single factor range analysis was conducted to identify the optimal mix proportion based on an orthogonal design, and the formation behavior of mineral crystals and microstructure characteristics were evaluated by X-ray diffraction and scanning electron microscopy respectively. According to the results, guar gum and borax form a dense boric acid crosslinked polymer through cross-linking reaction. The internal structure grew tighter and more continuous as the crosslinked polymer concentration grew. It improved the anti-permeability plugging action and viscosity of slurries by 36.1~94.3%. The optimal proportions of sodium bentonite, guar gum, polyacrylamide, borax, and water were 10%, 0.2%, 0.25%, 0.1%, and 89.45% respectively. All these works indicated that the improvement of slurry composition by using boric acid crosslinked polymers was feasible.

Morphological controls on flow conductivity and viscosity of bentonite–polymer composites

In this study, the authors investigate the morphological organisation of polymers in bentonite–polymer composites (BPCs) used in geosynthetic clay liners and controls on hydraulic conductivity and viscosity. A limited understanding of the relationship between the microstructure of BPCs and their macroscopic engineering behaviour motivates this study. The polymer resides in the inter-granular pores of bentonite based on synchrotron-based small-angle X-ray scattering measurements, scanning electron microscopy imaging and pore size analyses. On wetting, the polymer swells to produce a swollen hydrogel residing in the inter-granular pores. Enhanced tortuosity arising from hydrogel formation reduces the hydraulic conductivity of BPCs. The higher viscosity of the BPC slurry is attributed to the higher shear resistance emerging from swelling of the polymer fraction on wetting. These findings link the morphological features of BPCs to the observed hydraulic conductivity and viscosity of these materials.

Hydration behavior of geosynthetic clay liner with polymerized bentonite

The hydration behavior of a GCL with polymerized bentonite (PB-GCL) was investigated by laboratory column tests in terms of hydration gravimetric water content (w), apparent degree of saturation ([Formula: see text])and air permeability (kair), and compared with those of a GCL with natural bentonite as the core (NB-GCL). In cases of deionized water (DI water) and 0.1 M NaCl solution as pore water of the subsoil, the PB-GCL had higher final w values than the NB-GCL, but the values of [Formula: see text] are similar. The values of kair of the PB-GCL are approximately four orders lower than that of the NB-GCL. In addition, kair of the PB-GCL at [Formula: see text] = 40% is comparable with that of the NB-GCL at [Formula: see text] = 70%. In the case of the subsoil with 0.6 M NaCl solution, the final value of kair of the PB-GCL is about half of the NB-GCL. These results indicate that cation concentration in a subsoil has a considerable influence on the hydration behavior of a GCL. It is suggested that the PB-GCL has a better hydration performance than the NB-GCL.

Shear strength of bentonite–polymer composite geosynthetic clay liners and geomembranes

The interface shear strength between bentonite–polymer composite (BPC) geosynthetic clay liners (GCLs) and textured geomembrane (GM) was compared with the interface shear strength between conventional sodium (Na) bentonite GCLs (NaB GCLs) and the same GM. Four BPC GCLs were used that had polymer loading ranging from 0.12 to 1.09 g/kg. The peak shear strength of the BPC GCL/GM interface ranged from nearly identical to the interface strength of the NaB GCL/GM to as much as 14.4 times lower. The lower strengths of the BPC GCL/GM interface are attributed to the hydrated polymer gel that migrated out of the GCL and into the interface. The hydrated gel has very low shear resistance, reducing both peak and large-displacement interface shear strengths. The reduction in shear strength increased with increasing polymer loading of the BPC GCL and increasing normal stress. Inclusion of a woven slit-film geotextile in the interior of one of the BPC GCLs reduced migration of the polymer into the interface, leading to higher interface shear strength than obtained with the same BPC GCL without a slit-film geotextile.

