Insights into the water retention behaviour of GMZ bentonite pellet mixture

Bentonite pellets are recognized as good buffer/backfill materials for sealing technological voids in high-level radioactive waste (HLW) repository. Compared to that of a traditional compacted bentonite block, one of the most important particularities of this material is the initially discrete pellets and the inevitable heterogeneous porosity formed, leading to a distinctive water retention behaviour. In this paper, water retention and mercury intrusion porosimetry (MIP) tests were conducted on pellet mixture (constant volume), single pellet (free swelling) and compacted block (constant volume) of GMZ bentonite, water retention properties and pore structure evolutions of the specimens were comparatively investigated. Results show that the water retention properties of the three specimens are almost similar to each other in the high suction range (>?10 MPa), while the water retention capacity of pellet mixture is lower than those of the compacted block and single pellet in the low suction range (<?10 MPa). Based on the capillary water retention theory (the Young–Laplace equation), a new concept ‘saturated void ratio’ that was positively related to water content and dependent on pore size distribution of the specimen was defined. Then, according to the product of saturated void ratio and water density in saturated void, differences of water retention properties for the three specimens at low suctions were explained. Meanwhile, MIP tests indicate that as suction decreases, the micro- and macrovoid ratios of pellet mixture and compacted block decrease as the mesovoid ratio increases, while all the void ratios of single pellets increase. This could be explained that upon wetting, water is successively adsorbed into the inter-layer, inter-particle and inter-pellet voids, leading to the subdivision of particles and swelling of aggregates and pellets. Under constant volume condition, aggregates and pellets tend to swell and fill into the inter-aggregates or inter-pellets voids. While under free swelling condition, the particles and aggregates in a single pellet tend to swell outward rather than squeezing into the inter-aggregate voids, leading to the expansion of the pores and even formation of cracks. Results including the effects of initial conditions (initial dry density and fabric) and constraint conditions (constant volume or free swelling) on the water retention capacity and pore structure evolution reached in this work are of great importance in designing of engineering barrier systems for the HLW repository.

     

Investigation on healing behavior of unsaturated GMZ bentonite pellet mixture based on compressibility

Acta Geotechnica - Bentonite pellet mixture has been considered as an important candidate buffer/backfill material in deep geological repository for disposal of high-level radioactive waste. During...     

Model suggested for an important part of the hydro-mechanical behaviour of a water unsaturated bentonite

The hydro-mechanical behaviour of a clay-based buffer material for nuclear waste disposal has been investigated in a laboratory program. In this program, the main focus was on the influence of confinement on water uptake and swelling pressure during suction decrease. The laboratory program and some of the results are presented by Dueck [Dueck, A., 2006. Laboratory results from hydro-mechanical tests on a water unsaturated bentonite. submitted for publication.].

The results from the laboratory tests were used to find a relationship between water content, void ratio, swelling pressure and suction. Two equations for swelling pressure represent the outline of the model.

In the first equation, the swelling pressure developed during water uptake is normalised by a pressure corresponding to the swelling pressure at saturation. This is done in order to be independent of void ratio. A relationship between the normalised swelling pressure and the degree of saturation is suggested.

The second equation describes a relationship between the swelling pressure, the water content and the actual suction (or relative humidity). The equation is based on a thermodynamic relationship and includes the retention curve (i.e. water content vs. suction under free swelling conditions).

The model can be used for a state where two of the four variables; water content, void ratio, swelling pressure and suction are known and can thus be useful to evaluate field measurements and model late stages of the wetting process. An example of an application is given. The equations are mainly based on results from tests with increasing degrees of saturation under constant void ratio but are also suggested for use with increasing void ratio.     

Effects of salt solutions on the hydro-mechanical behavior of compacted GMZ01 Bentonite

In this study, the effects of salinity of infiltrating solutions on the swelling strain, compressibility, and hydraulic conductivity of compacted GMZ01 Bentonite were investigated. After swelling under vertical load using either distilled water or NaCl solutions with concentrations of 0.1, 0.5 M, and 1 M, laboratory oedometer tests were conducted on the compacted GMZ01 Bentonite. Based on the oedometer test results, hydraulic conductivity was determined using the Casagrande’s method. Results show that the swelling strain of highly compacted GMZ01 Bentonite decreases as the concentration of NaCl solution increases. The compression index C _c ^* increases and then turns to decrease with an increase in the vertical stress or a decrease in the void ratio for different solutions, and the C _c ^* decreases as the concentration of NaCl solution increases. The secondary consolidation coefficient C _α increases linearly with the increase of the compression index C _c ^* . Furthermore, a bi-linear relationship between the swelling index C _s ^* and the secondary consolidation coefficient C _α can be characterized clearly. The hydraulic conductivity increases as the concentration of NaCl solution increases, however, this increase can be prevented if a high confining stress is applied.     

