Clay-water System Research Articles

A new approach for identification of dispersivity class of soils by combining physical and chemical tests

The safety and performance of embankment dams after impounding water are highly related to the characteristics of the selected dam material. Impermeability is achieved by using clay soils as the core material in embankment dams. It has been observed that there are many collapsed dams, especially when dispersive soils with very low internal erosion resistance are used in the impermeable zone. For this reason, such soils should be determined at the design stage and not used in the construction of embankment dams. The dispersion mechanism is known to be controlled by clay mineralogy and cations in the pore fluid of soil. The physicochemical behaviour of the clay-water system is crucial to clarify the dispersion mechanism. Physical and chemical tests used for determining the dispersibility potential of soils are extensive and time-consuming methods. An alternative method has been proposed within the scope of this study. Three user-friendly and practical models based on Gene Expression Programming (GEP) were produced using chemical and double hydrometer test results to predict dispersion percentage (D%). In addition, a new classification criteria was proposed to identification of dispersivity class based on pinhole test data. If the dispersion percentage is less than 12% is soil is classified as non-dispersive, the dispersion percentage is between 12% and 30% and greater than 30%, it is defined as intermediate and dispersive, respectively. With this proposed criterion, more consistent and successful results were obtained for the dispersivity class.

Modified Kozeny-Carman equation for estimating hydraulic conductivity in nanoscale pores of clayey soils with active surfaces

Hydraulic conductivity is a critical parameter for studying the behavior of clay-water systems. However, accurately estimating saturated hydraulic conductivity k in clayey soil by using the Kozeny-Carman (KC) equation is challenging due to the neglect of its active surface properties and nanoscale pores. Clay surfaces can have diverse characteristics resulting from various physicochemical processes such as isomorphous substitution and are typically characterized by the Cation Exchange Capacity (CEC). These properties can significantly impact fluid transport through the clay matrix. To address this issue, this study modifies the KC equation by incorporating an adsorbed water proportion φ that considers the occurrence state of pore water and its correlation with pore diameter, making it applicable to clayey soils. Molecular dynamics (MD) models are established to investigate the influence of surface interaction (CEC) and nanopore size (r) on saturated hydraulic conductivity k of seepage flow in clayey soils. Based on the MD results, a specific function is proposed to quantify the adsorbed proportion φ, which is linearly related to the root of CEC and reciprocal of pore size (r), and then incorporated into a proposed hydraulic correction ratio k* to modify the classical KC equation. The modified KC equation is validated using experimental data from literature, showing a high R2 value of 0.96. This result demonstrates that the proposed correction ratio k* can extend the application scope of the classical KC equation to clayey soils.

Modeling microstructural mechanical behavior of expansive soil at various water contents and dry densities by molecular dynamics simulation

Microstructural behaviour plays an important role in building up the constitutive model of expansive soil. However, the responses of microstructural mechanical behaviour of expansive soil subjected to different water contents under varying dry densities are still poorly understood. In this study, we computed the bulk modulus and shear modulus of montmorillonite under various water contents and dry densities through molecular dynamics (MD) simulations. The mechanism that controls the changing of elastic properties subjected to water content and dry density changes was revealed by energy variation and water density field evolution. Results show that the microscopic bulk modulus is strongly influenced by dry density and water content while the dependence of shear modulus with water content and dry density is not as significant as bulk modulus. The comprehensive micromechanical studies demonstrate the critical water contents identified from either energies (bond, angle, and interfacial energies) or bulk/shear modulus are all highly related to the water content with full microscopic water filling ratio. The microscopic water filling ratio of the interlayer space thus can be considered as the key fundamental factor to control the variation of bulk/shear modulus through changing the energies in clay-water system.

Specific cation effects on surface reactions of HgCl2 in clay-water systems

The significant complexity of the chemical forms and reactions of HgCl2 in clay-water systems make elucidating the corresponding interaction mechanisms and activities challenging. The specific ion effect and its mechanisms for HgCl2 adsorption at charged clay surfaces were investigated in this study. Experimental results showed that (1) at a fixed background concentration, the adsorption capacity and adsorption energy of HgCl2 on montmorillonite surface decreased in the order Li+> > Na+ > K+, and HgCl2 adsorption exhibited a strong specific cation effect. (2) For the same background cation (Li+, Na+ or K+), the adsorption capacity and energy of HgCl2 decreased as the cation concentration increased. An analysis of the origin of the adsorption energy for HgCl2 showed that this energy linearly increased with the surface electric field strength of montmorillonite. On the basis of orbital asymmetric hybridization theory, we speculated that specific adsorption of the HgCl2 molecule occurs via “polarization-induced covalent bonding” adsorption between O (clay surface) and Hg (HgCl2) atoms. The adsorption energy linearly increased with the surface electric field strength in the clay-water system. These findings provide useful information on modifying the physical, chemical and biological activities of HgCl2 in clay-water systems.

