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Investigation on the hydraulic behaviour of nanoparticles based LBF exposed to salt solutions Sarkar, Grytan 2015

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INVESTIGATION ON THE HYDRAULIC BEHAVIOUR OF NANOPARTICLES BASED LBF EXPOSED TO SALT SOLUTIONS  by Grytan Sarkar  B.Sc., Khulna University of Engineering and Technology, Bangladesh, 2009  A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF  MASTER OF APPLIED SCIENCE in THE COLLEGE OF GRADUATE STUDIES (Civil Engineering)  THE UNIVERSITY OF BRITISH COLUMBIA (Okanagan)  May 2015  © Grytan Sarkar, 2015  ii Abstract In this study, the effect of pore fluid chemistry on the hydraulic behaviour of the light backfill (LBF i.e. 50-50 bentonite-sand compacted at a dry density of 1.24 Mg/m3) was investigated. The experimental results showed that pore fluid reduce the swelling pressure and increase the hydraulic conductivity of LBF. The change in the pore structure such as pore size, pore distribution and pore connectivity due to the presence of pore fluid was also investigated using Xradia Micro XCT-400. The X-ray source, detector, and a small portion of LBF specimen (5.5 mm in diameter) were placed closely to acquire a good quality image with a voxel size of 1.15 x 1.15 x 1.15 μm. The scanned images were de-noised and segmented to study the pore space. An algorithm was developed to compute the volume porosity and pore-size distribution of the scanned samples. Additionally, the interconnected pore components and absolute permeability of the LBF samples were analyzed using Avizo software. The results of microstructure analysis demonstrate that the porosity, pore size, volume of interconnected pores and hydraulic conductivity of the LBF increased due to pore fluids.  The effectiveness of using nanoparticles of bentonite to minimize the impact of saline solutions on hydraulic responses of LBF, were studied. Both the mechanical attrition (planetary ball mill) and the synthesis process (ultra-sonication, centrifuging and filtering techniques) were integrated to prepare the nanoparticles. The particle size analysis of nanoparticles through Scanning Electron Microscopy (SEM) showed particles range between 30 to 140 nm. Additionally, the chemical and mineralogical composition of nanoparticles analyzed by Energy Dispersive Spectroscopy (EDS) and X-Ray Diffraction (XRD) exhibited similar behaviour of the parent bentonite. Subsequently, two different percentage of nanoparticles (1 % and 2 % of dry bentonite) were carefully mixed with the bentonite-sand  iii mixture to prepare nanoparticles based LBF. Nanoparticle based LBF showed excellent performance in reduction of hydraulic conductivity, when LBF is exposed to both fresh water and saline solutions.    iv Preface This thesis poses a new technique to improve the sealing system in the deep geological barrier in the Canadian Shield with the continuous support from Dr. Sumi Siddiqua.   A version of Chapter 2 was submitted to Environmental Geotechniques. The paper is entitled Preparation of Wyoming Bentonite Nanoparticles and authored by Sarkar G., Dey A., and Siddiqua S. It is consisting of conjunct results with Dey A., and Siddiqua S. Dey A. It is consisting of conjunct results with Dey A., and Siddiqua S. Dey A. investigates the effect of ball type, size, solvent type, rotational speed and duration of grinding on the particle size of the ground bentonite. I performed both the grinding and synthesis process to reduce the size of ground bentonite. I compiled the data in a graphical presentation and wrote the manuscript. Siddiqua S. contributed to design of the experimental study, continuous monitoring during laboratory work and edit the manuscript. A version of Chapter 3 was submitted to Engineering Geology. The paper is entitled Effect of fluid chemistry on the microstructure of light backfill: an X-ray CT investigation and authored by Sarkar G. and Siddiqua S. I performed various parts of the experimental investigation and analysis and wrote the manuscript. The experimental parts includes, sample preparation, geotechnical testing, scanning of sample and the analysis parts includes geotechnical data analysis, image analysis and development of algorithm. Siddiqua S. contributed to continuous monitoring of the work, giving valuable suggestion and assists to prepare the manuscript.   A version of Chapter 4 will be submitted to Geotechnical and Geological Engineering. The paper is entitled Study on the effect of fluid chemistry on the Nanoparticle  v based LBF and authored by Sarkar G., and Siddiqua S. I performed all sort of laboratory experiments such as sample preparation and geotechnical testing. Finally, I analyzed data and wrote the manuscript. Siddiqua S. helps to design of experiments, monitoring during testing and assist to write the manuscript.    vi Table of Contents Abstract .................................................................................................................................... ii Preface ..................................................................................................................................... iv Table of Contents ................................................................................................................... vi List of Tables .......................................................................................................................... ix List of Figures .......................................................................................................................... x List of Symbols ..................................................................................................................... xiii Acknowledgements ............................................................................................................... xv Dedication ............................................................................................................................. xvi Chapter  1: Introduction ........................................................................................................ 