碱性溶液饱和高庙子钙基膨润土膨胀特性及预测

建设高放废物地质处置库需要大量混凝土,而放射性核素衰变释放的大量热量会加速水泥老化,产生的碱性孔隙水会对膨润土的缓冲性能产生不利影响。以高庙子(GMZ)钙基膨润土为试验材料,选用NaOH溶液模拟碱性孔隙水,利用自主研制的耐腐蚀固结仪,完成不同浓度和不同干密度下的膨胀力实验。结果表明,在相同浓度碱性溶液饱和后试样的最终膨胀力随初始干密度的增大而增大;初始干密度相同时,低浓度碱性溶液对膨润土膨胀力有强化作用,高浓度碱性溶液对膨润土膨胀力有弱化作用;在0.3、0.5、1.0 mol/L NaOH溶液环境下试样的膨胀力在达到最大值后会出现下降,膨胀力衰减的程度与溶液浓度有关而与干密度无关。通过X射线衍射试验(XRD)分析矿物成分和晶层间距得到,蒙脱石含量和方英石含量随碱溶液浓度升高而降低,长石含量随着碱溶液浓度升高而上升;NaOH溶液会使钙基膨润土钠化,钠化程度受溶液浓度影响。最后利用蒙脱石孔隙比的概念,对试样的蒙脱石孔隙比和最终膨胀力结果在双对数坐标系中线性拟合,给出预测碱性溶液饱和钙基膨润土膨胀力的表达式。     

碱金属离子及碱溶液对膨润土性能影响的研究进展

基于前人有关膨润土在碱性环境下的室内试验和数值模拟的研究结果 ,重点阐述了碱金属离子及碱溶液对膨润土矿物成分、微观结构、膨胀性和渗透性等方面的影响,讨论了温度、pH值与溶液浓度等对上述过程的影响。碱金属离子及碱性溶液不仅可交换膨润土中蒙脱石层间阳离子,而且可溶解膨润土中的蒙脱石,生成非膨胀性矿物,并随着温度和pH值升高,蒙脱石被溶解程度增加,从而导致了膨润土膨胀力减小,且随着温度、溶液浓度及pH值的增加,膨胀力削弱程度加强;同时,膨润土与碱金属离子及碱溶液接触时孔隙增多,进而渗透性增强,随着温度和溶液浓度增加,渗透性也明显增强。数值模拟可实现膨润土与碱性溶液长期接触时发生的矿物成分及渗透性变化的预测。高温、不同pH值的低碱性溶液及其耦合作用对膨润土的矿物化学成分、微观结构、膨胀力和渗透性的影响及机理研究应该是今后研究的重点。     

盐溶液对膨润土膨胀性的影响

膨润土因其优良的膨胀性和低渗透性常被用作深层地质处置库的缓冲和回填材料,但其膨胀性会受到地下流体溶液浓度的影响。针对这种现象,利用单轴固结仪研究了压实商用膨润土在不同上覆荷载和不同浓度NaCl溶液作用下的膨胀变形特性。结果表明：随着NaCl溶液浓度升高,该膨润土的膨胀性显著降低,且最大膨胀应变与上覆荷载之间在双对数坐标系下存在良好的线性关系。利用有效压力的概念和计算方法,实现了用唯一一条曲线来描述不同盐溶液浓度作用下膨润土的膨胀变形行为,同时证明了有效压力理论的合理性。     

盐溶液饱和高庙子膨润土膨胀特性及预测

甘肃北山被首选为修建高放废物处置库的地区。考虑到该地区地下水中含有总溶解固体（TDS）,选择NaCl-Na2SO4作为溶解固体。以高庙子钠基膨润土为试验材料,利用单向固结仪,进行不同TDS浓度盐溶液和蒸馏水饱和的膨胀试验。根据蒙脱石孔隙比的概念,统一整理了饱和高庙子膨润土在盐溶液和蒸馏水饱和下的膨胀特性。结果表明,当TDS浓度为12.3 g/L（预选处置库区的最高离子浓度）时,在双对数坐标中蒙脱石孔隙比与膨胀力关系和干密度与膨胀力关系均呈直线且平行于蒸馏水的试验结果;对于给定干密度试样,膨胀力的对数与TDS浓度呈线性关系。根据以上试验结果,给出了由高庙子钠基膨润土的设计干密度和离子浓度计算相应膨胀力和膨胀变形的表达式。     

