Bentonite Samples Research Articles

Penentuan Parameter Rangkak pada Bentonit sebagai Bahan Penyangga (Buffer Material) dengan Uji Geser Langsung

A preliminary research to investigate the creep behavior of bentonite as buffer material in a high-level nuclear or radioactive waste disposal hole as bentonite needs to maintain the stability of the waste canister for a long time, to avoid the displacement caused by the weight of the canister itself and the thermal, hydrological, mechanical and chemical (THM-C) processes of the near filed. The increase of bentonite time-dependent strain or creep needs to be taken into account for the safety of the disposal hole. The creep parameters obtained from the angular strain rate equation, through stress-controlled direct shear test using compacted SPV200 bentonite samples. The test results show an increase in angular strain rate with increasing of the applied stress.

Effects of bentonite heating on U(VI) adsorption

Engineered barrier systems designed to isolate high-level radioactive waste utilize bentonite, a montmorillonite-rich material, to restrict contaminant transport due to its low hydraulic conductivity and high adsorption capacity. High temperatures (100–200 °C) near waste canisters resulting from radioactive decay may alter the clay's ability to adsorb contaminants. In this study, we examine U(VI) adsorption onto two different bentonite samples subjected to (1) 18 years of in situ heating during an underground experiment and (2) short-term (7-week), high temperature (300 °C) heating in the laboratory. Results show that U(VI) adsorption was lower for field-heated bentonite located closest to the heater, which experienced temperatures of approximately 95 °C, compared to a control sample, which experienced temperatures of approximately 20 °C, over a range of aqueous chemical conditions. On average, Kd values for U(VI) adsorption were 31% lower for 95 °C heated samples. By contrast, U(VI) adsorption onto intermediate-heated bentonite (50 °C) was indistinguishable from the adsorption onto the cold-zone (20 °C) sample. U(VI) adsorption onto lab-heated bentonite was also lower than onto control bentonite over the pH range 4.5–8.0, with an average decrease in Kd values of 50% after heating. Lower U(VI) adsorption to field-heated bentonite persisted after bentonite was purified to isolate the clay fraction. This allows us to rule out changes in pore-water chemistry or accessory mineral composition as causes of the lower adsorption. No evidence of montmorillonite illitization was observed in the heated samples. While some of the lower U(VI) adsorption in the lab-heated bentonite can be explained by changes in aqueous U(VI) speciation, we propose that lower U(VI) adsorption to field-heated bentonite may be primarily due to changes in the montmorillonite edge structure. The observed changes in U(VI) adsorption to bentonite after heating have implications for U(VI) diffusive transport through engineered barriers and must be considered when designing radioactive waste disposal repositories.

Fractal characteristics of pore structure of compacted bentonite studied by ESEM and MIP methods

In this paper, we aim to clarify microstructure of bentonite from Cerny vrch deposit in the Czech Republic. We adopt results of ESEM and MIP experiments performed at various suctions along wetting and drying paths on bentonite samples compacted from powder to two different initial dry densities. The data were used for quantification of fractal dimension characteristics of pores of different sizes. Two different methods of calculating fractal dimension were used for MIP data, and one method was used for evaluation of ESEM images. Fractal dimensions obtained from MIP data, combined with the measured pore size density functions, allowed us to identify two different pore families: micropores and macropores. Macropores can be further subdivided into fine macropores and coarse macropores based on fractal analysis. The pore systems were further distinguished by different responses to suction changes and to compaction effort. In general, we observed slight increase in fractal dimension with increasing suction and with increasing dry density.