高庙子膨润土中蒙脱石碱性溶蚀的矿物学证据

高放废物地下处置库运营过程中,衬砌混凝土在地下水作用下产生的强碱性溶液,会扩散进入膨润土缓冲屏障,长期将使屏障性能发生退化。采用不同 pH 值的 KOH 溶液模拟衬砌混凝土溶出的碱性溶液,在室内进行为期一年的常温接触扩散试验,开展X射线荧光光谱（X-ray fluorescence spectroscopy,简称XRF）、X射线衍射（X-ray diffraction,简称 XRD）、扫描电镜（scanning electron microscope,简称 SEM）和能谱（energy dispersive spectrometry,简称 EDS）试验,研究强碱性溶液缓慢扩散对高庙子膨润土矿物的影响。XRF试验结果表明,当 KOH 溶液的 pH>12.6时,膨润土中 Si 元素的含量开始减少,即含Si 矿物蒙脱石、石英、方石英等发生了溶解;同时,K元素含量增多,表明膨润土与碱溶液发生离子交换反应,溶液中大量的K⁺进入蒙脱石层间。XRD 试验结果表明,在 pH=12.6时,蒙脱石矿物的 001峰开始右移、变宽,峰值强度大大减弱;在pH>13时,衍射角θ整体向右偏,晶面间距由原始膨润土样的1.385 3 nm（13.853 Å）减小为1.221 0 nm（12.210 Å）,表明蒙脱石矿物晶层被压缩。随着 KOH 溶液 pH 值增大,膨润土中蒙脱石、石英等矿物的含量明显减少,长石类矿物的含量逐渐增加,伊利石和斜发沸石的含量也呈现略微增多的趋势。SEM 试验结果表明,随着溶液 pH 的增大,蒙脱石部分晶层会发生重叠,进而产生部分裂隙和孔洞,加速蒙脱石的溶蚀进程。在为期一年的接触扩散试验中,pH=13.8的 KOH 溶液扩散深度超过7.5 mm,并且在碱溶液与膨润土接触面上观察到新生成的伊利石微晶,证实强碱性溶液会导致蒙脱石溶解及伊利石化。     

An experimental study of the water transfer through confined compacted GMZ bentonite

GMZ bentonite has been considered as a possible material for engineered barrier in the Chinese program of nuclear waste disposal at great depth. In the present work, the hydraulic conductivity of this bentonite was determined by simultaneous profile method. A specific infiltration cell equipped with five resistive relative humidity probes was designed for this purpose. The water retention properties were studied under both confined and unconfined conditions; the results show that at high suctions (> 4 MPa) the water retention capacity is independent of the confining condition, and by contrast, at low suctions (< 4 MPa) the confined condition resulted in significant low water retention. Furthermore, the microstructure was investigated at mercury intrusion porosimetry (MIP) and Environmental Scanning Electron Microscope (ESEM) in different states: on oven-dried powder, bentonite slurry, as-compacted and wetted samples. It has been observed that the soil powder is constituted of aggregates of various sizes; these aggregates are destroyed by full saturation at a water content equal to the liquid limit; compaction at the initial water content of 11–12% and a dry density of 1.7–1.75 Mg/m³ led to a microstructure characterized by a dense assembly of relatively well preserved aggregates; saturation of the compacted sample under constant volume condition defined a non-homogeneous microstructure with the presence of well preserved aggregates. This non-homogeneous microstructure would be due to the non uniform distribution of the generated swelling pressure within the soil sample upon wetting. The hydraulic conductivity determined has been found decreasing first and then increasing with suction decrease from the initial value of about 80 MPa to zero; the decrease can be attributed to the large pore clogging due to soft gel creation by exfoliation process, as observed at ESEM.     

Effect of salt concentration on desiccation cracking behavior of GMZ bentonite

Bentonite has been proposed for use as an engineering barrier and buffer in nuclear waste repositories and has been used frequently in municipal waste landfills. The cracking behavior and deformation properties of this material can be influenced by the chemistry of pore water. In the present work, the influence of salt concentration on the desiccation cracking behavior of GMZ bentonite was investigated with laboratory experiments. Image processing techniques and SEM tests were performed on the specimens which had undergone the desiccation testing in order to analyze the cracking mechanisms. Results show that the water evaporation process can be identified by a steady rate stage, a falling rate stage and a residual stage. The water evaporation rate is strongly affected by the salt concentration of the pore water; higher salt concentrations result in lower evaporation rates; the final water content is strongly impacted by a high initial salinity; otherwise the water contents are very similar for the residual stage. During desiccating, most of cracks appeared at the steady evaporation stage. The cracking morphology and patterns were greatly affected by the salt concentration of the pore water; and larger crack lengths and lower crack densities were obtained as the initial salinity was increased.     