Stick-slip behavior of a clayey crustal fault

Recent studies have revealed that slip on clay-rich fault planes, which develop in the shallow crust and can cause huge earthquakes, is controlled by their flow properties rather than their frictional properties. Here, we simulate a seismogenic crustal fault and show that such a fault plane may behave as a thixotropic yield stress fluid. We then conduct numerical experiments using a simple spring-slider model that incorporates this thixotropic property, and we successfully reproduce spontaneous stick-slip behaviors that correspond to a variety of seismogenic processes, ranging from regular earthquakes with high slip rates to slow earthquakes with lower slip rates. This finding suggests that the seismic activity on a shallow clay-rich crustal fault may be governed by the time-evolving microstructure of the clay-water system that exists along the fault plane.

Determination of Dispersive Erosion Resistance in Fine-Grained Soils with Newly Developed Test Equipment

Abstract Dispersive soils cause great damage to hydrophilic earth structures and unprotected slope surfaces because of their high erosion sensitivity. In fine-grained soils, erosion resistance decreases with increased dispersibility. The dispersion mechanism of clays is controlled by the clay mineralogy and by the physicochemical repulsive forces in the clay–water system. The erosion resistance of dispersive soils can be determined by surface and internal erosion tests. In this study, the internal erosion resistance of soils was determined by using new test equipment that allowed the flow to pass through a hole when the dispersion mechanism in fine-grained soils was activated. The experiments for this study were performed under a single hydraulic head on two different natural dispersive soils with similar clay mineralogy. In this experimental system, both uniform and fully developed flow conditions were achieved. Time-dependent flow rates obtained from the experimental system can be used to determine hydraulic parameters, such as energy grade line, at very low error rates with the help of basic theorems of pipe hydraulics in theoretical hydraulic models, which were formed using a physical hydraulic model. Moreover, the erosion rates were quantitatively determined by using the continuity equation, and critical shear stresses were qualitatively compared for internal erosion developed by the dispersion mechanism. The sand/clay ratio determined the erosion resistance and behavior of the dispersive soils.

Exchangeable cations (Cu2+, Zn2+) effects on unfrozen water content in clay-water system using 1H NMR method

The1H-NMR method has been applied to study unfrozen water content in three model montmorillonites in their natural state and after ion exchange for Cu2+ or Zn2+ cations, being toxic for the environment. The research indicates statistically significant effect of exchangeable cation type on the variation in unfrozen water content. The proposed empirical models are able to predict the unfrozen water content after the exchange for the Cu2+ or Zn2+ ions using the clay fraction as a soil parameter much better than some of the known models based on the specific surface area and, at the same time, on the specific surface area and the clay fraction.

Testing sensitive clays through time and length scales

In the Nordics, engineering of sensitive soils is of vital importance for realising the urban environment and the infrastructure connecting and underpinning it. The hydro-mechanical behaviour of sensitive clays is strongly affected by the geological deposition history in the area and subsequent human activities resulting in changes in stress and environmental loading. Finally, the characteristic physico-chemical identity of the clay-water system is expressed by the sensitivity of the material. A combination of classic geotechnical tests in the laboratory needs to be complemented with state-of-the-art technologies for material analyses, in order to deepen our understanding of the interaction between the colloidal nature of the clay and the observed response at the engineering scale. Until now these activities have been performed separately. The microstructural observations using various microscopy techniques were not directly combined with classic geotechnical tests. In contrast, the work presented herein showcases methodologies for simultaneous monitoring of fabric and mechanical probing under controlled conditions on samples of sensitive clay. In order to enable real-time fabric measurements of samples of sensitive clay two non-invasive techniques are utilised: X-ray Scattering (XS) and X-ray Computed Tomography (XCT). Furthermore, a bespoke apparatus for sample probing is designed and built at Chalmers University of Technology. The design of the apparatus is adapted to the special challenges of very soft samples and addresses issues of sample quality in soft soil testing (i.e., sample mounting, membraneless configuration). The intention of this work is to demonstrate the feasibility of expanding geotechnical testing outside the limits of the traditional geotechnical laboratory, combining geotechnics with state-of-the-art technologies for material analysis. This, in the end, provides a critical view on the perception of the material and its constituents that aims to contribute in improvement in geotechnical laboratory testing and the development of advanced constitutive models for sensitive clays.