The mineralogical characteristics between opaline silica in bentonite and α-cristobalite

The aim of this work was to distinguish opaline silica polymorphs from α-cristobalite based on their different crystallinity and dehydration. Opaline silica can be misidentified as α-cristobalite using X-ray diffraction (XRD) analysis only because of the similar characteristic XRD patterns. In order to distinguish opaline silica polymorphs from α-cristobalite, centrifugal classification was applied to three different raw bentonite samples. After centrifugal classification, all the separated samples were characterized by XRD, Fourier transform infrared (FTIR) spectroscopy and thermogravimetric Analysis (TGA). It was observed that the crystallinity of the controversial silica polymorph in bentonite was significantly poorer than the high temperature α-cristobalite. The FTIR spectra showed there were no strong marked absorbance characteristic bands of α-cristobalite in all samples from bentonites. Furthermore, Thermogravimetric Analysis (TGA) of the samples directly proved that the silica polymorph in bentonite samples were a form of hydrated silica which contains hydroxyl(-OH) in the structure. Based on these studies, it was concluded that the China north bentonite samples contained mainly the Opaline silica not the α-cristobalite.

The role of geological origin of smectites and of their physico-chemical properties on aflatoxin adsorption

Since 2013, bentonite in the form of dioctahedral smectite is an additive authorised in the EU as a substance for the reduction of the contamination of feed by aflatoxins. Several studies indicate a big difference in the effectiveness of smectites in sequestering aflatoxins. A clear correlation between mineralogical and physico-chemical properties of smectites and aflatoxin adsorption has not been well established. In the effort to identify the most critical mineralogical, chemical, and physical properties that affect aflatoxin adsorption by smectites, 29 samples of bentonites obtained from different sources around the world were evaluated. “As received” samples were divided into two main groups, i.e. hydrothermal (n=14) and sedimentary (n=15) bentonites depending on their geological origin. The characterization studies showed that all samples contained dioctahedral smectite as major mineral; a moderate CEC value (60-116 cmol/kg); the presence of iron; a small organic matter content; a near-neutral pH; and a fine and uniform particle size (<45μm). They differed substantially in their sodium, calcium and magnesium contents, and in the swelling properties depending on the geological origin. Several in vitro adsorption studies showed that they also differed in a significant manner in adsorbing aflatoxin B1 (AFB1). A correlation between geological origin and AFB1 adsorption capacity was found (p<0.001), being sedimentary smectites significantly more effective than hydrothermal ones in adsorbing the toxin at different pH values. The extent of AFB1 adsorption by all samples was negatively and linearly correlated to the extent of desorption, and sedimentary smectites were significantly more effective than hydrothermal smectites in keeping bound the adsorbed fraction of the toxin (p < 0.001). In addition, correlation studies using the Pearson statistical method showed a significant relationship among some physico-chemical properties of smectites and the amounts of adsorbed toxin. In particular, AFB1 adsorption by smectites correlated positively with sodium content and swell index, but negatively with d001-value, magnesium and calcium contents. In conclusion, it seems that the geological origin of smectite is a useful guide for the selection of bentonites for AFB1 detoxification. Sedimentary bentonites containing sodium/swelling-smectite should be preferred to hydrothermal samples as potential aflatoxin binders. Taking into account the geographical origin of our samples, this approach should be applicable to bentonites worldwide.

Preparation of MnO2/bentonite nanocomposite with enhanced photocatalytic activity under sunlight irradiation

In this work, MnO2/bentonite nanocomposite was constructed through a facile precipitation method and characterized using powder X-ray diffraction (XRD), scanning electron microscopy (SEM), and UV–Vis diffuse reflectance spectra (DRS). Removal of methylene blue (MB) from aqueous solution with MnO2/bentonite nanocomposite as photocatalyst under direct sunlight irradiation was investigated and compared with bare MnO2 and bentonite samples. The rate of photocatalytic degradation of MB by MnO2/bentonite nanocomposite reached about 87% after 2 h of irradiation under sunlight, which is much higher than that of the bare MnO2 and bentonite samples under similar experimental conditions. It was observed that the photodegradation kinetics of the dye followed the pseudo first order, according to the Langmuir–Hinshelwood kinetics model. Finally, the stability and reusability of the photocatalyst towards MB were investigated for five cycles. Since the dye pollution is a serious problem, this study may offer a highly efficient and cost effective photocatalyst for wastewater treatment under sunlight irradiation.

