Thermal-Mechanical Behavior of Compacted GMZ Bentonite

The THM behavior of compacted GMZ bentonite has been investigated using a suction-temperature controlled isotropic cell. The results obtained were compared with the existing results on other reference bentonites (MX80, FEBEX, FoCa, and Kunigel-V1). It has been observed that the coefficient of thermal expansion of the compacted GMZ bentonite is 2×10^-4°C^-1, similar to the values of compacted MX80 and FEBEX bentonites. The heating tests of the GMZ bentonite also show that the suction is an important parameter that governs the thermal volumetric behavior of unsaturated soils. Unlike temperature, suction has a significant effect on the compressibility parameters. Examination of the mineralogy of various bentonites showed that a good correlation can be generally established between the montmorillonite content and the cations exchange capacity (CEC) or the specific surface area (S). Nevertheless, both the basic geotechnical properties and the swelling potential seem to depend not only on the montmorillonite content but also on other factors such as the nature of base exchangeable cations. The quartz content of the GMZ bentonite is relatively high (11.7%). This could explain its relatively large values of thermal conductivity.     

Swelling pressure characteristics of compacted Chinese Gaomiaozi bentonite GMZ01

Gaomiaozi bentonite (GMZ01) has been decided upon as the first option for use as buffer/backfill materials in the deep disposal of high-level radioactive waste in China. The basic functions of the materials used in the waste repositories request among others a sufficient swelling pressure and low hydraulic conductivity in order to provide long-term stability to the barrier system under environmental pressure and behavior of the waste loads. As such, it is necessary to investigate the influence of initial dry density on the swelling properties of Gaomiaozi bentonite (GMZ01) in order to achieve better design of buffer/backfill materials. In this study the swelling pressure of GMZ01 has been studied and analyzed by multi and one-step wetting constant volume tests with five different dry densities (1.15, 1.35, 1.50, 1.60 and 1.75 mg/m³). Results show that swelling pressure changes with time nonlinearly, while there is a linear relationship between time/swelling pressure and time. Curves of swelling pressure and the amount of absorbed water varying with time can be classified into typical phases. For the GMZ01 tested here, the initial dry density is an important factor influencing the swelling pressure. The results show that there is an exponential relationship between swelling pressure and dry density. Moreover, comparison was done between the experimental swelling pressure results of used GMZ bentonite in this study and other bentonites cited in literature: (i) other GMZ׳s and (ii) different types of bentonites proposed as buffer/backfill materials (i.e., MX80, Kunigel, Montigel, and Calcigel). The effect on the microstructure of the density and the wetting under the constant volume condition (after the swelling pressure test) has been investigated by studying the results of pore size distribution for GMZ01 by using the mercury intrusion porosimetry (MIP) test and the environmental scanning electron microscope (ESEM) photos. Finally, two different theoretical concepts were used to estimate the swelling pressure (the modified DDL and thermodynamics approaches). The results of the two methods show that the swelling pressure results compare relatively well with the experimental data for the GMZ bentonite.     

关于JG／T193—2006（《钠基膨润土防水毯》有关问题的讨论

对建材行业标准JG／T193—2006《钠基膨润土防水毯》中有关膨润土的诸如标准的制定、标准的命名、膨润土吸蓝量、钠基膨润土、人工钠基膨润土的变质以及膨润土的耐久性等一些有争议的问题进行了讨论,并就有关膨润土防水毯国家标准的编制提出了一些建议。     

Effect of water chemistry on the hydro-mechanical behaviour of compacted mixtures of claystone and Na/Ca-bentonites for deep geological repositories