压实膨润土在盐溶液中的膨胀变形特性

采用蒸馏水以及浓度不同的NaCl溶液作为浸泡溶液,对商用膨润土进行了膨胀变形试验。结果表明在相同的荷载作用下,膨胀变形随浸泡溶液浓度的增加而减小。这是因为浸泡溶液的浓度越大,溶液的渗透吸力也越大,渗透吸力的增加将引起施加在黏土颗粒上有效应力的增加,使得在外部施加荷载相同时膨润土的膨胀变形随溶液浓度的增加而减小。当压实膨润土在盐溶液中膨胀时,其有效应力pe可分为两部分,即施加荷载部分p和渗透吸力部分p。对于浸泡于蒸馏水和NaCl溶液中的膨润土试样,其膨胀后的蒙脱石孔隙比em与有效应力pe之间的关系可采用同一关系曲线表示,此时施加于黏土颗粒上的有效应力的渗透吸力部分p=(p/)0.66。     

阳离子交换量和盐溶液浓度对膨润土膨胀变形的影响

膨润土遇水产生的膨胀变形以及膨胀力是由于颗粒间的斥力引起的,而该斥力大小与土体的固定负电荷密度（采用阳离子交换量CEC来表征）以及孔隙溶液的浓度有关,已有的对膨润土膨胀变形研究多针对孔隙溶液而非土体的阳离子交换量。现以钠基膨润土为研究对象,采用Li+固定法来控制膨润土的固定负电荷密度,以达到降低膨润土阳离子交换量的目的,再采用Li Cl为盐溶液,获得了上述不同阳离子交换量的膨润土在不同浓度的盐溶液作用下的膨胀指数。结果表明：随着溶液浓度升高,土样的膨胀指数降低;当浓度小于1 mol/L时,下降较为显著,之后则缓慢降低;随着阳离子交换量的减少,土样的膨胀指数也呈降低趋势。这是因为土颗粒的阳离子交换量和溶液浓度是影响土样与孔隙溶液物理化学作用的决定性因素,阳离子交换量越小,浓度越高,土颗粒之间的斥力降低,土样的膨胀指数则会降低。采用考虑颗粒间物理化学作用的粒间应力来描述试验结果,表明对于该种膨润土来说,不同阳离子交换量以及不同盐溶液作用下的回弹变形和回弹应力几乎在一条曲线上,这也进一步验证了粒间应力在膨胀土模拟中的有效性和适用性。     

极细颗粒黏土渗流离子效应的试验研究

采用常规固结仪对孔隙液离子浓度对极细颗粒黏土渗流固结特性的影响进行试验研究,在相同试验条件下进行了4组孔隙液含NaCl浓度不同的膨润土试样固结性质的测试,得到了各试样的固结曲线,求得相应的固结系数,再由固结系数求得相应的渗透系数。通过对各组试样的渗透系数对比分析,结果表明,孔隙液离子浓度对土体的渗流性质有重要影响,土体的渗透系数随孔隙液离子浓度的增加而增大,存在明显的离子效应。试验结果分析认为,黏土颗粒表面的扩散双电层厚度随离子浓度变化而变化,可能是产生离子效应的主要原因。试验结果有助于分析土体渗流离子效应的起因,对改进和完善现有渗流固结计算理论有一定指导意义。     

化学作用下高庙子膨润土屏障性能演化行为

深地质处置库运营过程中,受地下水化学成分、可能泄露的核素等近场化学条件影响,膨润土工程屏障性能可能发生衰减。围绕化学作用下膨润土屏障性能的演化行为,以我国北山处置库预选场地为工程背景,以内蒙古高庙子膨润土为研究对象,首先介绍了高庙子膨润土的基本物理性质及北山预选场地近场化学条件;在此基础上,详细总结了化学作用下高庙子膨润土水力、力学及阻滞等屏障性能演化行为相关研究成果。结果表明:化学作用对膨润土持水、渗透特性的影响可分别依据渗透吸力及双电层理论解释;化学作用下膨润土膨胀力的衰减与其水化膨胀机制有关,化-力耦合及化-水-力耦合作用下膨润土变形特性因渗透固结、化学软化效应而发生改变;膨润土的吸附与扩散特性受核素浓度、离子环境、pH值等因素影响。最后,提出化学作用下膨润土屏障性能演化研究应重点关注的方向。     