Calcium carbonate precipitation in compacted bentonite using electromigration reaction method and its application to estimate the ion activity coefficient in the porewater

ABSTRACT In the safety assessment of radioactive waste disposal, it is critical to understand the porewater chemistry in compacted bentonite in order to predict long-term migration behavior of radionuclides in the engineered barrier. This study estimates the activity coefficients of dissolved ions in the porewater of compacted bentonite from the concentrations of ions at which CaCO3 precipitation occurred. Solutions containing CaCl2 and NaHCO3 were introduced under electrical potential gradient from the opposite sides of the compacted Na-bentonite packed at the dry density of 1.0kg/dm3. After the electromigration, the spatial distribution of ions along the compacted bentonite sample was determined. Sequential extraction method was developed to distinctly determine the concentrations of free ions in the porewater and in solid phase in bentonite. The results show that the exchangeable Na+ ions were progressively replaced by the incoming Ca2+ ions, and the compacted bentonite sample can be divided into three zones: Ca-, Ca-/Na-, and Na-bentonite zones. Precipitates of CaCO3 were observed both in Ca- and Ca/Na-bentonite zones. The experimentally determined activity coefficients were at least smaller by a factor of 3 compared to the theoretical approximation calculated using PHREEQC code assuming dilute-solution conditions with no electrostatic interactions between ions and bentonite surface.

Paenibacillus lutimineralis sp. nov., Isolated From Bentonite.

Strain MBLB1234T was isolated from bentonite samples collected at Guryong mining area located in Pohang, Republic of Korea and was taxonomically characterized by a polyphasic approach. This strain was a Gram-stain-negative, motile, endospore-forming, facultative anaerobic, catalase-positive, oxidase-negative, and rod-shaped bacterium. Strain MBLB1234T was able to grow at 20‒45°C (optimum, 37°C), pH 6.0‒10.0 (optimum, 7.0-8.0), and 0‒5.0% (w/v) NaCl (optimum, 0.5%). Genome size was 6,497,679bp with a G + C content of 46.4mol%. The genome was predicted to contain 5233 protein-coding genes, and 135 rRNA genes consisted of 10 5S rRNAs, 10 16S rRNAs, 10 23S rRNAs, and 105 tRNAs. Phylogenetic analysis based on the 16S rRNA gene sequences revealed that strain MBLB1234T clustered with Paenibacillus motobuensis JCM 12774T and P. aceti JCM 31170T with 98.3-98.5% and 97.2-97.4% sequencing similarity, respectively. The major fatty acids of strain MBLB1234T were anteiso-C15:0 (35.7%), anteiso-C17:0 (17.8%), iso-C17:0 (14.5%), and C16:0 (11.0%). The polar lipids were diphosphatidylglycerol, phosphatidylglycerol, phosphatidylethanolamine, phosphatidylmethylethanolamine, and one unidentified phospholipid, six unidentified aminophospholipids, and one unidentified lipid. The predominant isoprenoid quinone was menaquinone-7. DNA-DNA hybridization values between strain MBLB1234T and P. motobuensis JCM 12774T and P. aceti JCM 31170T were 34 and 38%, respectively. Average nucleotide identity value between strains MBLB1234T and P. aceti L14T was 82.3%. Based on characteristics of genomic, phenotypic, chemotaxonomic, and phylogenetic analyses, strain MBLB1234T represents a novel species of the genus P. , for which the name P. lutimineralis sp. nov. is proposed. The type strain is MBLB1234T (= JCM 32684T = KCTC 33978T).

