Nanoclays are basically the silicates that are present in the form of layers whereas possessing thickness and diameter in the nonmetric form which can be equivalent to 50-200nm. These are currently known to be one of the best materials to have been launched in the industries as they are benefiting them at a greater rate. The sustainable production of nano clays is a hard process and takes several steps to complete. However, once attained these do wonders in different fields as their uses and advantages are enhanced.
With 50-200 nm of diameter and non-metric thickness, nano clays are a kind of layered silicates. There are various applications of these nanoplates like membrane coating, adsorption of toxins, sterilizing effect, and antibacterial activity (Wilson, 2003). The barrier, physical, and mechanical characteristics of the polymers can be enhanced by incorporating the nano clays into the polymeric matrixes.
Nanoclays example, saponite, kaolinite, and montmorillonite (MMT), are utilized in the food system as fillers. A huge amount of attention has been gained in the food industry by MMT because of their major enhancement in performance, simple process-ability, availability, and cost-effectiveness. They are layered silicates that have two coordinated tetrahedral silicon atoms, that are fusing to an edge-shared octahedral sheet of magnesium or aluminum oxide.
Hydrophilic characteristics are possessed by natural MMTs because of inorganic cations’ presence on the surface of MMTs; thus in combination with hydrophobic polymers, they are not effective. The characteristics of nano clays have been improved by using some modifications.
Organic nano clays
Better interactions with organic polymers, hydrophobic surface, increased interlayer spacing, and other such characteristics are possessed by the organically modified nano clays like Cloisite 20A and Cloisite 30B. Also, they possess antimicrobial activity against both Gram-negative and Gram-positive bacteria. The antimicrobial effect is because of organically modified nano clay's quaternary ammonium groups that are interacting with the bacterial cell and disrupting their cell membranes, thus resulting in cell lysis (Hong and Rhim, 2008).
In 2008, Hong and Rhim did an investigation on the antimicrobial activity of nano clays of 3 kinds, for instance, Cloisite 30B, Cloisite 20A, Cloisite Na+ (unmodified form), against 4 pathogenic bacteria like E. coli O157:H7, Salmonella typhimurium, Listeria monocytogenes, and S. aureus. According to these authors, the strongest antimicrobial activity is possessed by Cloisite 30B followed by Cloisite 20A against all tested bacteria.
Unmodified nano clays
No antibacterial activity was shown by the unmodified nano clays. In comparison with Cloisite 20A, Cloisite 30B has higher antimicrobial activity because of its more hydrophilic nature because then it is capable of easily adsorbing bacteria and interacting with them (Hong and Rhim, 2008).
Other than the direct antimicrobial effects of nano clays, nano clays also possess indirect antimicrobial effects with polymers. For instance, in C-S nano clays nanocomposites, nano clays can adsorb bacteria from solution and result in bacteria’s better interaction with antimicrobial polymer (Wang et al., 2006).
There are two main challenges when it comes to synthesizing them. One is the nanofiller’s homogeneous dispersion within the polymer matrix, and the other is the chemical compatibility between the nanofiller and the polymer matrix at the nanoscale. The interfacial interaction between polymer matrix and nano clay fillers, along with the nano clay's dispersion quality, possesses a major influence on the performance of polymer/nano clay composites.
The final bulk characteristics and morphology of the polymer/nano clay composites are determined by these intercorrelated features, for instance, gas barrier, shape memory abilities, self-healing, heat distortion temperature, thermal stability, elastic modulus, and strength.
One practical method for the improvement of the interfacial interactions between nano clay fillers and the polymeric matrix is surface functionalization of the nano clays, enabling the transfer of the interfacial stress to the nano clays from the polymer. For example, the covalent modification of the halloysite nanotubes’ outer surfaces improves the dispersibility of the halloysite nanotubes into the polymer matrix, enhancing the tensile characteristics and thermal stability of the resulting polymer/nano clay composites.
Different combinations of nano clays and the polymer matrix can be a result of Synthesis approaches like an exfoliated structure, an intercalated structure, and an immiscible structure. Nanoclay dispersion aggregates in the polymer matrix in the immiscible structure and the polymers are separated from the clay layers.