高庙子膨润土水化膨胀特性及其微观机理研究

膨润土具有遇水膨胀的特性,是高放核废料深地质处置库理想的缓冲回填材料。膨胀特性是其作为缓冲材料最重要的性能之一,同时受多方面因素的影响。本文以我国首选缓冲材料高庙子膨润土为研究对象,以含水率和干密度为控制变量,以恒体积法为试验方法,研究了高压实高庙子膨润土的水化膨胀特性,采用压汞试验法(MIP)对膨润土微观结构进行了研究,并以此对水化膨胀特性进行了解释。膨胀力试验结果表明,高庙子膨润土的膨胀力发展形式和最大膨胀力均受试样含水率和干密度影响,干密度较小时,水化曲线呈明显的双峰结构,干密度较大时,水化曲线形态与含水率相关,随着含水率增大,双峰结构逐渐消失。MIP试验结果表明,高庙子膨润土的孔径分布同样受含水率和干密度影响,随着含水率和干密度降低,集合体间大孔隙体积增多。膨润土的水化膨胀曲线受集合体间大孔隙影响显著。大孔隙较多时,膨润土集合体能迅速膨胀形成临时结构,当膨胀力超过临时结构的极限荷载时发生坍塌,膨胀力回落,内部结构重组后继续水化达到最大膨胀力,因此其水化膨胀曲线呈明显的双峰结构。随着大孔隙量减少,水化膨胀曲线由双峰结构演变成一条平滑曲线。     

地质系统热-水-力耦合作用的随机建模初步研究

热-水-力(THM)耦合作用是岩石力学与环境地质中的重要基础理论问题,核废料地质处置库周围的缓冲材料和围岩中的热-水-力耦合现象将影响其力学稳定性、热传导性和渗透性,进而影响放射性核素在裂隙岩体中的迁移规律。核废料或放射性废料的地下深埋处置是国际上正在研究的永久性隔离的有效方法之一。因此,对核废料地质处置法安全性评估的一个重要内容就是对裂隙岩体中力学稳定性与构造应力、地下水渗流及热载荷等的耦合作用之数值模拟和评估。这已成为当前刻不容缓的重要的环境影响评价课题。笔者研究了温度场-渗流场-应力场中热传导系数和渗透率以及岩体力学参数的空间变异性,用实验方法研究三场耦合效应及裂隙岩体的场性能等效处理,试图建立热-水-力耦合作用的随机性数学模型及可视化数值模拟方法,为核废料地质处置安全性评估提供直观的新方法。     

碱−热环境下MX80膨润土导热性能试验研究

以美国怀俄明州钠基（MX80）膨润土为研究对象,采用热探针法测定碱−热环境下MX80膨润土的导热系数,探讨了温度、碱液强度和干密度对试样导热系数λ的影响规律,并选择部分试样进行了X射线衍射（X-ray diffraction,简称XRD）和扫描电镜（scanning electron microscope,简称SEM）试验,揭示了碱−热环境下MX80膨润土导热性能演变的微观机制。试验结果表明：MX80膨润土的导热系数λ随碱液含量和干密度的递增而增大;在不同碱液含量条件下,膨润土的导热系数均随温度升高而增大,且碱液含量越高,导热系数的温度效应越显著;干密度较小时,膨润土导热系数λ随温度升高而增大的影响越明显,主要原因是温度促进了试样内部的水汽潜热传输;同一温度和干密度条件下,热传导系数λ随着碱液pH值的升高而降低6,且pH值越高,则λ降幅越明显,主要原因是强碱溶液腐蚀了膨润土的蒙脱石成分,减少了试样内石英含量,增大了膨润土试样的孔隙率,进而降低了膨润土导热系数,这与测试试样的XRD和SEM结果相吻合。     

Erosion of bentonite particles at the interface of a compacted bentonite and a fractured granite

Compacted bentonite has been considered as a candidate buffer material in the underground repository for the disposal of high-level radioactive waste. An erosion of bentonite particles caused by a groundwater flow at the interface of a compacted bentonite and a fractured granite was studied experimentally under various geochemical conditions. The experimental results showed that bentonite particles could be eroded from a compacted bentonite buffer by a flowing groundwater depending upon the contact time, the flow rate of the groundwater, and the geochemical parameters of the groundwater such as the pH and ionic strength.