1 1.1 Background ............................................................................................................... 1 1.2 Radioactive Waste .................................................................................................... 4 1.3 Deep Geological Repository ..................................................................................... 6 1.4 Engineered Barrier System (EBS) ............................................................................ 7 1.5 Clay-water Interaction System .................................................................................. 9 1.6 Motivations and Objectives of the Study ................................................................ 11 1.7 Overall Methodology .............................................................................................. 12 Chapter  2: Preparation of Wyoming Bentonite Nanoparticles ....................................... 14 2.1 Background ............................................................................................................. 14 2.2 Nanotechnology Used in Geotechnical Engineering .............................................. 16 2.3 Materials and Methods ............................................................................................ 19 2.3.1 Materials ............................................................................................................. 19 2.3.2 Mechanical Attrition ........................................................................................... 19  vii 2.3.3 Extraction of Nanoparticles ................................................................................ 21 2.4 Results and Discussions .......................................................................................... 23 Chapter  3: Effect of Fluid Chemistry on the Microstructure of Light Backfill: An X-ray CT Investigation ............................................................................................................. 34 3.1 Background ............................................................................................................. 34 3.2 Materials and Methods ............................................................................................ 36 3.2.1 Materials ............................................................................................................. 36 3.2.2 Swelling Pressure and Consolidation Testing ..................................................... 37 3.2.3 X-ray CT Observation ......................................................................................... 38 3.2.4 Image Analysis .................................................................................................... 41 3.2.4.1 Preprocessing of Scanned Images ............................................................... 41 3.2.4.2 Analysis of Porosity and Pore Size Distribution ......................................... 43 3.2.4.3 Analysis of Pore Connectivity .................................................................... 45 3.2.4.4 Analysis of Hydraulic Conductivity ........................................................... 45 3.3 Results and Discussion ........................................................................................... 47 3.3.1 Swelling Pressure ................................................................................................ 47 3.3.2 Porosity ............................................................................................................... 48 3.3.3 Pore Size Distribution ......................................................................................... 49 3.3.4 Pore Connectivity ................................................................................................ 50 3.3.5 Hydraulic Conductivity ....................................................................................... 53 Chapter  4: Performance Study on the Hydraulic Behaviour of Nanoparticle-based LBF Exposed to Pore Fluid ........................................................................................................... 56 4.1 Background ............................................................................................................. 56  viii 4.2 Materials and Methods ............................................................................................ 59 4.2.1 Bentonite and Bentonite-nanoparticles ............................................................... 59 4.2.2 Sand ..................................................................................................................... 60 4.2.3 Sample Preparation ............................................................................................. 60 4.2.4 Experimental Study ............................................................................................. 61 4.3 Results and Discussions .......................................................................................... 62 4.3.1 Void Ratio ........................................................................................................... 63 4.3.2 Swelling Pressure ................................................................................................ 64 4.3.3 One Dimensional Constrained Modulus ............................................................. 65 4.3.4 Compression and Swelling Indices ..................................................................... 66 4.3.5 Hydraulic Behaviour of LBF .............................................................................. 68 Chapter  5: Conclusions and Further Studies .................................................................... 