The geometry of the clay layers is altered as the polymer chains make an intercalated structure in between the clay layers. The alteration that causes all of this includes modification in the interlayer spacing, variation in the layers’ stacking mode, and diminishing the electrostatic forces between the clay layers. All of these alterations result in a significant improvement in the thermal and mechanical characteristics of the composites.
Polymer chains in the exfoliated structure fully separate the nano clay stacks, thus providing excellent mechanical characteristics and polymer processability. In-situ polymerization method, solution-blending method, and melt-blending method are the three main synthesis procedures for polymer/nano clay composites. However, the in-situ polymerization method is the most broadly utilized method of synthesis, where the grafted amounts of organics were adjusted and changing the conditions of polymerization controlled the clay interlayer spacing.
Combining in-situ polymerization with effective coupling methods, for instance, mini emulsion, photopolymerization, tandem preparation, radical-mediated polymerization, and click chemistry, enables nano clays' effective dispersion in the polymer matrix in the form of individual platelets, which is a major challenge inherent to the production of polymer/nano clay composites. There has been successful implementation of all these methods to chemically modify clay surfaces with polymeric or low molecular grafts.
In-situ polymerization, melt-blending, and solution-blending are the three synthesis methods for which numerous approaches are available. Obtaining nano clay's uniform dispersion in the polymer matrix is a significant task in preparing polymer/nano clay composites. The solution-blending method yields favorable dispersion of clay layers in the polymer matrix, as compared to melt blending, because of its high agitation power and low viscosity. However, along with high economic potential, melt blending is friendly to the ecosystem and is industrially viable.
In-situ polymerization method
The most broadly implemented synthesis method is the in-situ polymerization method, as it gives uniform dispersion and if we change the conditions of polymerization, it is easily modifiable. Various novel synthesis methods have been proposed recently for designing polymer/nano clay composites with excellent characteristics. Here, various advanced synthesis methods are reviewed and relationships between the polymer/nano clay composites' mechanical and physical characteristics, structure formation, and the strategy of their synthesis, are discussed.
Modeling mechanical properties of nano clay-based composites
The mechanical characteristics of the nano clay-based polymer composites can be predicted by using various theoretical composite models that have been developed. All these models have the following key parameters which include orientation, filler volume fraction fp, filler orientation filler/matrix stiffness ratio Ep/Em, and filler aspect ratio. In earlier studies and even now, Halpin–Tsai equation can provide suitable estimates for effective stiffness, and the best results for high aspect ratio fillers are given by the Mori–Tanaka type model. Mori-Tanaka Model's accuracy against the Finite element method (FEM) was compared by Hbaieb et al.
A very good prediction of elastic modulus with 1-5 wt% weight fraction was given by the Mori–Tanaka model. As it isn't responsible for the interaction between the fillers, a lower value of elastic modulus is given by Mori-Tanaka at a higher weight percentage. A multiscale model was used by N. Sheng et al. for explaining the mechanical characteristics, the total elastic modulus, and the hierarchical morphology of the nano clay composites.
Mori–Tanaka theory and Halpin–Tsai equations were used by D. R. Paul and T. D. Fornes, for the evaluation of nylon 6 polymer composites reinforced with layered aluminosilicates and glass fibers. A Three-phase model was developed by Luo et al. which had 3 phases; the intercalated nano clay clusters, the exfoliated clay nanolayers, and the matrix material. Ellipsoidal geometry was for both clusters and clay nanolayers in their work.
The primarily silt-sized soil particles and a little clay in 15-20 percent range accumulate to form Loess, which is aeolian sediment. There can be a presence of a major quantity of sand-sized particles in some cases, and there is a common observation of light calcium carbonate cementation of particles. Loess soils’ wind-blown depositional formation yields a comparatively loose soil structure that has a low density. The weak cementation maintains this unstable, loose state that occurs because of the production of calcium carbonate (calcite).
