Friction characteristics are critical mechanical properties of clay, playing a pivotal role in the structural stability of cohesive soils. In this study, molecular dynamics simulations were employed to investigate the shear behavior of undrained montmorillonite (MMT) nanopores with varying surface charges and interlayer cations (Na+, K+, Ca2+), subjected to different normal loads and sliding velocities. Consistent with previous findings, our results confirm that shear stress increases with normal load. However, the normal load-shear stress curves reveal two distinct linear regions, indicating segmented friction behavior. Remarkably, the friction coefficient declines sharply beyond a critical pressure point, ranging from 5 to 7.5 GPa, while cohesion follows an inverse trend. The elevated friction coefficient at lower pressures is attributed to the enhanced formation of hydrogen bonds and concomitant changes in density distribution. Furthermore, shear strength was observed to increase with sliding velocities, normal loads, and surface charges, with Na-MMT exhibiting superior shear strength compared to KMMT and Ca-MMT. Interestingly, the friction coefficient shows a slight decrease with increasing surface charge, while ion type exerts a minimal effect. In contrast, cohesion is predominantly influenced by surface charge and remains largely unaffected by ion type, except under extreme pressures and velocities.

This research compares the stabilization efficiency of kaolinite and montmorillonite clayey soils using two industrial and agricultural by-products, namely fly ash (FA) and rice husk ash (RHA), activated by sodium hydroxide (NaOH). To this end, various proportions of FA and RHA (i.e., 0%, 5%, 10%, 15%, and 20%), along with NaOH solutions at 2 M and 4 M concentrations, are utilized to treat both low-and high-plasticity clayey soils. The resulting geopolymers are then subjected to a wide range of mechanical and micro-structural tests, including standard compaction, unconfined compressive strength (UCS), ultrasonic pulse velocity (UPV), swelling potential, scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), and Fourier transform infrared spectroscopy (FTIR). Results show that incorporating both FA and RHA into kaolinite and montmorillonite clays up to their respective optimal contents significantly enhances all their mechanical properties. However, FA-based geopolymers exhibit superior mechanical properties compared to RHA-based ones under similar additive contents and curing conditions. Accordingly, the optimal FA content is found to be 15%, while for the RHA-based geopolymers, the peak UCS is observed at 15% and 10% RHA for kaolinite and 10% and 5% RHA for montmorillonite when treated with 2 M and 4 M NaOH solutions, respectively. The results also suggest that FA is more effective than RHA in controlling the swelling potential of both kaolinite and montmorillonite soils. Microstructural analyses further corroborate the findings of macro-scale experiments by showcasing the comparative occurrence of geopolymerization, as well as the formation of cementitious gels, and synthesis of new chemical products.

To enhance the barrier performance of biomass films, carboxymethyl cellulose (CMC) was combined with montmorillonite (MMT) modified by stearyltrimethylammonium bromide (STAB) and loaded with Fe3O4 particles as a nano-filler, and a CMC/m-OMMT mulch film was fabricated using magnetic field orientation. The characterization of m-OMMT was conducted through Fourier-transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), and vibrating sample magnetometry (VSM), which confirmed the successful intercalation of STAB into the MMT structure, along with the effective loading of Fe3O4 particles onto the MMT matrix. A comprehensive investigation into the mechanical properties of CMC/m-OMMT films revealed that, in the dry state, the films exhibited a tensile strength of 29 MPa and an elongation at break of 64 %. A series of barrier performance tests were conducted on the films. The findings demonstrated that the incorporation of MMT and the application of a magnetic field substantially enhanced the water contact angle, increasing it from 86 degrees to 112 degrees. Additionally, water vapor permeability increased by approximately 30 %, soil erosion was reduced by about 22 %, and UV resistance was notably improved by 94 %. Moreover, scanning electron microscopy (SEM), thermogravimetric analysis (TGA), and biodegradation tests on the CMC/m-OMMT/40mT films revealed that the magnetic field effectively oriented the MMT nanosheets within the composite matrix. This study presents a novel approach for enhancing the barrier properties of biomass-based mulch films.

The mechanical behavior of expansive soil in geotechnical engineering is significantly sensitive to loading rates, hydration, confining pressure, etc., where most engineering problems are attributed to the existence of montmorillonite in expansive soil. Here, the hydration, confining pressure, and loading rate effect on the mechanical behavior of montmorillonite were investigated through the triaxial tests and molecular dynamics (MD) simulation method, revealing their fundamental mechanism between the microscale and macroscale. The average basal spacing of hydrated montmorillonite system, the diffusion coefficient and density distribution of interlayer water molecules were calculated for the verification of MD model. The experimental results indicated that the stress-strain relationship of montmorillonite was the strain-hardening type. The failure stress did not increase monotonously with the increase in loading rate, and there were two obvious critical points. The failure stress of the soil sample increased with the increase of the confining pressure, and the decrease of the water content, where their fundamental mechanism between microscale and macroscale were adequately discussed. Furthermore, the stress-strain response, total energy evolution, deformation evolution of atomistic structure, and broken bonds evolution were analyzed to deeply understand the fundamental deformation mechanism at the microscale. The multi-scale studies could effectively examine the macroscopic mechanical behavior of expansive soil and elucidate its microscopic mechanisms.