73 5.1 Conclusions ............................................................................................................. 73 5.2 Limitations of this Study ......................................................................................... 74 5.3 Recommendations for Future Study ....................................................................... 74 References .............................................................................................................................. 76   ix List of Tables Table 3.1: Experimental results of the samples ...................................................................... 47 Table 4.1: Mineralogical composition of bentonite and bentonite nanoparticles. .................. 59 Table 4.2: Chemical composition of bentonite and bentonite nanoparticles. ......................... 60 Table 4.3: Test scheme for swelling pressure and one dimensional consolidation test of LBF. ..................................................................................................................... 61   x List of Figures Figure 1.1: Inventory of Nuclear fuel projection in Canada from 2008 to 2013(International Nuclear Societies Council, 2012). ........................................... 5 Figure 1.2: Multiple barrier system (Adapted from Siddiqua et al. 2011) ............................... 8 Figure 1.3: Clay water interaction system (Adapted from Siddiqua et al. 2011) ................... 10 Figure 1.4: Overall methodology of this study ....................................................................... 13 Figure 2.1: Effect of ball types and size on the grinding of nanoparticles at a speed of 400 rpm for 30 minutes. ....................................................................................... 24 Figure 2.2: Effect of solvent on the grinding of nanoparticles using zirconia ball having diameter of 5mm at a speed of 400 rpm for 30 minutes. ..................................... 25 Figure 2.3: Effect of rotational speed on the grinding process using zirconia ball having diameter of 2 mm for 90 minutes. ........................................................................ 26 Figure 2.4: Effect of grinding period on the particle size of ground particles using zirconia ball having diameter of 2 mm at a speed of 800 rpm. ............................ 27 Figure 2.5: Effect of centrifugation time on the particle size of ground particles at a speed of 4000 rpm. ............................................................................................... 28 Figure 2.6: Microstructure of (a) bentonite, (b) ground bentonite before synthesis, and (c) ground bentonite after synthesis ..................................................................... 29 Figure 2.7: Particle size analysis of nanoparticles before and after synthesis. ....................... 30 Figure 2.8: Elementary analysis of the bentonite and its nanoparticle using EDS ................. 31 Figure 2.9: X-Ray diffraction patterns of Wyoming bentonite and its nanoparticles ............. 33 Figure 2.10: Crystal composition of Wyoming bentonite and its nanoparticles ..................... 33  xi Figure 3.1: (a) Sample mounted on the rotation stage of the Xradia Micro XCT-400; (b) Sample Holder ...................................................................................................... 39 Figure 3.2: Two-dimensional X-ray CT images of (a) dry bentonite-sand mixtures; (b) DW saturated bentonite-sand LBF sample after one dimensional consolidation test. ................................................................................................. 41 Figure 3.3: Image preprocessing steps for the dry sand, bentonite, bentonite-sand mixtures and DW saturated LBF. ........................................................................ 43 Figure 3.4: Determination of pore size distribution (a) picking a pore from binary image; (b) possible movement along neighboring pixels; (c) labeling the connected black pixel. .......................................................................................... 45 Figure 3.5: Particle size distribution of dry materials (bentonite, sand, bentonite-sand mixture) and DW and salt solutions saturated LBF. ............................................ 49 Figure 3.6: Results of the connected component analysis of DW saturated LBF samples (a) 3D Microstructure of the representative samples; (b) interconnected pore with a volume ranging from 1 to 10 μm3; (c) connected pore with a volume ranging from 10 to 100 μm3; (d) interconnected pore with a volume ranging from 100 to 1000 μm3; (e) interconnected pore with a volume larger than 1000 μm3; (f) interconnected pore space showing different sizes of pore volumes. ................................................................................................... 52 Figure 3.7: The effect of pore fluid on the number fractions and total porosity volume contributions of connected pores. ........................................................................ 53 Figure 3.8: Performance study of the X-ray CT analysis on the variation of hydraulic conductivity with the pore fluid type and concentration. ..................................... 55  xii Figure 4.1: Variation of void ratio with respect to applied axial stress for the LBF samples prepared according to Table 4.1. ............................................................ 64 Figure 4.2: Change in swelling pressure due to the inclusion of bentonite-nanoparticles for LBF samples prepared with DW and saline water. ........................................ 65 Figure 4.3: Change in 1D constrained modulus due to the inclusion of bentonite-nanoparticles for LBF samples prepared with DW and saline water. .................. 66 Figure 4.4: Change in compression index (CC) due to the inclusion of bentonite-nanoparticles for LBF samples prepared with DW and saline water. .................. 