A gel formation of the bentonite was observed to be a dominant process in the erosion of bentonite particles although an intrusion of bentonite into a rock fracture also contributed to the erosion. The concentration of the eroded bentonite particles eroded by a flowing groundwater was increased with an increasing flow rate of the groundwater. It was observed from the experiments that the erosion of the bentonite particles was considerably affected by the ionic strength of a groundwater although the effect of the pH was not great within the studied pH range from 7 to 10. An erosion of the bentonite particles in a natural groundwater was also observed to be considerable and the eroded bentonite particles are expected to be stable at the given groundwater condition.

The erosion of the bentonite particles by a flowing groundwater did not significantly reduce the physical stability and thus the performance of a compacted bentonite buffer. However, it is expected that an erosion of the bentonite particles due to a groundwater flow will generate bentonite particles in a given groundwater condition, which can serve as a source of the colloids facilitating radionuclide migration through rock fractures.     

水-岩化学作用等效裂纹扩展细观力学模型

从化学腐蚀下岩石细观结构的变化出发,研究水化学溶液对岩石中裂纹的腐蚀作用。运用地球化学矿物-水反应的溶解动力学,考查水-岩反应化学溶液中不同时间段离子浓度的变化,从理论上探讨化学腐蚀下等效裂纹扩展的定量化分析方法。通过质量守恒定律计算出裂纹在迹长及隙宽方向的变化,将水化腐蚀等效成有效裂纹长度,建立了水-岩化学作用下等效裂纹扩展的计算公式并计算应力强度因子。通过对砂岩的研究结果表明,水-岩化学作用对岩石裂纹的扩展有显著影响,并且具有时间效应。同时,水化学溶液的离子浓度及pH值大小对裂纹的扩展存在影响,其中pH值的变化对岩石的断裂力学效应更加显著。当溶液的酸性越强或碱性越强,腐蚀越大,裂纹扩展越快;pH值为中性时,腐蚀作用得到较大缓解,裂纹发展缓慢。为定量研究裂隙岩体在水岩化学作用下的细观机理提供了一种新的思路。     

极细颗粒黏土渗流的微电场效应分析

采用渗流固结法试验结合颗粒表面电位分析,研究极细颗粒黏土的渗流的微电场效应,在相同试验条件下完成了5种含不同百分比的人工高岭土与人工膨润土的试样的渗流特性测试。测试结果表明,微孔渗流的微电场效应对极细颗粒黏土的渗流特性有相当显著的影响,随着孔隙液离子浓度的升降或土颗粒表面电位的增减,试样的渗透系数会随之发生改变;在相同的孔隙液离子浓度下,随着膨润土相对含量的增加,试样的渗透系数随之降低。对科威特软土的渗流固结试验证实了人工土的微电场效应的产生机制与变化规律同样适用于天然软土。试验结果分析认为,在黏土-水-电解质系统的相互作用下,黏土矿物通过土颗粒表面的结合水影响试样的渗流特性,而土颗粒表面电荷的微电场作用是极细颗粒黏土渗流特性改变的内在原因之一。     

Mechanisms and Models for Bentonite Erosion Used for Geologic Disposal of High Level Radioactive Waste: A Review