Construction resting on soil and rocks containing montmorillonite (MMT) are prone to damage induced by swelling, which involves a significant release of energy. It is often desirable to enhance these soils to mitigate swelling potential, regulate volume changes, and manage energy release. Experimental findings suggest that increasing temperature is one method to improve these soils, with water content, initial volume, and boundary conditions also influencing the swelling mechanism. This study utilizes ab initio molecular dynamics calculations to explore changes in volume and energy within MMT unit cells at the nanoscale due to temperature variations. The response of unit cells of MMT with varying dimensions and quantities of water molecules to temperature is assessed under constrained and unconstrained conditions. Results indicate that the volume changes and energy release of unit cells in response to temperature are contingent upon the presence of water molecules. In unit cells containing water molecules, both energy and volume decrease with rising temperature, whereas in unit cells devoid of water molecules, energy decreases while volume increases as temperature rises. Given the inherent association of soils with water in natural settings, it can be deduced that increasing temperature presents a viable method for enhancing naturally occurring MMT-dominated soils. Density functional theory calculations demonstrate that alterations in the volume and energy of MMT stem from shifts in interactions among the minerals, cations, and water molecules, as well as intrinsic structural defects like isomorphic substitution and peroxy links within the unit cells. These modifications induce variations in charge carriers and electrical properties, consequently influencing volume and energy changes within MMT unit cells. Additionally, it was observed that the failure of peroxy links can significantly impact the optimal temperature selection for the thermal enhancement of MMT.

Soils contaminated with per- and poly- fluoroalkyl substances (PFAS) require immediate remediation to protect the surrounding environment and human health. A novel animated clay -polymer composite was developed by applying polyethyleneimine (PEI) solution onto a montmorillonite clay-chitosan polymer composite. The resulting product, PEI -modified montmorillonite chitosan beads (MMTCBs) were characterized as an adsorptive soil amendment for immobilizing PFAS contaminants. The MMTCBs exhibited good efficiency to adsorb the PFAS, showing adsorption capacities of 12.2, 16.7, 18.5, and 20.8 mg g -1 for PFBA, PFBS, PFOA, and PFOS, respectively, which were higher than those obtained by granular activated carbon (GAC) (i.e., an adsorbent used as a reference). Column leaching tests demonstrated that amending soil with 10% MMTCBs resulted in a substantial decrease in the leaching of PFOA, PFOS, PFBA, and PFBS by 90%, 100%, 64%, and 68%, respectively. These reductions were comparable to the values obtained for GAC-modified soil, particularly for long -chain PFAS. Incorporating MMTCBs into the soil not only preserved the structural integrity of the soil matrix but also enhanced its shear strength (kPa). Conversely, adding GAC to the soil resulted in a reduction of the soil ' s mechanical properties.

Controlled release of pesticides in response to environmental stimuli using hydrogels as carriers is a feasible approach to improve the effective utilization rates of pesticides. In this regard, modified carboxylated cellulose nanocrystal (CCNC)-based hydrogels with appropriate biocompatibilities and high specific surface areas have broad prospects. Accordingly, in this study, a pH -responsive hydrogel loaded with the pesticide thiamethoxam (TXM) (PEI-CCNC@A-MMT/TXM) was constructed by synergistically introducing CCNC modified with polyethyleneimine (PEI) into cost-effective acidified montmorillonite (A-MMT) via electrostatic self -assembly followed by combination with sodium alginate (SA) by emulsion - gel method via ionic crosslinking. PEI-CCNC@AMMT efficiently improved the mechanical properties of the SA hydrogel and ensured the stability and TXM loading efficiency of this hydrogel; however, the hydrogel stress increased from 9.48 to 41.44 kPa under 20 % compressive strain when the mass ratio of A-MMT to PEI -modified CCNC (PEI-CCNC) was increased from 0 to 0.8. PEI-CCNC@A-MMT/TXM exhibited significant controlled -release characteristics with the change in pH; specifically, with an increase in pH from 5.0 to 9.0, the cumulative release ratio of TXM increased from 53.62 to 94.86 wt % within 48 h of the addition of PEI-CCNC@A-MMT/TXM to the phosphate buffered saline solution. Fitting the six models to the release curves proved that swelling, dissolution, and diffusion acted together during TXM release, and release mechanisms for TXM under different pH conditions were proposed. The release behaviors of PEI-CCNC@A-MMT/TXM in soil indicated that this hydrogel effectively prolonged the release of TXM, and only 91.53 wt % TXM was released within 240 h after the hydrogel entered the soil. The bacterial activity revealed that the hydrogel did not destroy the microbial environment of the soil and demonstrated high biocompatibility. This study provides a promising strategy for regulating the pesticide release behavior, improving pesticide utilization, and reducing environmental pollution of pesticides via introducing low-cost AMMT and green CCNC into the SA hydrogel and applying this hydrogel as a pesticide carrier.