67 Figure 4.5: Change in swelling index (CS) due to the inclusion of bentonite-nanoparticles for LBF samples prepared with DW and saline water. .................. 68 Figure 4.6: Change in coefficient of consolidation (CV) due to the inclusion of bentonite-nanoparticles for LBF samples prepared with DW and saline water. .................................................................................................................... 70 Figure 4.7: Change in hydraulic conductivity (k) due to the inclusion of bentonite-nanoparticles for LBF samples prepared with DW and saline water. .................. 72  xiii List of Symbols E    Specific energy, kJ/kg p    Power input, kW β    net energy transmitted, % t    Ultrasonic time, s V    Sample volume, L Ts     Initial total solid concentration, kg/L tc    Centrifuging time, min ɳ    Viscosity coefficient, Pa.s, kPa R1 Distance from the surface of the liquid in the centrifuge to the center of the axis of centrifuge, cm R2  Distance from the particle in the centrifuge to the center of the axis of centrifuge, cm N     Centrifuge speed, rpm r     Radius of the particle in the centrifuge, μm Δd    Difference in density between the particles, g/cm3 v    Velocity through the porous media, m/s μ    Dynamic viscosity, kg/m.s L    Length of porous media, m K    Permeability of porous media, m2 P    Pressure difference applied to the sample, Pa ?⃗? .    Divergence operator ?⃗?     Gradient operator  xiv V⃗     Velocity of the fluid in the fluid phase of the material ∇2    Laplacian operator P    Pressure of the fluid in the fluid phase of the liquid, k    Hydraulic conductivity, m/s ρ    Density of fluid, kg/m3 g    acceleration due to gravity, m/s2 Cc    Compression index Cs    Swelling index Cv    Coefficient of consolidation, m2/s   xv Acknowledgements I would like to express my gratitude to my supervisor Dr. Sumi Siddiqua for taking a chance on me, without her continuous support, expertise and motivation, this thesis would not become in reality.  I would also like to thank my committee members, Dr. Kasun Hewage and Dr. Sunny Li for their crucial suggestion on my research projects, and my external examiner Dr. Keekyoung Kim, for taking the time to read and critique my thesis. Thanks to the Nuclear Waste Management Organization (NWMO) and National Science and Engineering Research Council (NSERC) for funding the research. I also must acknowledge Dr. Lukas Bichler, and Dr. Andre Phillion, for providing their lab facilities and training for the instrumentations.  I am really grateful to Fahmida Islam, Ashish Dey, Tim Abbott and Dean Courtney for their enduring help in the laboratory for their assistance during nanoparticle experimentation. I am also thankful to Dr. Maziar Shah Mohammadi and Sadegh Hasanpour for their assistance with the imaging experiments and analysis. I want to thanks all of the students of my research group, Md. Zillur Rahman, Md. Rafizul Islam and Amin Bigdelhi, for their enthusiastic support during my experimental work. I would also like to thank the laboratory technicians, especially Michelle Dawn Tofteland, Michele Cannon, Ryan J. Mandau and Russell LaMountain, for their endless help. I would like to express my greatest gratitude to my parents, brother and sister, without their unconditional love, caring and passions I can’t stay a day from thousands of mile away.  xvi Dedication  To my parents 1 Chapter  1: Introduction 1.1 Background Generation of radioactive waste is increasing day by day due to human activities, especially from nuclear power plant, mining and milling, chemical industries and some from educational and research activities. The safe disposal of this waste is now a major concern (Thomas et al., 1998). Currently, all the radioactive waste is temporarily stored in large containers on the surface but a long run sustainable solution is needed. Deep geological repository is one of the possible solutions for the safe disposal of high-level radioactive waste. In the deep geological concept, waste canisters are placed 500 to 750 m below the ground surface in a relatively impermeable rock (Pusch, 1982). Moreover, the concept of a multi-barrier system consisting of waste canisters, geological and engineered barriers makes the containment system more effective and sustainable (Gierszewski et al. 2004; NWMO. 2005). The geological barrier is mainly the host rock, which can be crystalline, sedimentary, clay stone, volcanic and salt rock (International Atomic Energy Association, 2009). The engineered barrier is composed of compacted clay and / sand buffer. Different types of clay such as Boom clay (Delage et al., 2010; Hemes et al., 2015), Callovo-Oxfordian clay (Bardelli et al., 2014),  Opalinus clay (Bock, 2015; Gens et al., 2009; Schuster et al., 2004; Stroes-Gascoyne, 2002), Avolena clay (Yong et al., 1997), Foca and Serata Clays (Vaunat and Gens, 2005; Villar, 2004; Yahia Aissa, 1999) and compacted bentonite (Baumgartner et al., 2008; Saba et al., 2014) are generally used for the sealing of radioactive waste.  Bentonite has been proposed as a barrier material in many countries for their future deep geological repositories because of its high swelling capacity and low hydraulic conductivity compared to other clays (Kawaragi et al., 2009). However, compacted bentonite  2 materials crack during the period of desiccation because of their high compressibility, which can be reduced by mixing a percentage of silt or clay.  The swelling pressure of compacted bentonite-sand mixtures primarily depends on: (i) the type and percentage of the clay content used in the backfill, (ii) the degree of compaction and (iii) the pore fluid present in the repositories (Morshedi and Sameni, 2000; Nelson and Miller., 1992). Generally, Na-bentonite and Ca-bentonite are two widely used ones in the engineered barrier system. Na-bentonite is broadly used in USA, Canada (NWMO., 2005), Japan (PNC, 1992), Sweden (SKB, 1983), and Finland (Vieno et al., 1993), while Ca-bentonite materials are popular in France (Coulon et al., 1986), Switzerland (NAGRA, 1985), Korea (Kang et al., 2003) and Spain (Linares et al., 1989).   