大气条件下高庙子钠基膨润土化学缓冲特性研究

深地质处置目前被国际上公认为是处置高放废物的最有效可行的方法。我国采用多重工程屏障系统和适宜的地质体共同作用来确保与生物圈的安全隔离。缓冲材料是高放废物重要的工程屏障材料之一,我国选用高庙子钠基膨润土作为缓冲材料的基础材料。膨润土作为缓冲材料的一个重要性能表现为缓冲孔隙水的化学变化。介绍了GMZ-1钠基膨润土大气条件下与蒸馏水的反应试验,并对试验结果进行了讨论。批式试验反应溶液中钠离子来源于钠基膨润土层间阳离子和矿物溶解,镁离子来源于钠基膨润土层间阳离子,钾离子和钙离子来源于矿物溶解,相关研究认识对于高放废物处置库近场核素迁移研究和评价工程屏障的长期稳定性具有重要意义。     

An unsaturated hydraulic conductivity model for compacted GMZ01 bentonite with consideration of temperature

As one of the most important properties of compacted bentonite used as buffer/backfill materials, hydraulic conductivity is influenced by various factors including temperature, microstructure and suction (or degree of saturation), etc. Based on the readily available results of both temperature-controlled water-retention tests and unsaturated infiltration tests under confined (constant volume) conditions, influences of temperature and microstructure variations on unsaturated hydraulic conductivity of the compacted Gaomiaozi (GMZ01) bentonite were analyzed. Then, a revised unsaturated hydraulic conductivity model considering temperature effects and microstructure changes was developed. With this proposed model, prediction and comparison were made on the unsaturated hydraulic conductivity of the compacted GMZ01 bentonite at 20 °C. Results show that water-retention capacity of compacted GMZ01 bentonite decreases as temperature increases and the degree of the temperature influence depends on suction. Under confined conditions, influence of hydration on microstructure of compacted GMZ01 bentonite depends on pore size. The proposed model can well describe the variations of unsaturated hydraulic conductivity with suction at different temperatures. However, further improvement of the proposed model is needed to account for the phenomenon of inter-aggregate pores clogging that occurred at the initial stage of hydration of compacted GMZ01 bentonite under confined conditions.     

Numerical modelling on water retention and permeability of compacted GMZ bentonite under free-swelling conditions

In this study, a solid–liquid–gas coupled equation was established to simulate water retention characteristics of highly compacted GMZ bentonite. Then, modelling results were compared with laboratory test results. Results indicate that GMZ bentonite has a strong moisture expansion (or a limit drying shrinkage) characteristic. The control equation can simulate the water absorption and deformation characteristics very well at high relative humidity (or low suction). Environmental scanning electron microscope (ESEM) observation reveals the course grain soil texture of the surface under low relative humidity (RH), while the surface of GMZ bentonite becomes smooth (more fine-grained soil texture) as RH increases. Differences were found between the porosities calculated by macroexperiment results and microscopic observations with ESEM method. This is because only the interaggregate pores can be observed by ESEM photographs. Additionally, we find that the simulated effective porosities are close to the results calculated by microscopic tests, while the effective porosity is considered as the main flow channel of flow. Further, the intrinsic permeability, the effective water and gas permeability are calculated based on the proposed model. The modelling results coincide well with the laboratory experimental results and support the reliability of the proposed model.     

Water retention and compression behavior of MX80 bentonite pellet

Acta Geotechnica - Bentonite pellet/powder mixture has been considered as a possible sealing material in deep geological disposal of radioactive waste. As the hydro-mechanical behavior of such...     

Effective stress incorporating osmotic suction and volume change behavior of compacted GMZ01 bentonite

Acta Geotechnica - Investigation of osmotic suction effects on the volume change behavior of clay is of great importance in evaluating the performance of engineering barrier in deep geological...     

Research on water retention and microstructure characteristics of compacted GMZ bentonite under free swelling conditions

In this study, water retention tests under free swelling conditions were performed to investigate the water intake (or loss) behaviour of compacted GMZ bentonite. First, the water retention characteristics were investigated, and then the microscopic pore structure was observed by environmental scanning electron microscope (ESEM). The results indicate that GMZ bentonite has a strong swelling (or a limited shrinkage ability) due to water intake (loss). The suction behaviour of GMZ bentonite is similar to MX80 bentonite and FEBEX bentonite. We also find that the confinement conditions can affect the suction behaviour of the material, especially at high relative humidity (RH). Additionally, a mathematic model can fit the mass change data very well. Microscopic tests show that the granular sensation of GMZ bentonite is obvious for a sample at low RH. With the increase in RH, the surface of GMZ bentonite becomes more smooth. The differences in the porosities calculated by the macroscopic and microscopic tests can be attributed to image resolution. The inter-laminar pores and intra-aggregate pores cannot be observed by the ESEM method. In addition, ESEM observation can provide an intuitive basis for the further research of the seepage property of GMZ bentonite.     

A hybrid discrete-continuum approach to model hydro-mechanical behaviour of jointed rocks

A hybrid discrete-continuum approach has been presented in this paper to simulate water flow in the near and far fields of deformable fractured rocks. In the near field, the discrete model is used; while in the far field, the equivalent continuum model is employed. The discrete element method (with the static relaxation algorithm) is used in the near field and the boundary integral equation method in the far field. Along the interface of these two domains, both mechanical and hydraulic compatibility conditions are satisfied. Fully coupled hydro-mechanical analysis can be conducted in the combined near and far fields. Application to a dam foundation problem has demonstrated the capability of the developed approach.