The performance of the bentonite buffer in nuclear waste repository concept relies to a great extent on the buffer surrounding the canister having sufficient dry density. Loss of buffer material caused by erosion remains as the most significant process reducing the density of the buffer. In the worst case, the process is assumed to last as long as the free volume between the pellets in the pellets filled regions is filled with groundwater. Erosion rate and mass erosion are calculated based on the erosion model, and the measures are presented to prevent the geological disaster due to bentonite erosion. The groundwaters may solubilise the smectite particles in the bentonite and carry them away as colloidal particles. A dynamic model is developed for sodium gel expansion in fractures where the gel soaks up groundwater as it expands. The model is based on a force balance between and on smectite particles, which move in the water. Attractive van der Waals forces, repulsive electric diffuse layer (DDL) forces, gravity and buoyancy forces and forces caused by the gradient of chemical potential of the particles act to move the particle in the water. The effect of the fracture width and the frictions between particles and water and surrouding rock is analysed based on erosion model. The DDL forces strongly depend on the type of clay minerals and the type of ion and concentration in the water surrounding the particles. In the designed safe use of nuclear waste disposal (tens of thousands of years to hundreds of thousands of years), the safety of nuclear waste disposal is affected by the hydrodynamic and chemical effects, and bentonite erosion. Due to the bentonite erosion, the buffer/backfill layers become loose, and their permeability increases, which causes the nuclear element diffusion and convection, and even the nuclear disaster. In this paper, the mechanisms, models, experiments and control measures of bentonite erosion were systematically summarized. The current deficiencies of bentonite erosion were pointed out, and new methods were put forward to carry out the research for bentonite erosion. The measures were presented to prevent the geological disaster due to bentonite erosion through changes. The project is not only academic innovation, but also has a large practical significance. The research results of this project can be widely applied to the design, construction and maintenance of the bentonite buffer in nuclear waste repository.     

Effects of Post-compaction Residual Lateral Stress and Electrolyte Concentration on Swelling Pressures of a Compacted Bentonite

The effects of post-compaction residual lateral stress and salt concentration in the hydrating fluid on swelling pressures of compacted MX80 bentonite is brought out in this paper. In order to release the residual lateral stresses, following the static compaction process during preparation of specimens, compacted bentonite specimens were extruded from the specimen rings and then inserted back prior to testing them for swelling pressures in isochoric condition. The swelling pressure tests were carried out at several dry densities of the bentonite with distilled water and solutions of NaCl (0.1 and 1.0 M) as the hydrating fluids. With water, the test results showed that specimens that underwent extrusion and insertion processes exhibited about 10–15 % greater swelling pressures as compared to the specimens those that were compacted and tested. The influence of saline solutions was found to reduce the swelling pressure of the bentonite, but their impact was less significant at high compaction dry densities.     

Simultaneous application of iron and aluminum anodes for nitrate removal: a comprehensive parametric study

Exposure to high concentration of nitrate through drinking water poses a threat to human health and environment. Electrocoagulation (EC) is an alternative water treatment process that involves electrogeneration of coagulant agents. In the present study, EC was exerted for the nitrate removal in a batch reactor using aluminum and iron anodes simultaneously. The effects of the main parameters including electrical current, initial pH, NaCl dosage, initial nitrate concentration and presence of turbidity on NO₃ ^? removal were investigated. NO₂ ^? as a by-product was monitored during electrolysis, and nitrate–nitrite index was calculated. The results indicated that optimum condition was pH of 5, 300 mA electrical current, 100 mg/L NaCl and electrolysis time of 40 min, under which removal efficiency was 81.5 %. Nitrite anion was generated during electrolysis of nitrate solution which increases nitrate–nitrite index at the first reaction time, and it was eliminated after 20-min electrolysis time. Reaction kinetic of nitrate removal in the absence and presence of turbidity was first-order and zero-order, respectively.     

微生物-膨润土联合矿化防渗模型试验研究

砂土地基发生渗漏时容易引发地基塌陷和工程结构破坏等工程安全问题。利用微生物-膨润土联合矿化的方法开展了大尺寸砂柱的防渗模型试验,研究了砂土的颗粒粒径、浆液的液固比与注浆次数等因素对砂柱的渗透特性、内部侵蚀特性及膨润土与碳酸钙沉淀分布的影响,分析了处理后试样的封堵稳定性和微观结构,进一步评估了微生物-膨润土联合矿化方法的处理效果。结果表明,通过微生物-膨润土联合矿化方法可以有效地提高砂土的防渗效果与封堵稳定性。经过1～3次注浆后,试样的渗透系数最大可以降低4个数量级,并且渗透过程中的侵蚀速率也得到了成倍的降低,最低达到了0.51 g/(s·m2)。针对膨润土与碳酸钙沉淀在砂土封堵中起到的作用,分析了微生物-膨润土联合矿化方法的防渗机制。该研究成果证实了微生物-膨润土联合矿化方法应用于砂土防渗工程的可行性,为微生物注浆技术解决砂土地基渗漏问题提供了重要参考。