EFFECT OF DURATION OF THE BENTONITE ACID MODIFICATION ON THE ACTIVITY OF PD(II)-CU(II) CATALYTIC COMPOSITIONS IN THE REACTION OF CARBON MONOXIDE OXIDATION

The effect of duration of the bentonite acid modification on its physicochemical and structural parameters and also on the catalytic properties of bentonite anchored palladium-copper complexes in the reaction of low-temperature carbon monoxide oxidation with air oxygen has been studied. For all acid modified bentonite samples, 1H-Bent-τ (τ = 0.5, 1.0, 3.0, 4.0, and 6.0 h), some regularities of changes as compared with natural bentonite, N-Bent, in the bentonite phase composition and X-ray spectral parameters of montmorillonite that is a samples, regardless a contact time, τ, the montmorillonite crystalline structure does not change however its aluminosilicate layers constrict for both bentonite acid modified samples and Pd(II)-Cu(II)/1H-Bent-τ compositions. After acid treatment, the number and positions of IR bands characterizing structural groups Al-Al-OH, Al-Fe 3+ -OH, Si-O-Si, and Si-O-Al of Mont phase do not change however their intensities, except for Si-O-Si, decrease pointing to the loss of Al 3+ and Fe 3+ cations. Palladium-copper complexes can be anchored not only on Mont structural groups but also on surfaces of quarts, amorphous SiO 2 , and calcite if any. It has been found that the changes in physicochemical and structural parameters of the bentonite samples result in alterations in the catalytic activity of Pd(II)-Cu(II)/S— compositions (S— is N-Bent or 1H-Bent-τ) in the reaction of carbon monoxide oxidation with air oxygen. The composition based on 1Н-Бент-0.5 characterizing by the minimum pH value of its aqueous suspension shows the maximum catalytic activity: CO conversion is 78 %.

Sensitive and cost effective disposable composite electrode based on graphite, nano-smectite and multiwall carbon nanotubes for the simultaneous trace level detection of ascorbic acid and acetylsalicylic acid in pharmaceuticals

In this study, we used nano-smectite (SMT) and multiwall carbon nanotube (MWCNT) as a recognition surface over the disposable pencil graphite electrode for the low-level detection of ascorbic acid (AA) and acetyl salicylic acid (ACA). Firstly, nano-semectit was obtained by the purification of natural bentonite clay. XRD patterns and Scanning electron microscopy (SEM) were employed to ensure the purity of the bentonite samples. Pencil graphite rodes were immersed into suspensions of SMT, MWCNT or the both.The surface morphology of the sensor was investigated by cyclic voltammery (CV), electrochemical impedance spectroscopy (EIS) and SEM techniques. The effect of pH, percentage composition of modifiers, immobilization time, accumulation time and accumulation potential was optimized for reaching the best electroanalytical response for AA and ACA. The sensor developed proved to give good recovery, notable electro-catalytic activity and appropriately distant signals for the simultaneous determination of the analytes. Common contaminates, uric acid, l-cysteine, dopamine, glucose, Na+, Cl- and citric acid were investigated for their interference effects. Uric acid, citric acid and dopamine were found to interfere to some extent. The electrode exhibited wide working ranges and a fairly satisfactory detection limit of 0.096 and 0.241 μM for AA and ACA, respectively. The electrode system proved to be practical for the analysis of pharmaceutical tablets and the recovery results are very satisfactory.

Dynamic Undrained Shear Strength Behavior of Bentonite–Grouted Sand

In this paper the results of dynamic and static triaxial tests on uniformed graded sand of Firoozkooh (131) has been tested and examined, which has been injected by bentonite slurry in different concentrations. The bentonite slurry inside the soil texture, can lead to changes the mechanic and hydraulic properties of soil. The results show that the bentonite being in sand leads to increase the liquefaction resistance. It was seen that when the slurry concentration is less than 7.5% it cannot have a significant impact on the liquefaction resistance, but by increasing the concentration of the slurry to 10%, liquefaction resistance increase effectively. The severity of liquefaction resistance changes for samples with low slurry concentrations was slow, and with increasing of slurry concentration was drastic. In addition it was observed that with increasing the concentration of slurry, the maximum dynamic modulus of elasticity increases. Due to obtained results, the soil liquefaction resistance through time and due to thixotropic process of bentonite, increases faster but after a specified period of time the rate of increase was slower. The static triaxial tests on injected bentonite samples were also compatible with dynamic testing results.