Index tests for geosynthetic clay liners containing bentonite–polymer composites

The hydraulic conductivity of geosynthetic clay liners (GCLs) containing bentonite–polymer composites (BPCs) is controlled by a pore clogging mechanism related to the swelling of sodium bentonite (NaB) granules and formation of polymer gel in the pore space. Three index testing procedures that are influenced by bentonite swelling and rheology of the polymer gel were evaluated in this study for screening BPC GCLs for hydraulic conductivity: viscosity, liquid limit by fall-cone penetration and active water content. Experiments were conducted using polyacrylamide (PAM) polymer, dry-blended mixtures of PAM and NaB and a BPC from a commercially available GCL. These materials were contacted with inorganic solutions representing leachates from a range of waste containment scenarios. For the BPC that was evaluated, hydraulic conductivities less than 10−10 m/s were obtained when the viscosity exceeded 220 mPa s and the active water content exceeded 300%. Hydraulic conductivities less than 10−11 m/s were achieved for viscosity exceeding 350 mPa s, fall-cone liquid limit exceeding 190 and active water content exceeding 300%. Additional testing is needed to assess the generality of these index tests and the identified thresholds when applied to other BPC GCLs and other permeant liquids.

Hydraulic Performance of Geosynthetic Clay Liners Comprising Anionic Polymer–Enhanced Bentonites

The hydraulic performance of geosynthetic clay liners (GCLs) comprising anionic polymer–enhanced bentonites (EBs), or EB-GCLs, based on permeation with 500 mM NaCl and 167 mM CaCl2 at an average effective stress of 27 kPa and hydraulic gradients of ∼300 was evaluated. The EBs included powdered sodium bentonite (NaB) enhanced with one of seven anionic polymers, including linear, noncrosslinked poly(acrylic acid) (PA) with low, medium, and high molecular weights, sodium carboxymethylcellulose (CMC) with either low viscosity or high viscosity, a covalently crosslinked sodium polyacrylate, and an in situ polymerized bentonite polymer composite. The NaB was enhanced with polymer at mass loadings of 5%, 8%, or 10% using dry-sprinkling, dry-mixing, and/or wet-mixing methods. Hydraulic conductivity tests at a low hydraulic gradient (∼35) also were conducted to determine the effect of seepage forces. Overall, the hydraulic performance of the EB-GCLs to the NaCl solution was better than that to the CaCl2 solution. Dry sprinkling (DS) with PA generally provided the best hydraulic performance, with hydraulic conductivity values based on permeation with 500 mM NaCl and 167 mM CaCl2 of 1.3×10−11 and 4.0×10−11 m/s, respectively, for the high-molecular-weight PA added at 5% via DS and 2.9×10−11 and 4.7×10−11 m/s, respectively, for the medium-molecular-weight PA added at 8% via DS. The CMC viscosity grade did not affect the hydraulic conductivity of the EB-GCLs. All EB-GCLs eluted polymer during permeation, regardless of mixing method or polymer type. Polymer elution was correlated with preferential interaggregate flow paths in multiple EB-GCL specimens, indicating the importance of polymer retention in maintaining low hydraulic conductivity of EB-GCLs. Because standard termination criteria for the measurement of the hydraulic conductivity of conventional unenhanced GCLs (e.g., as per current ASTM standards) do not address the potential impact of polymer elution from EB-GCLs, caution should be exercised when adapting these criteria to measure the hydraulic conductivity of EB-GCLs such as those evaluated in this study.

Effect of effective stress on hydraulic conductivity of bentonite–polymer geosynthetic clay liners to coal combustion product leachates

Experiments were conducted to evaluate the hydraulic conductivity of five geosynthetic clay liners (GCLs) permeated with four synthetic coal combustion product (CCP) leachates (ionic strength = 473–4676 mM). One of the GCLs contained conventional sodium bentonite (Na-B), and the other four GCLs contained a bentonite–polymer (B-P) composites with polymer loading ranging from 0.5 to 12.7% (B-P-0.5–B-P-12.7). Hydraulic conductivity tests were conducted at 20 kPa until reaching hydraulic and chemical equilibrium and then the effective confining stress was increased incrementally from 20 to 100, 250 and 500 kPa. The hydraulic conductivity of B-P GCLs with high polymer loading showed a faster decreasing trend than that of Na-B and B-P GCLs with lower polymer loading as the effective confining stress increased. Additionally, the hydraulic conductivity of GCLs permeated with a synthetic CCP leachate with ionic strength of 473 mM decreased significantly as the effective confining stress increased, whereas the GCLs permeated with an aggressive CCP leachate (e.g. ionic strength = 4676 mM) had limited reduction in hydraulic conductivity at elevated effective confining stress.