Volume change behavior of reconstituted marine clay: effect of initial and leaching pore fluid salinity

Because the salinity of marine clay changes continuously in response to the migration of seawater toward land or freshwater from river to sea, the intrinsic pore fluid salinity of the clay-water system may change periodically. Recent studies indicate that pore-water salinity changes the intrinsic properties of reconstituted homoionic artificial clay. However, the effect of seawater on the intrinsic properties of marine clay is unknown. The aim of this study was to investigate changes in the compression parameters of marine clay for the leaching salinity of seawater with respect to tap water. The results show that ratio $${e}_{0}/{e}_{\mathrm{L}}$$ is more sensitive to salinity than $${e}_{100}^{*}$$ . Although the Iv − log p for all samples at different intrinsic/leaching salinities were located on the intrinsic compression line (ICL); some deviation from the ICL was observed at low-stress levels. In high-salinity reconstituted samples, the osmotic gradient caused the low-salinity water to penetrate the clusters and separate individual particles, increasing the size of intra-cluster pores and settlement during consolidation. The distance between the particles at any stress level was larger than in diffuse double layer theory because of a deviation from the assumed parallel fabric in the theory.

Peridynamic modelling of clay erosion

Experimental observations of the erosion of clays indicate a high degree of coupling between the hydro-chemical processes in the clay–water system and the mechanics of erosion. In contrast with the volume of experimental work and the significance of swelling clay erosion in many geotechnical engineering problems, limited advances have been made with the predictive modelling of this phenomenon. The critical erosion step, missing in existing models, is the particles’ detachment from the moving boundary of the expanding clay – a discontinuous process. This paper presents, for the first time, a non-local formulation for clay erosion, which brings together clay swelling, particle detachment and detached particle transport, into a single modelling tool. The detachment criterion, an essential element of the integrated erosion model, is first tested against a series of experimental benchmarks to show the excellent agreement between calculated and experimental results. The integrated erosion model is subsequently validated by comparison with experimental data for the behaviour of compacted bentonite under several eroding environments. The results: (a) show clearly the capacity of the model to capture concurrently the free swelling of clay, the detachment of clay particles and the transport of detached particles in the eroding environment; and (b) support strongly the applicability of the model to account for the hydro-chemical conditions (composition and velocity) of the eroding environment. The proposed multi-physics non-local formulation, successfully validated for hydro-chemical effects on clay erosion, provides a robust framework for incorporating a wide set of additional couplings.

Wettability of clay aggregates—A coarse-grained molecular dynamic study

Clay minerals play a vital role in environmental pollution remediation due to their special wettability performance. However, limited by experimental technology and computational costs, wettability mechanism of clay aggregates at mesoscale remains unclear. In this work, the coarse-grained molecular dynamic simulation method has been used as a novel strategy to discover the mesoscale wettability of clay aggregates with water. Through visualization analysis, we have observed that the water droplet firstly got in touch with the clay surface, then some water molecules flowed into clay pores while some adsorbed onto clay platelets edge. With further invasion of water, clay platelets rearranged and reoriented to fit themselves. The clay-water system reached equilibrium under three mechanisms, including normal blocking, non-normal diffusion and interlayer adsorption. The final inhomogeneous equilibrium water droplet had an average contact angle value of 81.5°, which is in good agreement with preceding experimental measurement. These findings help for a better understanding on the wetting property of clay mineral surface and fill a gap of the wettability study on mesoscale level.

Microstructural Investigations on Plasticity of Lime-Treated Soils

The surface charge distribution of clay particles governs the interparticle forces and their arrangement in clay-water systems. The plasticity properties are the consequences of the interaction at the microscopic scale, even if they are traditionally linked to the mechanical properties of fine-grained soils. In the paper, the plasticity modifications induced by the addition of lime were experimentally investigated for two different clays (namely kaolinite and bentonite) in order to gain microstructural insights of the mechanisms affecting their plastic behavior as a function of the lime content and curing time. Zeta potential and dynamic light scattering measurements, as well as thermogravimetric analyses, highlighted the mechanisms responsible for the plastic changes at a small scale. The increase of the interparticle attraction forces due to the addition of lime increased the liquid and plastic limits of kaolinite in the short term, without significant changes in the long term due to the low reactivity of the clay in terms of pozzolanic reactions. The addition of lime to bentonite resulted in a decrease of interparticle repulsion double layer interactions. Rearrangement of the clay particles determined a reduction of the liquid limit and an increase of the plastic limit of the treated clays in the very short term. Precipitation of the bonding compounds due to pozzolanic reactions increased both the liquid and plastic limits over the time.