In the French deep geological disposal for radioactive wastes, compacted bentonite/claystone mixtures have been considered as possible sealing materials. After emplacement in place, such mixtures are hydrated by the site solution as well as the cement solution produced by the degradation of concrete. In this study, the effects of synthetic site solution and cement solution on the hydro-mechanical behaviour of compacted mixtures of claystone and two types of bentonites (MX80 Na-bentonite and Sardinia Ca-bentonite) were investigated by carrying out a series of swelling pressure, hydraulic conductivity and mercury intrusion porosimetry (MIP) tests. It was found that for the MX80 bentonite/claystone mixture hydrated with synthetic site solution, the swelling capacity was reduced compared to the case with deionised water owing to the transformation of Na-montmorillonite to multi-cation dominant montmorillonite by cation exchanges. For the Sardinia bentonite/claystone mixture, the similar increasing rate of swelling pressure was observed during the crystalline swelling process for different solutions, suggesting insignificant cation exchanges. Additionally, the cations in the synthetic site solution could reduce the thickness of diffuse double layer and the osmotic swelling for both MX80 bentonite/claystone and Sardinia bentonite/claystone mixtures. The large-pore volume increased consequently and enhanced water flow. In the cement solution, the hydroxide could also dissolve the montmorillonite, reducing the swelling pressure, and increase the large-pore volume, facilitating the water flow. Furthermore, the decrease of swelling pressure and the increase of hydraulic conductivity were more significant in the case of low dry density because of more intensive interaction between montmorillonite and hydroxide due to the high permeability.     

The effects of technological voids on the hydro-mechanical behaviour of compacted bentonite–sand mixture

Compacted bentonite-based materials are often used as buffer materials in radioactive waste disposal. A good understanding of their hydro-mechanical behaviour is essential to ensure disposal safety. In this study, a mixture of MX80 bentonite and sand was characterised in the laboratory in terms of water retention property, swelling pressure, compressibility and hydraulic conductivity. The effects of the technological voids or the voids inside the soil were investigated. The technological voids are referred to as the macro-pores related to different interfaces involving the buffer material, whereas the voids inside the soil are referred to as common macro-pores within the compacted bentonite/sand mixture. The results obtained show that at high suction, the amount of water absorbed in the soil depends solely on suction, whereas at low suction it depends on both suction and the bentonite void ratio. There is a unique relationship between the swelling pressure and the bentonite void ratio, regardless of the sample nature (homogeneous or not) and the sand fraction. However, at the same bentonite void ratio, a higher hydraulic conductivity was obtained on the samples with technological voids. The effect of sand fraction was evidenced in the mechanical yield behaviour: at the same bentonite void ratio, the bentonite–sand mixture yielded at a higher pre-consolidation stress.     

Bentonite transformations in strongly alkaline solutions

Strongly alkaline solution pH causes changes to the mineralogy of bentonites which might impact on their performance as environmental barriers. The long term effect of solution pH on the performance of bentonite barriers such as in Geosynthetic Clay Liners needs to be studied from the viewpoint of solubility and stability of the mineral phases present at extreme pH values. Changes to bentonite mineralogy brought about by extended reaction with 1 M sodium hydroxide solutions at 20–25 °C reveal that certain components of bentonites, namely smectite, opaline silica and quartz, are subject to dissolution in alkaline solution. Associated with dissolution is the formation of hydrous hydroxy-aluminosilicate as well as hydrous carbonate mineral phases. It is postulated that these precipitates, formed from reaction of bentonite with alkaline leachates can result in pore filling, which is responsible for recently measured lower hydraulic conductivity of some bentonites to high pH leachates.     

高庙子膨润土的土水特征曲线

近年来，用于高放废物深地质处置工程屏障的高压密膨润土越来越受到关注。经过全国范围内的比较和筛选，内蒙古兴和县高庙子膨润土矿床被确定为我国高放废物处置库缓冲材料的首选矿床。采用渗析法和水汽平衡法来控制吸力技术，通过上水特征曲线测定试验、环境扫描电镜和乐汞试验，研究高庙子高压密膨润土在不同吸力下的持水特性及其微观结构特征。利用压汞试验结果推算膨润土在恒体积条件下的土水特征曲线，并与实测数据进行比较。研究结果表明：在不同吸力作用下，膨润土的持水特性与其微观结构之间有着密切关系。     

非饱和土的非线性模型及其应用

提出了一个较完整的非饱和土的非线性模型。该模型涉及土的变形和水量变化两个方面, 共包含１３ 个参数。测定全部参数只需用两种三轴试验。应用所提模型预估三轴不排水试验中的吸力变化,理论计算与试验结果比较接近。     