Effects of water content on shear properties of bentonite–polymer composite structure

Understanding mechanical behavior at the soil–structure interface is fundamental for safety in engineering and is therefore an important consideration in structural design. Buildings can be deformed or severely damaged by shear forces from the action of earthquakes, waves, or winds. It is therefore necessary to understand the conditions which lead to structural failure at the soil–structure interface under shear action. We investigated the strain and displacement of bentonite–polymer specimens at various moisture contents. Shear failure was investigated using novel shear test equipment consisting of a two-dimensional soil fracture test device and computerized digital image correlation (DIC) technology. Analysis of the shear test results revealed the mechanical properties of bentonite–polymer composite structure under shear stress. Experiments showed that there were four stages in the failure of the bentonite–polymer samples: an increasing stress stage, a pre-damage stage, an abrupt damage stage, and a stable stage. At the instant the shear stress reaches peak value, the strain in the bentonite was greater than that in the polymer at the same position. In the failure stage, the bentonite at the middle and upper levels of the bentonite–polymer interface cracked first. Increased moisture content in the bentonite weakens the shear performance at the bentonite–polymer interface, and brittle failure was most significant when the moisture content was optimal. • A novel type of shear observation equipment was used in this work. • The four stages of shear failure of bentonite-polymer specimens were observed. • The influence of water content on the shear failure mode of bentonite-polymer composite specimens has been deeply studied. • It is found for the first time that the bentonite-polymer specimens exhibit brittle failure under the optimal water content.

A New Soil Conditioner for Highly Permeable Sandy Gravel Stratum in EPBs

Full-face water-rich gravel stratum is a large challenge during tunnel excavation with earth pressure balance shields (EPBs) because of accidents such as water spewing from the screw conveyor and ground collapse. Slurry and polymer have been used as conditioning agents to avoid such problems and thus ensure a successful tunneling. However, limited improvement of sandy gravel was achieved when traditional soil conditioner were applied. This study proposes a new conditioner (modified slurry) consisting of bentonite slurry, viscosity modifier, sodium silicate and polymer, which will enhance the properties of sand gravel stratum. Low reaction time, high apparent viscosity, good plastic behavior and low permeability were employed for investigating the optimum ratio of the ingredients. The proposed modified slurry has a good performance in conditioning sandy gravel soils and can be the reference for EPBs’ excavation in highly permeable, non-adhesive coarse-grained soil stratum.

Effect of Polymerization on the Swelling Properties of a Sodium Bentonite and its Mechanism

The effect of polymerization on the swelling properties of a sodium bentonite has been investigated experimentally using free swelling index (FSI). Using free radical polymerization method in cationic solution with acrylate acid as the monomer (M) and potassium persulfate as initiator (I), the optimum conditons for higher FSI value in 0.6 M NaCl solution were pH of 5.5 and I/M ratio of 0.005. The polymerized bentonite (PB) had much higher FSI values than that of untreated bentonite (UB) in deionized water and 0.6 M NaCl solution. However, in the 0.6 M CaCl2 solution, the FSI value of PB was slightly lower than that of UB and the reason is not clear yet. The X-ray diffraction (XRD) test shows that polymer chains did not enter the interlayer of sodium bentonite crystals and it is postulated that the polymers only wrapped around the particles of the bentonite.

Effect of pH Variations on the Yield Stress of Calcium Bentonite Slurry Treated with pH-Responsive Polymer

The pH-responsive polymers, such as polyacrylamide (PAM), show distinct conformational states according to the pH of their environmental groundwater. Therefore, the interactions between clay–polymer and polymer–water molecules, which determine the yield stress of bentonite–polymer composites, can be affected by the pH of groundwater. This study aims to evaluate the effect of pH variation on the yield stress of calcium bentonite treated with PAM. The yield stresses (τy) of untreated and PAM treated clays were measured with varying volume fractions of solid (VF = 10–23%) and under varying pH conditions (pH = 7.6–9.6). In addition, the zeta potential was measured for both untreated and treated clays to figure out the change in the surface charge of the mineral surface due to PAM treatment. The results of this study demonstrate that τy for treated clay is higher than that for untreated clay at a given VF, because van der Walls attraction dominates electrostatic repulsion in the case of treated clay. Due to the change in conformational states of PAM and the consequent change in surface charge that comes with varying pH, the pH-dependent change in τy of treated clay is significantly different from that of untreated clay.