The swelling pressure of bentonite also depends on the initial dry density of bentonite and / bentonite-sand mixtures.  The higher swelling pressure and low hydraulic conductivity of the materials can be obtained by increasing the initial dry density (Siddiqua et al., 2011). Based on the percentage of the bentonite and initial dry density of bentonite-sand mixtures, the Canadian repository concept designed the following sealing systems for disposal of radioactive and chemical wastes : (i) highly compacted 100% bentonite (HCB), (ii) highly dense bentonite sand buffer (BSB), (iii) dense backfill (DBF), (iv) low to medium dense light backfill (LBF), and (v) low to medium dense gap fill (GF) (Gierszewski et al., 2004; Siddiqua et al. 2011; Siddiqua et al., 2014).  In the Canadian context, chemistry of pore fluid existing in the locations of host rock at the repository depth is another critical component that needs attention. Based on the annual report of Nuclear Waste Management Organization (NWMO., 2005), the host rock at the repository depth (> 500 m) in the Ontario region contains a total dissolved solids (TDS)  3 concentration in the range of 10 to >100 g/L. The TDS in this region mainly consist of NaCl and CaCl2, which can diminish the swelling pressure and increase the hydraulic conductivity of Na-bentonite based materials to a great extent (Siddiqua et al., 2011, 2014). Although, there is no significant loss in the swelling pressure of Ca-bentonite based materials  in saline solutions (Karnland et al., 2005); still Na-bentonite is preferred over Ca-bentonite as   the swelling pressure of Na-bentonite is 4 times greater than Ca-bentonite (Koch 2008). Therefore, it is necessary to explore alternative techniques to minimize the influence of saline solutions on hydraulic properties of bentonite based sealing materials. Particles that have diameters less than 100 nm are known as nanoparticles, which hold a high surface to volume ratio i.e. specific surface area compared to their micro size counterparts. High specific surface area of nanoparticles may increase cation exchange capacity, decrease the gravitational force on the particles, and lower the bulk density (Majeed and Taha, 2013). Nanoparticles also show discontinuity due to the quantum confinement effects in materials with delocalized electrons, which affect the optical, magnetic and electrical behaviour of materials (Roduner, 2006; TRS – Royal Society and Royal Academy of Engineering, 2004). Thus nanoparticles play a vital role to change the properties of materials. Although Taha and Taha, 2012 observed that there is an improvement of maximum dry density (MDD), optimum moisture content (OMC), liquid limit (LL), plastic limit (PL), unconfined compressive strength (UCS) and hydraulic conductivity of a mixture of Malaysians UKM soil and sodium bentonite at a nano-clay content (i.e. Nanoclay Cloisite Na+ produced by Southern Clay Products, Gonzales, TX, USA) of 0.1 %; no reports were found that demonstrated detailed investigation on the hydraulic behaviour of nanoparticle- 4 based sealing materials exposed to saline solutions. Therefore, it is important to investigate the efficacy of using nanoparticles in the composition of sealing materials.  In this research project, LBF was selected out of the other types of Canadian system of sealing materials. In the first stage of this investigation, nanoparticles of Na-bentonite were prepared using both the mechanical attrition and synthesis process. The properties of nanoparticles such as particle size distribution, mineralogical characteristics and chemical compositions were computed. Secondly, the effect of pore fluid on the hydraulic properties of LBF was studied using one-dimensional consolidation tests. Also, the microstructure of LBF was analyzed using X-ray computed tomography (X-ray CT) technique to get a better understanding on the change in hydraulic behavior due to the pore fluid chemistry. Finally, nanoparticles were incorporated in the LBF matrix and the hydraulic behaviour of LBF observed under the influence of pore fluid chemistry. 1.2 Radioactive Waste Radioactive wastes are the materials containing or contaminated with radionuclides with a concentration higher than those established by the laws (International Nuclear Societies Council, 2012). The report of Inventory of radioactive waste in Canada, 2012 presented that the first radioactive waste in Canada was found since early 1930 after the uranium mine began at Port Radium in the Northwest Territories for medical use. Radioactive waste in Canada is generated directly from mining, milling, refining and conversion of uranium. It also generates from nuclear fuel fabrication before using in reactors and during operation of reactors, different nuclear research, and radioisotope manufacturer. These wastes can be categorized as high-level radioactive waste (HLRW), low and intermediate level radioactive waste (LLRW) and uranium mine and mill tailings. This report  5 also revealed that nuclear fuel wastes are mostly discharged from 22 CANDU power reactors of which 20 are located in Ontario and the others two are in Québec and New Brunswick. In addition, a very little portion of wastes are generated from research reactors in Atomic Energy of Canada Limited (AECL) and other sources. On the other hand, the LLRW wastes are generally generated for the production of electricity and different research and development works in medicine, education, agriculture and industry. For example, AECL has the research facilities in two locations: the Ontario facility is to develop reactor, apply nuclear/ environmental science and LLRW management; the Manitoba facility is to dispose the nuclear fuel waste and develop reactors (International Nuclear Societies Council, 2012). A 5 years record of projected liquid and dry nuclear used fuel stored in Canada (Figure 1.1) depicts that the generation of radioactive waste has been increasing every year and need to store in a safest place.    