This paper discusses efforts made by past researchers to steady the expansive (problematic) soils using mechanical and chemical techniques - specifically with EPS beads, lime and fly ash. Administering swelling of problematic soils is critical for civil engineers to prevent structural distress. This paper summarizes studies on reduction of swelling potential using EPS, lime and fly ash individually. Chemical stabilization with lime and fly ash are conventional methods for expansive soil stabilization, with known merits and demerits. This paper explores the suitability of different materials under various conditions and stabilization mechanisms, including cation exchange, flocculation, and pozzolanic reactions. The degree of stabilization is influenced by various factors such as the type and amount of additives, soil mineralogy, curing temperature, moisture content during molding, and the presence of nano-silica, organic matter, and sulfates. Additionally, expanded polystyrene (EPS) improves structural integrity by compressing when surrounded clay swells, reducing overall swelling. Thus, EPS addresses limitations of chemicals by mechanical means. Combining EPS, lime and fly ash creates a customized system promoting efficient, long-lasting, cost-effective and eco-friendly soil stabilization. Chemicals address EPS limitations like poor stabilization. This paper benefits civil engineers seeking to control expansive soil swelling and prevent structural distress. It indicates potential of an EPS-lime-fly ash system and concludes by identifying research gaps for further work on such combinatorial stabilizer systems.

Montmorillonite (Mt) is a ubiquitous swelling clay mineral and major component of soft rocks, sediments, and soils with an inherent capability to sorb metal cations. This unique feature renders Mt important for the enrichment and mobilization of environmentally important metal cations, retardation of heavy metals and radionuclide ions, the evolution of clay mineral itself, soils and sediments, and other geological processes. Understanding the interfacial interactions of Mt with metal cations at the molecular level is of fundamental importance in all these processes, but still remains elusive, due to the chemical and structural complexity of Mt surfaces and the diverse chemistries of metal cations. In this Review, we aim to provide the reader with a comprehensive overview of the adsorption modes of metal cations on basal and edge surfaces of Mt, local chemical environments of the cation binding sites, the driving forces for metal sorption, and factors influencing the dynamics of cation uptake onto Mt surfaces. Various surface complexation models [i.e., nonelectrostatic model (NEM), constant capacitance model (CCM), diffuse layer model (DLM), and triple-layer model (TLM)], advanced spectroscopic techniques (i.e., NEM, CCM, DLM, and TLM), and atomistic simulation methods (i.e., MD, DFT, and FPMD) have been used in conjunction with macroscopic adsorption experiments to gain detailed insights into the interfacial interactions of metal cations on Mt. Mt adsorbs metal cations via three independent pathways: (1) cation exchange; (2) surface complexation; and (3) nucleation and surface precipitation. The principal driving force for cation exchange is electrostatic interaction, while chemical bonding governs the two other mechanisms that depend on the basal and edge surface properties of Mt. The siloxane cavities on the tetrahedral basal plane exhibit the strongest adsorption sites for cation exchange and are greatly affected by the the degree of Al3+/Si4+ tetrahedral substitutions. At the amphoteric edge surfaces bearing hydroxyl groups, metal cations could form mono/multidentate surface complexes on Mt [010] and [110] edges. Ionic strength, pH, the presence of competing cations, temperature, and layer charge have been shown to affect the adsorption mechanisms and quantity of adsorbed cations. The updated information on the interfacial interactions of metal cations with Mt basal and edge surfaces presented in this review provides an improved understanding of the enrichment of metals, formation of metal ores, and natural biogeochemical cycles, as well as may promote technological and engineering applications of this important clay mineral in environmental remediation, geological repository, petroleum exploration and extraction, and extraterrestrial research.

In the present study, three-axis dynamic tests were performed on different samples of soil, and the effect of montmorillonite based carbon nanotubes (CNT) and amorphous silica dioxide nanoparticles presence, density, hydrostatic pressure and moisture percentage on the elastic modulus and equivalent damping ratio of soil was investigated. Considering that in the present research, experimental studies and tests were carried out on three samples of sandy soil reinforced with 2 % volume fraction CNT, sandy soil reinforced with 8 % nanosilica and sandy soil reinforced with 0.5 % CNT and 4 % nanosilica. Also, the optimum moisture percentage has been determined for these types of soils. In general, with the increase of hydrostatic pressure and compaction of sand, the elastic modulus increases, and the amount of increase is different according to the type of nanoparticles. For 1 % applied strain and soil sand, with the increase of hydrostatic pressure from 100 kPa to 300 kPa, the elastic modulus of sand with 2 % CNT and 8 % nanosilica increases by 36 % and 214 %, respectively. This shows the favorable effect of silica nanoparticles on increasing the Young's modulus of sand by increasing the amount of hydrostatic pressure. The effect of CNT on improving the elastic modulus of sand in dry state is much higher than the effect of nanosilica. At 1 % strain and 4 % compaction, adding 2 % CNT to sand increases the elastic modulus of sand about 4 times compared to sand reinforced with 8 % nanosilica.