Synthesis and Characterisation of Carboxymethyl Cellulose Based Bentonite Polymer Blend

Polymer blend of carboxymethyl cellulose (CMC) and bentonite clay were prepared in the presence and absence of the cross linking agent glutaraldehyde. The synthesized compounds were characterized using FT-IR, XRD, TGA and DSC analysis. The percentage of yield obtained is higher for the polymer blend prepared with glutaraldehyde as cross liking agent. The result of FTIR studies showed the various peaks corresponding to the functional groups present in the two polymer, which clearly indicates the presence of strong cross linking between the polymer and cross linking agent. Similarly, thermal gravimetric analysis revealed that enhanced thermal stability for the polymer blends along with glutaraldehyde. X-ray diffraction studies prove that the polymer blends have higher crystallinity when blending with glutaraldehyde.

Barrier properties of a geosynthetic clay liner using polymerized sodium bentonite

The hydraulic and swelling properties of a polymerized bentonite (PB), and the self-healing capacity of a geosynthetic clay liner (GCL) using the PB as the core material (PB-GCL) were investigated experimentally. Five different test liquids included of deionized water, NaCl solutions (0.1 M and 0.6 M) and CaCl2 solutions (0.1 M and 0.6 M) were used in this study. The PB exhibited a higher free swelling index (FSI) than that of the untreated bentonite (UB) for all test liquids. For permeability test, under a given void ratio (e), the value of k of the PB is much lower than that of the UB in NaCl and CaCl2 solutions. The PB-GCL specimens demonstrated a higher self-healing capacity than that of the corresponding GCL specimens using the UB (UB-GCL). Specifically, when using a 0.6 M CaCl2 solution for a 20-mm-diameter damage hole, the UB-GCL specimen provided a zero healing ratio (healed damage area/total damage area), but the PB-GCL specimen demonstrated an approximately 76% healing ratio. The results from this study indicate that the PB-GCL provided better barrier performance against cationic liquids with higher cation valence and concentrations compared to that of the UB-GCL.

Attenuation of Cr/Pb in bauxite leachates by bentonite-polymer composite geosynthetic clay liners.

Three commercially available bentonite–polymer composite geosynthetic clay liners (BPC GCLs) were selected for hydraulic conductivity testing, respectively permeated by two types of bauxite leachates with high alkalinity (pH > 12) and high ionic strength (620.3 mM). The influence of BPC GCLs on the attenuation behavior of Cr/Pb in the bauxite leachates was analyzed. The BPC GCLs with a low hydraulic conductivity (k < 10−10 m s−1) retard the migration of Cr and Pb and the Cr had a higher mobility than Pb in the BPC GCLs. Scanning electron microscope (SEM) microstructure analysis showed that the migration and attenuation behavior of Cr/Pb mainly depended on the chemical properties of the leachates, polymer content and the microstructure of the polymer. Higher attenuation of heavy metals was obtained with bauxite leachates having higher ionic strength. Sufficient polymer content is needed to ensure BPC GCLs have adequately low hydraulic conductivity to suppress attenuation of heavy metals. The gelatinous structure associated with hydrated linear or crosslinked polymer diminishes when the polymer in a BPC is in contact with bauxite leachates. Compromising the hydrogel structure promotes polymer elution and leaves pore space open, resulting in attenuation of heavy metals.

Performance of Geosynthetic Clay Liner with Polymerized Bentonite in Highly Acidic or Alkaline Solutions

The hydraulic behavior of a polymerized bentonite (PB) and the self-healing capacity of a geosynthetic clay liner (GCL) using the PB as core material (PB-GCL) in corrosive solutions (pH ranging from 1 to 13) were investigated through a series of laboratory tests [i.e., free swelling index, swelling pressure, permeability (k) (consolidation), and leakage rate tests]. The test results indicate that the PB had a higher swelling capacity than that of the corresponding untreated bentonite (UB). Particularly, for pH greater than 12.5 solutions, PB had a higher free swelling index (FSI), higher swelling pressure, and lower k value than that of the PB with deionized water. PB-GCL with a damage hole had lower permittivity for the damage hole (ψhole) and a higher self-healing capacity than that of the GCL using UB as the core (UB-GCL). Based on the test results, it is suggested that PB-GCL can be used as an effective barrier material to contain acidic and alkaline liquids.

Polymer Elution and Hydraulic Conductivity of Bentonite–Polymer Composite Geosynthetic Clay Liners

AbstractHydraulic conductivity tests were conducted on two commercially available bentonite–polymer composite (BPC) geosynthetic clay liners (GCLs) containing the same dry blend of sodium bentonite...