Figure 1.1: Inventory of Nuclear fuel projection in Canada from 2008 to 2013(International Nuclear Societies Council, 2012). 2008 2009 2010 2011 2012 20130.00.51.01.52.02.53.0Number of used fuel bundles (in millions)Projected time (Years) Dry storage Wet storage Total storage 6 1.3 Deep Geological Repository For the last thirty years to protect the environment and the human beings, the nuclear fuel wastes are safely stored in a special metal or concrete thick container that can absorb the radioactive emissions. The waste inside the container could be transferred into a new one after 100 years but it is an expensive way of storage. In addition scientists have developed new technologies that the wastes are reprocessed using specific type of reactor but some of the wastes are remained highly radioactive. Therefore, it is necessary to dispose these wastes in a safe and economic way without interrupting the container. A deep geological repository is a facility of safe, secure and permanent disposal of low to high level radioactive wastes. There are number of natural analogous consist of the natural reactor proved that a given waste disposal system is able to fulfill the requirements for long-term efficiency of deep geological repositories. For example natural nuclear reactor at Oklo in Africa, Ruprechtov in Czech Republic and Cigar Lake in Canada showed that radioactive materials produced by the natural spontaneously chain reaction penetrated the surrounding environment at a very slow rate (approximately 10 m per million years). The containers for disposal of radioactive wastes can be made with high resilient materials such as copper and it is discovered that the Greek and Egyptian shipwrecks lying on the seabed for more than two and a half thousand years remained practically undamaged by corrosion. In Canada nuclear fuel wastes are initially stored in a water-filled pool for about ten years and after that it is transferred to steel-lined concrete storage tank at the fuel reactor site. Although it is safe but requires continuous institutional control and therefore the NWMO has taken several research projects to seal these wastes at a depth of 500 to 1000 m in a stable low permeability rock. The concepts of deep geological repository is a part of the Adaptive Phased Management approach for long-term management of used fuel recommended to the federal government by the NWMO  7 comprises a system of multi-barrier system includes the waste form, nuclear waste container, buffer and backfill materials, other repository seals, and the geosphere (Hobbs et al., 2005).  1.4 Engineered Barrier System (EBS) The engineered barrier system (EBS) can be defined as the man-made confinement within the repository includes the waste form, canisters, backfill, buffer materials, seals and plugs (OECD, 2003). It is used to prevent and / or delay the percolation of the radionuclides from the radioactive waste to the repository host rock. Fuel Cycle Research & Development, 2011 mentioned that the design concept of EBS was found dates back to the late 1970’s and early 1980’s but the work of (Stula et al., 1980) is considered to be a pioneer in the performance analysis of various backfill. The goal of their study is to assess a general EBS design concepts and the need for ensuring adequacy and compliance with the performance and functional requirements, which represent various levels of complexity. Their work is now consistent with the advancement of multiple barriers concepts (Apted, 1997; McKinley et al., 2006). The design concepts of EBS is such that it will provide long-term safety in the order of thousands of years or at least during the first several hundred years after repository closure when the fission-product content is high, and where they might be mobilized by natural groundwater. The regulatory factors are generation of heat by the waste, potential interaction with the chemicals present in the waste and EBS, the mechanical behaviour of the surrounding rocks,  pH and redox conditions of the repository site and existing groundwater fluidity and chemistry (OECD, 2002).   8  Figure 1.2: Multiple barrier system (Adapted from Siddiqua et al. 2011) The NWMO has taken adaptive phased management (APM) approach to bury radioactive wastes as a long run solution (NWMO., 2005) under the Nuclear Safety and Control Act (NSCA) and selected by the Canadian Government in June 2007 (Crowe et al., 2013). A typical multi barrier concept was proposed for the repository in Canadian Shield is shown in Figure 1.2 (after Siddiqua et al. 2011). They proposed system consisting of six different types of bentonite based barrier systems such as highly compacted bentonite (HCB), bentonite-sand buffer (BSB), dense backfill (DBF), 70/30 bentonite sand (70/30BF), light backfill (LBF) and gap fill (GF) (Baumgartner et al., 2008). The HCB comprises of 100 % bentonite clay compacted at high density in the repository or prefabricated block. It is used as an outer shell of canisters. The BSB is prepared using bentonite sand mixture and installed at high dry density either in in situ compaction or prefabricated block forms. The HCB is surrounded by the BSB. A thick layer of DBF is applied around the BSB, which is prepared using a mixture of lake clay, crushed host rock and bentonite clay, either installed at high dry density by in situ compaction or prefabricated as blocks. Whereas LBF comprises of a mixture of bentonite and sand installed at a low-to-medium dry density. GF comprises  9 mainly bentonite clay, fabricated in the form of dense pellets, silica sand or some combination of the two, which are likely to be installed at low-to-medium dry density.  