Loess soils possess a fairly low natural water content, due to their depositional environment’s arid nature. According to estimations, 10% of the surface of the earth is covered by loess. There is the founding of large loess deposits in parts of South America, Europe, Africa, and Asia. Loess soil can also be found in the United States' south- and mid-western portions, and Alaska. When subjected to loading, significant volumetric compression was experienced by their unstable, open soil fabric, unsaturated loess deposits. Loading the soil by constructing a new structure on the loess deposit's top can induce this soil collapse
Changes in saturation
The soil structure can fail due to changes in the related suction and the degree of saturation. Commonly, this phenomenon is known as wetting-induced collapse. A leaking pipe, a change in groundwater level, irrigation, or precipitation events can be the reason for any kind of alterations in the degree of saturation. Followed by wetting, mechanical loading can be specifically problematic when civil engineering structures are being built on loess deposits.
Settlements associated with hydro-mechanical loading can be reduced by treating loess soils with stabilizing additives. Also, the erodobility, dispersivity, plasticity, stiffness, strength can be improved and it all depends on the technique that is used for stabilization. Bituminous materials, fly ash, lime, and cement, are included in the traditional chemical stabilizers. Lignin derivatives, ions, silicates, acids, resins, enzymes, and liquid polymers, are included in the non-traditional and new stabilizers. Also, various latest studies have used various types of polymers to enhance the engineering characteristics of the problematic cells.
There has been an upsurge recently in the utilization of nanomaterials and nanotechnology for various applications, like soil stabilization. In this study, we have investigates the usage of nano clay which is an engineered material that is used to enhance the engineering characteristics of the loess soils. The retrieved samples were used for examining the effectiveness of nano clay stabilization. Various traditional geotechnical engineering tests (pinhole tests, collapse potential, unconsolidated undrained triaxial, unconfined compressive strength, standard compaction, and Atterberg limits) were done on stabilized nano clay specimens. Then, this study’s results were used at the Gonbad dam irrigation channel site for field stabilization of loess.
The usage and formation of nano-scale particles and the usage of nanotechnology have majorly increased in the last decade, resulting in advancements in many disciplines, for instance, engineering, materials science, and medicine. Various researchers have been attracted to the application of nanomaterials and nanotechnology from the geoenvironmental, geotechnical, and civil engineering disciplines.
Geoenvironmental and Geotechnical perspectives
From the perspective of Geoenvironmental and geotechnical engineering, a huge amount of potential is possessed by the nano clays in various waste containment and soil improvement applications as environmentally friendly additives. Layered mineral silicates have nanoparticles that are known as nano clays, and they are categorized into various classes like halloysite, hectorite, kaolinite, bentonite, montmorillonite, etc., and it is determined by their nanoparticle morphology and chemical composition.
Generally, nano clays are produced by using different techniques like energetic stirring followed by ultracentrifugation, or centrifugation and cross-flow filtration, or centrifugation and freeze-drying, for extracting desired nanomaterials from natural clays. A layer is the basic structural unit of the nano clays, with individual layers that are made up of octahedral and/or tetrahedral sheets, and the way that sheets are arranged does play an important role when it comes to distinguishing and defining the nano clay minerals.
Effects of nanomaterials
The effects of numerous nanomaterials like nano clay, nano copper, and nano aluminum, have been studied by some researchers on the shrinkage and swelling behavior of fine-grained soils. According to the results, the effects on the stabilized soil’s maximum dry unit weight and optimum water content are negligible on the addition of nano clay. Moreover, it was observed that on adding nano clay, the shrinkage limit and plasticity index of the treated soil increased. According to the results of scanning electron microscopy (SEM), on adding even small amounts of nano clay, there comes changes in the soil fabric because of the interparticle void space being filled by the nano clay particles.
Increased surface area
The surface area between the clay particles and the surrounding environment was increased by the nano clay particles. Taipodia et al. used nanoparticles like KNO3, CaO, and CaCl2 to perform the stabilization of clayey and sandy soils. According to this study's results, with the addition of nanoparticles, there comes a decrease in permeability and an increase in the shear strength.
Development of stress-strain behavior
Nanoclay stabilization can aid in developing plastic (ductile) stress-strain behavior. After the nanomaterial has been added, a reduction of collapse behavior is indicated on doing the experimental investigation of the nanoparticle’s stabilizing effects like nano clay, nano copper, nano aluminum, and nano silicate. According to researchers, nano clay's addition enhanced the erosion resistance of the sandy loam soils. Zomorodian et al. used nano-silica and nano clay to study the strength behavior of kerosene contaminated sandy lean clay, and he also indicated an increase in the stiffness and uniaxial strength due to stabilization.