The effects of potentially toxic metals (copper and zinc) on selected physical and physico-chemical properties of bentonites

The purpose of this study was to determine the effect of copper or zinc ions, absorbed by soil on its physical and physicochemical properties. The change in these properties may reduce the soil usefulness as a mineral protective barrier, for example, on hazardous waste landfills. Parameters such as granulometric composition, effective particle size d10, empirical hydraulic conductivity, Atterberg limits, colloidal activity, specific surface area, sorption moisture content, and montmorillonite content were determined. The tests were carried out on model Na+ or Ca2+ samples of American bentonites (SWy-3, Stx-1b) and Slovak bentonite from Jelšový potok (BSvk), subjected to ion exchange for Cu2+ or Zn2+ ion. The content of elements was determined using inductively coupled plasma optical emission spectrometry (ICP-OES). Regression analysis showed a significant effect of Zn2+ ions on the reduction of sorption moisture content w95 and the increase in the hydraulic conductivity. Nearly complete negative correlation was obtained between the Cu2+ ion content and the specific surface area, sorption moisture content w50, and montmorillonite content (R = -0.99). It was observed that the significance of the influence of Cu2+ and Zn2+ ions on specific clay properties differed, which indicates different behavior of these metals in the clay-water system. The different nature of clays contaminated with Cu2+ and Zn2+ ions justifies the need to continue research on other potentially toxic metals and to further search for prediction equations of the cohesive soil hydraulic conductivity based on soil parameters that are most frequently modified as a result of their impact.

Flocculation behaviour of bentonite clay-water systems

ABSTRACTThe effect of polyacrylamide (PAM) as flocculant, NaCl electrolyte concentration and pH on the degree of flocculation of bentonite suspension has been investigated. It has been observed that the increase in PAM dosage reduces the residual turbidity and improves the settling rate of the bentonite particles. At higher pH conditions bentonite dispersions are highly flocculated having high settling rates and low supernatant turbidity. The combined effect of salt concentration and PAM dosage improved the overall flocculation efficiency of the system.

The behaviour of water on the surface of kaolinite with an oscillating electric field.

A quantitative understanding of oscillating electric field effects on the behaviour of water on the surface of kaolinite is vital for research in the field of clay–water systems. The behaviour of water molecules on the (0 0 1) and (0 0 −1) surfaces of kaolinite are systematically investigated in the absence or presence of an oscillating electric field using molecular dynamics simulations. The simulated results demonstrate that the applied oscillating electric fields parallel to kaolinite surface contribute to decreased amounts of adsorbed water molecules on the (0 0 1) surface of kaolinite. The oscillating electric field performs an inconspicuous effect on the adsorption of water on the (0 0 −1) surface of kaolinite. The behaviour of water on the surface of kaolinite will be impacted more severely by oscillating electric fields. Our results demonstrate that water molecules will rotate following the directions of the applied fields, which causes the decrease of hydrogen bonds, and thus, the weaker water–kaolinite interactions due to the applied field drive water molecules away from kaolinite surfaces. These results are of significance to understand the mechanisms of the oscillating electric fields affecting the behaviour of clay–water systems.

Influence of temperature on the swelling pressure of bentonite clay

The swelling pressures of bentonites have been studied extensively due to their potential use as a barrier in radioactive waste disposal repository. Due to the radioactive decay of the waste, the temperature will increase in the repositories. Several experimental studies of the temperature dependence of the swelling pressure of smectites have been published while theoretical studies have received less attention. Atomic level simulations have been proven to be useful in broadening the understanding of clay materials in various conditions. Therefore, molecular dynamics simulations were carried out to predict thermal effects on the swelling pressure of a sodium smectite clay. The study focused on two temperature regimes. In the first regime, the clay was placed in a frozen environment and heated while the second regime involves heating the clay-water system from room temperature. At low temperatures gradual melting is observed and the onset of the swelling takes place around 250 K. At temperatures in the range of 300 K–600 K a slight increase in the swelling pressure is observed. The results can be used to rationalize experimental findings in the swelling pressure studies of smectite clays.