1.5 Clay-water Interaction System The clay surface carries a net negative charge resulting from the isomorphous substitution and of a break in the continuity of the structure as described in Murthy 2002. He also mentioned that the concentration of charge on the surface of the clay depends on the chemical and mineralogical properties of the clay particles. However, the activity on the clay surface depends on the concentration of the charged particles and specific surface area of the particles. When water is added to clay, the cations and anions are floats around the clay particles. The water molecules are dipolar as the two hydrogen atoms are connected with the oxygen atom at an angle of 120°and, therefore, these two positive and negative atoms stayed in the opposite end of a water molecule. The dipole water molecules are attracted to the charged clay particles in three ways (i) negative charged surface of the clay particles and the positive ends of the dipoles; (ii) floating cations on clay surfaces and the negative end of the dipoles; (iii) hydrogen bond between the oxygen atom in clay surface and the oxygen atom in the water molecules. These water molecules are arranged in a definite pattern in the vicinity of the boundary between the clay surface and water. It is observed that more than one layer of water molecules sticks on the surface with considerable force and this attractive force decreases with the increase in the distance of the water molecule from the surface known as zone of influence (Figure 1.3: Clay water interaction system (Adapted from Siddiqua et al. 2011). Therefore, a layer of electrically attracted water molecules that forms around the clay particles is termed as diffuse double layer (DDL). The water remained in this zone (adsorbed water) bears the property of soil near to the surface and behaves like a viscous fluid at the  10 middle of the layer. The first theoretical model that describes  the existence of double layer at the surface of the metal being charged with an electrolyte was observed by Helmholtz 1879. It was then improved to a great extent by Gouy and Chapman (Chapman, 1913; Gouy, 1910) that describes a diffuse double layer of accumulated ion is found a bit distance from the solid surface due to Boltzmann distribution. The main objective of this model is to determine the electric potential at the middle of two parallel flakes. This model is widely used to relate clay compressibility to basic particle-water-cation interaction (Bolt and Miller, 1958; Bolt, 1955; Mesri and Olson, 1971; Mitchell, 1993, 1960; Olsen and Mesri, 1970; Schanz and Tripathy, 2009; Sridharan and Jayadeva, 1982; Sridharan and Rao, 1973; Stula et al., 1980; Tripathy et al., 2004; Van Olphen, 1977).  Figure 1.3: Clay water interaction system (Adapted from Siddiqua et al. 2011)  11 1.6 Motivations and Objectives of the Study The previous studies on LBF are limited to the hydraulic and mechanical behaviour of the material. No previous works were reported for the microstructural investigation of LBF under the presence of pore fluid chemistry; although microstructure plays an important role on the material’s performance. In this research, a non-destructive X-ray CT technique was used for the very first time to explore the 3D microstructure of LBF and to investigate the relationship between the pore connectivity and hydraulic conductivity.  In addition, this research further studied the effectiveness of nanoparticles of bentonite in the LBF matrix via performing one-dimensional consolidation tests.  . The overall goal of this research was to characterize the effect of pore fluids on the hydraulic responses of currently used LBF materials and explore solutions to minimize the effects. The specific objectives of this thesis were summarized as follows: 1. Development of the methodology to process and characterize the properties of nanoparticles of bentonite. The bentonite nanoparticles are to be processed in such a way that it does retain the properties of parent materials. 2. Experimental investigation of the effect of pore fluid chemistry on selling pressure and hydraulic conductivity of LBF and explore the microstructural behaviour of LBF using X-ray CT technique. The microstructural behavior of LBF comprises of 3D pore structure, porosity, pore size distribution and pore connectivity.  3. Study on the hydraulic performance of nanoparticle based LBF exposed to various salt solutions. A percentage of bentonite nanoparticles of the total bentonite content were added to prepare nanoparticle based LBF.   12 1.7 Overall Methodology The overall work of this study was presents into three sections because of the multidisciplinary nature of the work (Figure 1.4). The relevant background was incorporated in each section to provide a better understanding of the work. Chapter 2 presents a detailed preparation technique of bentonite nanoparticles. Bentonite nanoparticles were prepared using both the mechanical attrition and synthesis process. Mechanical attrition method was used to grind the micro-size bentonite particles and the controlling parameters in this process were the types and sizes of grinding ball, types of solvent, grinding speed and durations. However, the ground particle comprises of both the micro and nano-sized particles. The nano-size particles were separated using synthesis process, where the controlling parameters were sonication energy and centrifugation. The effect of pore fluid on the hydraulic behaviour of LBF, prepared with 50% sand and 50% bentonite, was determined from the oedometer tests and the microstructural behaviour of the LBF samples were computed using X-ray CT observations are described in  Chapter 3. A small portion of the LBF samples at the end of  a consolidation test was taken for the X-ray CT investigations. Chapter 4 presents the performance study on the hydraulic responses of the nanoparticle based LBF exposed to salt solutions. Two different percentage (1% and 2% of the  bentonite content) of bentonite nanoparticles were first throughly mixed with the dry bentonite. Afterward, the composite of bentonite and bentonite nanoparticles were further mixed with dry sand, which was saturated with distilled water and salt solutions and compacted to prepare nanoparticle based LBF. The swelling pressure and hydraulic conductivity of the samples were computed using consolidation apparatus.  