自由膨胀条件下高压密膨胀粘土微观结构随吸力变化特征

核废料处置问题是世界各国安全利用核能所关心的重大问题。作为用于封堵通往地下处置库的通道,以隔断处置库内核废料与外界环境联系的首选隔绝用材料——膨胀性粘土的特性正越来越受到岩土工程界的关注。利用压汞测试法和电镜扫描法方法,对吸力控制条件下高压密蒙脱石充分自由水化后微结构变化特征展开平行对比试验,以研究高压密蒙脱石在不同吸力条件下的体积变化特征。吸力的控制采用了渗析(液相)法。试验对不同密实度的压密膨胀土随吸力变化特征的进行了研究。研究结果表明:(1)压实膨胀土水化过程中的膨胀势与初始压实密度有关,初始压实密度越大,膨胀势越大;(2)体积膨胀量与水化过程的控制吸力之间存在近似的半对数衰减关系,且随着控制吸力的增大,衰减速率减弱;(3)体积的膨胀主要来源于膨胀土集合体结构中集合体之间的大孔的扩张。     

Geo‐environmental characteristics of modified and ordinary bentonitic soils exposed to MTBE

Methyl tertiary‐butyl ether (MTBE) is the most common oxygenated fuel additive used to increase the octane rating and enhance the combustion efficiency of gasoline. This product is characterized as a harmful compound by the US EPA and has been shown to be a health threatening contaminant for human beings. The most contingent route for MTBE to enter the environment is its leaking from underground storage tanks (USTs). To avoid soil and groundwater contaminations because of leaking of MTBE, we investigated the performance of both modified and ordinary bentonite clays as liner systems at USTs sites in this study. Modified bentonites are synthesized by changing the original metallic cations in the structure of bentonite particles, thus converting the bentonite from hydrophilic to organophilic nature. In this research both modified and ordinary bentonites were investigated to evaluate MTBE adsorption efficiencies. As permeability is a major parameter in measuring contaminants flow in soils, tests were conducted to measure the permeability characteristic of bentonites. X‐ray diffraction (XRD) analyses were also carried out to evaluate the adsorption of MTBE to bentonites. The results indicated that basal spacing of modified bentonite particles was increased from 28.64 to 42.32 Å, thus showing MTBE adsorption to the soil. In terms of permeability tests, modified bentonites showed much lower permeability values compared to ordinary bentonites (1.9 order of magnitude reduction) when exposed to MTBE. Modified clays are a viable option for use as liner system material at MTBE contaminated sites.     

膨润土的膨胀性衰减研究

利用地球化学软件Phreeqc模拟了高放核废料处置库的高压实MX80膨润土中发生的水文地球化学反应,基于长时间尺度的研究的需要,忽略热、力、流等因素的影响,主要考虑了K⁺、Ca²⁺、Na⁺、Mg²⁺四种阳离子的离子交换和矿物溶解沉淀反应,建立了膨润土中的一维离子运移反应模型。模拟了不同外界流体作用下,膨润土化学组分在时间和空间上的变化。结合化学组分变化结果,对膨润土膨胀性衰减情况进行了计算和评估。得到1万年后海水和地下水环境下的MX80膨润土膨胀性衰减系数分别达到0.79和0.91。     

Influence of heating and water-exposure on the liquid limits of GMZ01 and MX80 bentonites

Bentonite has been considered as a buffer material for embedding canisters with high-level radioactive waste（HLW） in deep geological repositories.GMZ bentonite deposit,located in Xinghe County,Inner Mongolia Autonomous Region,China was proposed as a buffer/backfill material for HLW repository in China.The liquid limits of natural Na-bentonite GMZ01 and commercial Na-bentonite MX80,which are previously heated at 80 ℃ and 95 ℃,respectively,and exposed to water for different times are measured.It is observed that the liquid limit of GMZ01 increases slightly at the beginning,and then decreases as the heating time increases,while the liquid limit of MX80 decreases with the heating time.The liquid limits of both GMZ01 and MX80 decrease with increasing water-exposure time.After the samples are heated at 80℃and 95 ℃ for several months,the mineralogical composition of GMZ01 does not exhibit evident change,whereas MX80 experiences some changes.In addition,the chemical composition,cation exchange capacity（CEC） and exchangeable cation of all the samples do not change significantly.     