Use of Microwaves to Activate Bentonite in the Processes of Sorption of Nickel Ions from Concentrated Aqueous Solutions

The physicochemical parameters of the process of sorption water purification from compounds of Nickelwith clay sorbent in static conditions are studied. The absorption isotherm of ions of this metal from concentratedaqueous solutions on samples of bentonite using ultrahigh-frequency irradiation (microwave) is analyzed. Mostcorrectly, it is described by the linear equation of Langmuir. It is shown that irradiation of this sorbent bymicrowaves in the process of nickel ion sorting almost 2 times increases the value of the marginal sorptioncapacity for this metal (in comparison with the method of "stimulation" - preliminary washing of bentonite withpure water under the action of microwaves). The results of the study of spent bentonite by scanning electronmicroscopy, energy dispersion and X-ray diffraction analysis indicate that the metal under action of themicrowave is deposited on the surface of the bentonite not only in the form of an adsorbed monolayer of ions, butalso in the form of a separate crystalline phase, nickel silicate. Changes in the acidity of the medium duringsorption purification under the action of microwave are observed: from pH ≈ 5.7 ... 6.5 (before exposure) to pH ≈6.7 ... 7.1 (after irradiation). Confirmed assumption about that under the action of microwaves in the aqueousmedium there is a partial destruction of the silicate framework of clay sorbents with the release of silicon ionsSiO32-. These ions undergo hydrolysis, as a result of which the pH of the solution increases. Also, the directdiscontinuity of the Si-O bonds in the near-surface groups ≡Si-OH is not excluded, which leads to an increase inthe number of "free" hydroxyl groups.

Eu(III) sorption onto various montmorillonites: Experiments and modeling

The sorption of Eu(III) onto montmorillonite samples from different deposits such as those of Kutch (India), 10th Khutor deposit (Russia), and Cortijo de Archidona (Spain) was studied over a wide range of experimental conditions by varying the total Eu concentration, pH, and ionic strength. Various bentonite samples showed quantitative sorption at all pH values at low ionic strengths of the solution because of ion exchange interactions. At a higher ionic strength (1 M NaClO4), the sorption is pH-dependent because of the interaction with edge sorption sites and it differed from sample to sample. The difference in the mechanism of interaction at different ionic strengths was confirmed by time-resolved laser-induced fluorescence. Thermodynamic modeling was performed with all experimental data for montmorillonite samples from different sources to create a generalized model for describing their interaction with Eu(III).

Crystal structure model development for soil clay minerals – I. Hydroxy-interlayered smectite (HIS) synthesized from bentonite. A multi-analytical study

During the acidic weathering of soils, aluminum (Al) is dissolved from primary silicates such as feldspars or chlorites and intercalated in layer silicates. Part of this Al polymerizes in the interlayer of former smectites or vermiculites and the resulting phases are called hydroxy-interlayered minerals. These phases are known for a long time but are still insufficiently understood in terms of chemical composition and crystallographical structure. In the present study a sample set consisting of a continuous series of hydroxy-interlayered smectite samples with different degrees of hydroxy-interlayering ranging from 0 to 96% mol/mol smectite/HIS was prepared from a bentonite sample by adding different amounts of a 0.1 mol/L AlCl3 solution and different reaction times. The products were investigated by a multi-analytical approach using X-ray diffraction, Infrared spectroscopy, X-ray Fluorescence spectrometry, Fe2+:Fetotal determination, Nuclear Magnetic Resonance spectroscopy, Simultaneous Thermal Analysis coupled with mass spectrometry and Cation Exchange Capacity to provide an in-depth characterization. X-ray diffraction indicated that the turbostratic disorder typical for smectite did not change during hydroxy-interlayering. Nuclear Magnetic Resonance spectroscopy proved the exclusive six-fold coordination of interlayer Al and based on chemical calculations the average size and charge of the interlayer structures were characterized. Hydroxy-interlayers evolved in-situ in the smectite interlayers by polymerization. These interlayer “polymers” were small oligomers, the average size of which increased from monomers to pentamers with increasing degree of hydroxy-interlayering while the average net charge of each oligomer decreased. An average structural formula of each interstratified smectite-hydroxy-interlayered smectite was calculated from X-ray Fluorescence data including Fe2+:Fetotal ratios. If all exchangeable cations are attributed to smectite layers and all hydroxy-interlayers are attributed to hydroxy-interlayered smectite layers, the degree of hydroxy-interlayering ranged from 0 to 96% mol/mol smectite/HIS. The analysis provides an improved structural model of hydroxy-interlayered smectite suitable for Rietveld refinement which can be used to quantify the amounts of hydroxy-interlayered smectite in natural samples.