In-Situ Determination of Specific Surface Area of Clays

There are many methods to determine the specific surface area (SSA) of clay minerals. Some of these methods lack accuracy and some are time consuming. There are no methods available for the determination of in situ SSA. In this paper a new approach to predict the SSA of clays is proposed, which has been verified in the laboratory. This method is based on the electrical properties of soils. In this paper the experimental set up and the estimation of specific surface area is presented on the basis of variation of the dielectric constant and the conductivity of the clay–water system when subjected to alternating current in the frequency range of 1–100 MHz. The dielectric constant and conductivity respond differently for different soils with varying surface area corresponding to their mineralogy. When dielectric constants and conductivity were measured for sandy soils, there were no effects with the change of input frequency. The electrical method can identify clays and separate them from fine grained silts. Data are presented which shows a linear correlation between the magnitude of dielectric dispersion and SSA for various clay mixtures. The clay structure determines the value of in situ dielectric dispersion of the clay (Δeo) and hence the SSA, making this a good in situ method to estimate SSA of clays.

The Amounts of Water Adsorbed to the Surface of Clay Minerals at the Plastic Limit

Abstract The paper presents results of a study on the amount of water associated with the solid phase of the clay water system at the plastic limit. Two model monomineral clays, namely kaolinite, and montmorillonite, were used in the study. The latter was obtained by gravitational sedimentation of Na-bentonite (Wyoming). The calculated mean number of water molecule layers on the external surface of montmorillonite was 14.4, and water in interlayer spaces constituted 0.3 of the water mass at the plastic limit. The number of water layers on the external surface of kaolinite particles was 63, which was related to the higher density of the surface electrical charge of kaolinite compared to that of montmorillonite. The calculations were made on the basis of the external surface area of clays and the basal spacing at the plastic limit measured by an X-ray diffraction test. The external surface area of clays was estimated by measuring sorption at a relative humidity p/p 0 = 0.5.

Molecular dynamics simulation of hydrated Na-montmorillonite with inorganic salts addition at high temperature and high pressure

The swelling behaviors of Na-montmorillonite (Na-Mt) in non-ambient environment have been the subject of considerable speculation due to the difficulties associated with their study. Inorganic salts are often used to minimize clay mineral hydration. The influence of inorganic salts addition (NaCl, CaCl2, KCl) on the hydration of Na-Mt at high temperature and high pressure (HTHP) was investigated by molecular dynamics simulation at 6GPa and different temperatures (200, 300, 400, 500 and 600K) in an isobaric isothermal ensemble (NPT). Water model and force field used in the clay-water system are SPC/E model and universal force field (UFF), respectively. The results show that after adding inorganic salts into Na-Mt, the mobility of the interlayer species increases, and the hydration shell of the interlayer cations decreases with a rise in temperature. The influence of temperature on the diffusion of water molecules is much greater than that of the ions present in the interlayer. Temperature increase and hydration degree play an important role on the diffusion behavior of the interlayer species. The size, valence, mass and hydration energy of cations present between the layers affect the structure of Na-Mt interlayer. The inorganic salts are able to inhibit the hydration of Na-Mt by reducing the mobility of the interlayer species and the basal spacing. After analyzing the effect of each salt on Na-Mt, it was found that, the most stable state of Na-Mt at HTHP is achieved with KCl addition, compared with NaCl and CaCl2. It is expected the results obtained from this study would help to understand the inhibition effect of inorganic salts on Na-Mt, and predict the swelling of Na-Mt at HTHP.

Clay hydration mechanisms and their effect on dustiness

Clays are employed in a wide variety of industries such as ceramic industry, manufacture of paper, rubber, etc. In this sense, it is well known that at industrial processes in which clayey materials are used, such as ceramic industry, in order to carry out some specific stages, the wetting of clays is commonly required. Moreover, it is also long established that wetting is an appropriated measure to reduce particulate matter emissions during clays storage and handling.The present study was undertaken to assess the influence of moisture on clay dustiness because, though the complex behaviour of the clay–water system has been known since antiquity, the mechanisms involved in clay hydration and their influence on dustiness are still not well understood. To encompass a wide range of specific surface areas, three clays and a kaolin were studied. Chemical and mineralogical analysis of these four raw materials was performed and their particle size distribution, flowability, true density, plastic limit, and specific surface area were determined. Raw materials dustiness was determined using the continuous drop method.As against what might intuitively be expected, the results showed that the relationship between moisture and dustiness was quite complex and strongly related to the hydration mechanisms. In this regard, to better understand the phenomena involved in the clay hydration process, a specific methodology was developed to estimate the critical points of the clay hydration process (regarding dustiness). This methodology can be readily applied to other clays or even to materials of different nature to predict the optimum moisture and, therefore, it could be employed to propose specific measures which could entail an improvement of outdoor and indoor air quality.