Effects of mineralogy on thermo-hydro-mechanical parameters of MX80 bentonite

Thermal conductivity, water retention curve and swelling behavior of two MX80 bentonite samples were studied in the present work. The difference obtained from these two MX80 bentonite samples was then analyzed in terms of mineralogical effects （effects of the proportion of quartz and montmorillonite）. It was concluded that the mineralogical effect was significant on the thermal conductivity and the swelling capacity; on the contrary, it was negligible on the water retention property.     

Laboratory investigation of GCL hydration from Lateritic subsoils

A laboratory investigation on the hydration behavior of GCLs from lateritic soils was conducted under isothermal and thermal conditions (tropical climate), varying subsoil moisture contents, GCLs bentonite particle size and mineralogy. GCL hydration levels from lateritic subsoils under isothermal conditions (55%) were similar to literature findings. A slight decrease in water content of some GCLs after long periods of contact with the lateritic soils indicates that equilibrium can demand long time in these soils. GCL with granular bentonites were less efficient to hydrate from lateritic subsoils. GCLs with activated-calcium bentonites maintained hydration levels in long-term. Nonwoven geotextile facing down favored capillary effects. Thermal cycles significantly influenced GCLs hydration from subsoils. Capillary connections developed during hydration under isothermal conditions due to suction gradient reductions. Post-hydration tests under isothermal conditions showed more alterations in GCLs swelling and cation exchange properties than thermal cycles test. An increase in the saturated hydraulic conductivity of GCLs was observed in both lateritic soils, mainly for isothermal condition, although continued attending hydraulic conductivity requirements for barrier applications.     

温控下高压实膨润土持水特性及预测研究

高庙子膨润土是我国核废料深地质处置的首选材料。作为缓冲密封材料,在使用过程中,会受到核素衰变放热的影响而改变土的持水特性、渗透性和力学特性,其工作性能也随之改变。为研究高温下高庙子膨润土的持水特性,文中采用在80℃侧限条件下进行实验研究。实验采用渗析法和水汽平衡法来控制吸力,测得在不同吸力点的含水量,从而得出土水特征曲线。对于特定吸力点的土样进行压汞试验,分析不同吸力点的孔隙特点,并由压汞试验得到PSD曲线来预测其土水特征曲线,并在PSD预测中考虑温度影响。最终将PSD预测曲线与实测曲线对比研究,发现考虑温度的PSD预测比较接近于真实的土水特征曲线。     

Impact of a synthetic leachate on permittivity of GCLs measured by filter press and oedopermeameter tests

Geosynthetic clay liners (GCLs) are used in landfill liner applications due primarily to their low hydraulic conductivity to water. The low hydraulic conductivity of GCLs comes from the structure of the clay in the bentonite. However, the interaction between clay and aggressive liquids may alter the structure of the clay and, thus, result in an increase in the hydraulic conductivity of the GCL. This paper presents the results of a project aimed at evaluating the impact of a synthetic leachate on the structure of four different bentonites used in the manufacturing of GCLs. The preparation of bentonite dispersions increased the interaction between the bentonites and the various liquids. The hydraulic properties of the dispersions also were tested using filter press tests to obtain flow curves. Results of these tests were correlated with the cationic concentration, electrical conductivity and pH of the dispersions, swell indexes of the bentonite extracted from the GCLs, and permittivities of the intact GCLs determined in oedopermeameter tests. The results showed that one bentonite was more sensitive to the synthetic leachate than the other bentonites. For example, the permittivities of the more sensitive bentonite based on the oedopermeameter tests and filter press tests were respectively 2.11 × 10⁻⁸ s⁻¹ and 5.6 × 10⁻⁸ s⁻¹, whereas the permittivities for other bentonites, including a natural sodium bentonite and two sodium-activated calcium bentonites, were respectively 5.7 to 6.5 × 10⁻⁹ s⁻¹ and 3.2 to 3.5 × 10⁻⁸ s⁻¹. The filter press test served as a quick and easy-to-use test to compare the performance of the various bentonites in containing a given liquid. However, the oedopermeameter test or direct permeation test is preferable to filter press tests or fluid loss tests for evaluating the long-term impact of a liquid on a bentonite.     