Atterberg limits were determined for the plasticity index, plastic limit, and liquid limit of the soils. Stabilized soil's plasticity index, plastic limit, and liquid limit are majorly increased by adding even very little amounts of nano clay to the loess soil (0.5–3%).
For instance, at a content of 3% nano clay, there was a 66% increase in the plasticity index, a 25% increase in the plastic limit, and a 36% increase in the liquid limit. Chemical interactions between moisture and the particles and water retention characteristics are majorly affected by an increase in the surface area. The behavior of nano clay-stabilized loess is the same as the behavior that has been seen by the other researchers i.e. an increase in the plastic and liquid limits. There are some of the studies that interestingly showed a reduction in the plasticity index with the addition of nano clay for numerous other soils, and the observed behavior was the opposite of this behavior.
Standard proctor compaction tests
Compaction tests were also done. Adding small amounts of nano clay to the loess (0.5-3%) can significantly affect the standard Proctor compaction curves for loess soil. With an increase in the content of the nano clay, the dry unit weight suffered a major and consistent decrease; as the content of nano clay increased from 0% to 3%, the maximum dry unit weight decreased from 15.20 kN/m3 to 13.43 kN/m3.
Optimum moisture content
By adding 0.5% nano clay, optimum moisture content decreased by about 0.6%, after which it increased in a consistent fashion by about 1.3% as there was an increase in the content of the nano clay from 0.5% to 3%. Nanoclay’s effect on the compaction curve's shape is a little more pronounced on the optimum dry side. In general, The results are similar to the results that were seen in past studies.
Unconfined compressive strength (UCS) tests
Unconfined compression tests were also done. Loess soils were amended with 2%, 1%, 0.5%, and 0.2% nano clay by dry weight to prepare specimens. As the content of the nano clay increases, a major increase comes in the UCS of the loess, with a two-fold increase in strength being showed by the 2% nano clay specimens in comparison with the strength of the natural loess soil.
According to observations, strain at failure faced a major increase for the mixtures in which nano clays were present in larger percentages, and a stiffer response was displayed by stabilized specimens that were proportional to the nano clay's amount that was utilized. This behavior is due to the nano clay particles’ high specific surface area as it makes the nano clay particles more reactive as compared to the loess soil particles. There are beliefs that the interparticle forces functioning between the clay particles in the nano clay and the soil matrix result in compressive strength and greater cohesion. In general, The results of the other researchers and these results are consistent.
Unconsolidated undrained triaxial compression (UU) tests
UU tests were also done. The mixture’s shear strength was increased by the net result of the change in cohesion and friction angle. Khalid et al. made similar observations.
Collapse potential tests
A collapse potential test was done on the loess soil on a precisely trimmed field specimen as the wetting-induced collapse was considered a main reason for the observed irrigation channel failure. The specimen was loaded in its natural condition on testing's commencement, inducing a few minor pore collapses. Then, the specimen was totally inundated for inducing extra pore collapse and unloaded and that is the final step in the testing process. CP ¼ De 1 þ e0 ð1Þ is the equation that was used for evaluating the collapse potential; here, natural void ratio is presented by e0, change in void ratio due to wetting is presented by De, and the collapse potential is presented by CP.
Field modification of loess soils with nano clay
Other than the laboratory tests, the applicability of nano clay stabilization at a field scale was assessed by conducting an exploratory pilot field study. A huge amount of attention has been given to them for the evaluation of surface scour and loess soil's erosion that had been amended with nano clay in an exposed channel. A six-meter section was selected from the irrigation channel for field evaluation.
Removal of concrete liner
A motorized excavator was used to remove the concrete liner and the exposed length was divided into 3 two-meter subsections, and for each subsection, the top 50 cm of the site soil was mixed with 2%, 1.5%, and 1% nano clay by weight for preparing three loess-nano clay mixtures.
The production of nano clays has massively increased over the past years as their uses have been added up in an enhanced form. The benefits of nano clays are vast and hence, they need to be produced at larger scales. Their sustainable production has been a challenging yet rewarding road to success.