Influences of degree of saturation and stress cycle on gas permeability of unsaturated compacted Gaomiaozi bentonite

Compacted bentonite is usually used as the backfill material in underground storage of radioactive waste due to its swelling properties and low permeability. To understand the effects of the degree of saturation and stress cycle on the barrier properties of compacted bentonite, gas permeability tests were performed on highly compacted bentonite with different dry densities, water contents and stress cycles. The testing results show that the degree of saturation and stress cycle both have significant influences on the gas permeability of unsaturated compacted bentonite samples. As the saturation or confining stress increases, gas permeability of the samples is decreased by no more than 3 orders of magnitude. Moreover, under the condition of the same dry density, the greater the degree of saturation, the faster the decline of gas permeability of samples during loading; under the condition of the same degree of saturation, it shows that larger initial dry density can cause gentler decrease of the gas permeability of samples. A slight recovery in gas permeability was observed during unloading process of confining stress, and the decline of gas permeability caused by increasing confinements is mainly irreversible. Different from saturated/nearly-saturated samples, gas migration in the low/medially saturated compacted bentonite sample is considered to be dominated by single-phase advection in the connected pores of the unsaturated zone. The present work implicates that the effect of the degree of saturation and the stress path on the gas permeability of the backfill material should be taken into account during the construction period and early stage of operation of the radioactive waste.

Effect of various temperatures to liquid limit, plastic limit, and plasticity index of clays

This research aims to study the liquid limit, plastic limit, and plasticity index of soil samples treated with elevated temperatures. Liquid limit and plastic limit are physical properties of soils. Bentonite, kaolin, and Pasir Panjang samples were heated in an oven from room temperature up to 100 °C before the test. The liquid limit and plastic limit of soil samples were examined by Casagrande’s cup test and fall cone penetrometer test. Results show that the liquid limit and plastic limit of soil samples decrease as temperature increases. Plasticity index varies with the change in liquid limit and plastic limit. The bentonite sample is the most sensitive to temperature change, whereas the kaolin sample is the most insensitive. The sensitivity of the Pasir Panjang sample is in between those of the two other samples.

Cultivable Microbial Diversity in Domestic Bentonites and Their Hydrolytic Enzyme Production

We have isolated and identified 72 bacterial strains from four bentonite samples collected at the mining areas located in Gyeongsangbuk-do, Republic of Korea, and measured their hydrolytic enzyme (α-amylase, protease, and cellulase) activities to identify the isolates with industrial-use potential. Most of the isolates belonged to the Bacillaceae, with minor portions being from the Paenibacillaceae, Micrococcaceae, and Bacillales Family XII at the family level. Of the strains isolated, 33 had extracellular α-amylase activity, 30 strains produced cellulase, and 35 strains produced protease. Strain MBLB1268, having the highest α-amylase activity, was identified as Bacillus siamensis (0.38 ± 0.06 U/ml). Bacillus tequilensis MBLB1223, isolated from Byi33-b, showed the highest cellulase activity (0.26 ± 0.04 U/ml), whereas Bacillus wiedmannii MBLB1197, isolated from Zdb130-b, exhibited the highest protease activity (54.99 ± 0.78 U/ml). These findings show that diverse bacteria of the Bacillaceae family adhere to and exist in bentonite and are potential sources of industrially useful hydrolytic enzymes.