Swelling pressure of compacted MX80 bentonite/sand mixture prepared by different methods

Compacted bentonite/sand mixture have been considered as possible sealing/backfilling material in the deep geological disposal of high-level radioactive nuclear waste. The swelling pressure of the compacted bentonite/sand mixture is of significance in the design of the geological repository, which requires good consistency between data obtained by laboratory and field measurement. In this work, a series of swelling pressure tests were performed on compacted MX80 bentonite/sand specimens prepared by methods commonly adopted in laboratory (As-compacted, Transferred) and those mimicking the real block manufacturing process (Trimmed, Inserted). Results shown that with identical dry density (especially when the dry density was larger than 1.70 Mg/m³), largest swelling pressure was obtained in specimens prepared by method Inserted, followed by method As-compacted, Transferred and Trimmed. The distinct difference between the swelling pressure could be largely attributed to the effects of residual post-compaction lateral stress. More interestingly, specimens prepared by methods Trimmed and Transferred followed a similar swelling pressure-dry density relationship. From this point of view, method Transferred without causing mass loss, change of bentonite content and possible technological voids effect was recommended in lieu of method Trimmed for specimen preparation in laboratory.     

Experimental study on hydro-mechanical coupling behaviours of highly compacted expansive clay

Highly compacted expansive clays have been usually considered as a possible material for sealing and backfill in deep geological disposal of radioactive waste. In this condition, the material is simultaneously subjected to water infiltration from the geological barrier and stresses generated by the swelling of engineered barriers in confined conditions. Its behaviour under hydro-mechanical loading is essential to the safe design of the whole storage system. In the present work, MX80 bentonite, a kind of expansive clay from Wyoming, USA, was studied. After compaction, its dry density was 1.8 Mg/m3 and its initial suction was 110 MPa. Firstly, the soil was humidified under controlled suction and free-swelling conditions. Significant swelling was observed. Secondly, four values of suction of 110, 39, 9 and 0 MPa were employed to perform isotropic compressive tests at constant suction conditions. That allowed studying the effect of suction on the yield pressure, elastic and plastic compressibility parameters. The results show that the elastic and plastic compressibility parameters increase when the suction decreases. The relationship between these parameters and the logarithm of suction can be linearly correlated. The yield stress drastically decreases upon wetting under free-swelling conditions, from 12 – 18 MPa (at an initial suction of 110 MPa) to 0.2 MPa at saturated state.     

Removal of Pb(ii) from aqueous/acidic solutions by using bentonite as an adsorbent

The ability of bentonite clay to remove Pb(II) from aqueous solutions and from nitric acid, hydrochloric acid and perchloric acid solutions (1.0-1 x 10(-5)) has been studied at different optimized conditions of concentrations, amount of adsorbent, temperature, concentration of electrolyte and pH. Maximum adsorption of Pb(II), i.e. > 98% has been achieved in aqueous solutions, while 86% is achieved from 1.0 x 10(-5) M HCl using 0.5 g of bentonite. The adsorption decreases by increasing the concentration of electrolytes. Flame atomic absorption spectrometer was used for measuring lead concentration. Isotherm analysis of adsorption data obtained at 25 degrees C, 30 degrees C, 40 degrees C and 50 degrees C showed that the adsorption pattern of lead on bentonite followed the langmuir isotherm and freundlich isotherm, respectively. DeltaH(o) and deltaS(o) were calculated from the slope and intercept of ln K(D) vs. I/T plots.     

膨润土防水毯应用若干问题谈

分析了膨润土防水毯应用中存在的一些问题，介绍了区分天然钠基膨润土与人工钠基膨润土的方法以及地下工程采用膨润土防水毡进行防水的设计施工要求。