Variations in the Texture Profile Analysis (TPA) Properties of Clay/Mineral-Medicinal Water Mixtures for Pelotherapy: Effect of Anion Type

A peloid’s liquid phase can be mineral, sea or salt-lake water. This study examines the interactions among three materials, two special clays (bentonite and sepiolite) and one common clay, and three chemically different mineral-medicinal waters. In all clay–water mixtures, the hardness and adhesiveness decreased with the amount of water in the mixture. For a given hardness or adhesiveness, sepiolite retained more water than the other clays, especially in the presence of sulphate-rich mineral-medicinal water (73%). In contrast, the common clay retained the least amount of water (26%), and the bentonite samples returned an intermediate value for the amount of retained water (52–53%). These differences had a strong influence on the thermal behaviour of the mixtures and, hence, should be taken into account for their use in thermotherapy applications. There were no significant differences in the instrumental texture of the clay pastes according to the predominant anion in the mineral-medicinal waters.

Experimental simulation of boundary condition effects on bentonite swelling in HLW repositories

On the basis of textural and hydro-mechanical characteristics, bentonite has been proven to be an effective buffer/backfill material for long-term containment of high-level radioactive waste (HLW) in deep geological repositories. Herein, the results of experiments performed to investigate the swelling equilibrium limit (SEL) of bentonite under various boundary conditions are presented. A special apparatus was employed to simulate various stress–strain boundary conditions, including constant volume (CV), constant vertical stress (CVS), and constant stiffness (CS). Bentonite samples were prepared with various initial dry densities ranging from 1.5 to 1.7 g/cm3 and vertically stressed to different levels. During wetting, they were subjected to different boundary conditions before the swelling strain or swelling pressure reached equilibrium. Test results indicate that stress–strain boundary conditions have significant effects on the measured swelling strain and swelling pressure of the tested bentonite. More specifically, the relationship between the sequence of swelling pressure under different boundary conditions is CV > CS > CVS, while the relationship between the sequence of swelling strain is CVS > CS > CV. In addition, the characteristics of SEL curves are governed by the initial dry density and vertical stress with the effect of dry density being more significant. Based on these results, several SEL curves were developed to index the effects of boundary conditions on swelling potential of bentonite. They can be used to evaluate the final stress and volume states of bentonite during fluid infiltration under the range of boundary conditions possible in HLW repositories.

Linkage between swelling pressure, total suction of saturated bentonites and suction of saturating aqueous solutions

In deep geological repositories, compacted bentonites have been proposed for use as barrier materials for isolating nuclear waste. The prevailing hydro-mechanical-chemical boundary conditions in the repositories can affect the swelling capacity of compacted bentonites. The present study examines the linkage between the swelling pressure of saturated bentonites, the total suction of saturated bentonites and the suction of hydrating fluids. An equation describing the linkage between these parameters was derived based on the thermodynamics of soil moisture. To validate the derived relationship, laboratory tests involving constant volume swelling pressure tests and total suction measurements after the swelling pressure tests were carried out on a selected Ca-Mg-rich bentonite. Seven bentonite samples with compaction dry density ranging from 1.20 to 1.83 Mg/m3 were hydrated with deionized water, whereas four samples with a dry density of 1.59 Mg/m3 were hydrated with 0.0001 to 2.0 M CaCl2 solutions. The total suctions of the saturated bentonite and the hydrating fluids were determined by using a chilled-mirror hygrometer. The total suction of the saturated bentonite was found to be affected by the compaction dry density and the suction of the hydrating fluids. The experimental results from the present study and from the literature demonstrated that the total suction of saturated bentonites minus the suction of the hydrating fluids used to saturate bentonites is approximately equal to the swelling pressure of saturated bentonites.