13  Figure 1.4: Overall methodology of this study 14  Chapter  2: Preparation of Wyoming Bentonite Nanoparticles 2.1 Background The word “Nanoparticles” derived from the Greek word dwarf means one billionth are the multidimensional particles having dimension less than 100 nm (Roco, 2005; Wilson et al., 2008). Generally, it consist different shapes such as spherical, tubular, or irregularly shaped and can exist in fused, aggregated or agglomerated forms (Aitken et al., 2006; Nowack and Bucheli, 2007). Although these particles are naturally present in some extents as soot, carbon black and organic colloids from the millions of years ago but its uses  in various fields was found from thousands years ago (Nowack and Bucheli, 2007). Recently, because of its potentiality in different field of research such as electronic, energy, environmental, pharmaceutical, biomedical, catalytic cosmetic and material applications, a huge amount of research grant is invested in the nanotechnology (Guzman et al. 2006). The idea of nanotechnology was first proposed by Richard Feynman, 1960 and his speech in the American Physical Society in Pasadena was entitled as “Plenty of Room at the Bottom” (Feynman, 1960). The invention of scanning electronic microscopy (SEM), X-ray diffraction (XRD) and atomic force microscopy (AFM) leads the technology to a great extent. The unique properties of nanoparticles lead to the development of novel materials, which are different from their natural state. The main advantage of using nanoparticles is the large surface to volume ratio as it can potentially improve the catalytic process and interracially driven phenomena (wetting and drying) (DeCastro and Mitchell, 2002) Based on the sources, nanoparticles can be classified as natural and anthropogenic. The anthropogenic nanoparticle also called engineered nanoparticles are produced by combustion, 15  synthesized, mechanically attrition or intentionally and different way according to their uses. Engineered nanoparticles are fullerenes and CNT, dendrimers, both pristine and functionalized and metals (quantum dot, nanoiron, nanogold, nanosilver) and metal oxides such as TiO2 and Ag. Whereas, natural nanoparticles exist in airborne combustion by products, soil sediments, ocean surface micro layers, natural water body, humic substances, metal oxides, clay minerals, volcanic ash, viruses and diesel exhaust particles (Aitken et al., 2006; Calabi-Floody et al., 2011; Murr et al., 2004; Reddy et al., 2002; Waychunas et al., 2005). Natural nanoparticles play an important role in sorption capacity for metallic and anionic contaminants, enhancing contaminant transport, biogeochemical reactions and kinetics, and in geocatalysis (Hochella et al., 2008; Ryan and Elimelech, 1996; Waychunas et al., 2005; Wilson et al., 2008). It can be further classified according to their chemical composition such as carbon containing and inorganic nanoparticles. This carbon containing nanoparticles again subdivided into biogenic, geogenic, atmospheric and pyrogenic. Nanoparticles are widely used for the removal of pollutants such as poly chlorinated biphenyl (PCB), trichloroethane (TCE), perchlorate, chromium, lead, nitrate, copper, and zinc (Ellen et al., 1995; Liao et al., 2007). Reddy et al., 2002 stated that nano iron particles (NIP) can be used to decontaminate contaminants in soils and groundwater within a very short time.  In addition, the hydrocarbons in contaminated soil can be removed by using synthesize nano zero-valent iron (nZVI) and the synthesize nanoparticles can be produced by using ultrasonic waves and sodium borohydride and NaBH4 as strong reducing agents (Lim and Okada, 2005; Masciangioli and Zhang, 2003; Schrick et al., 2002). Majority of engineering nanoparticles are produced by USA (49%) and European Union (30%) (Aitken et al., 2006) 16  Bentonite nanoparticles can be categorized as an improved geotechnical material. A very few research was found in the field of bentonite nanoparticles and it showed that it can be produced artificially by mechanical attrition. Some of the literature showed synthesis process can be a useful tool for the separation of natural nanoparticles present in the soil. In the current study, sodium rich Wyoming bentonite was used to prepare nanoparticles using mechanical attrition and synthesis process. The main objective of this study is to prepare nanoparticles using both the mechanical attrition and synthesis process without affecting the properties of the parent soil. In addition, this study emphasis that no chemicals were used for degrading and dispersing the bentonite samples as it may change the properties of original soil. In the first stage, mechanical attrition was performed using planetary ball mill (Pulverisette 7) and different parameter associated with the grinding process was selected. The particle size was analysed after the completion of each grinding using a zetasizer. In the second stage, the bentonite-water aqueous solution was dispersed using ultra-sonication and the larger particles were settled down using centrifugation techniques. The particle size and chemical composition of the ground particles were determined using SEM and EDS respectively. In addition the mineralogical content of the original and ground particles were determined using XRD analysis.   2.2 Nanotechnology Used in Geotechnical Engineering Nanotechnology is a very new and modern approach in geotechnical engineering field. Nanoparticles in soil may exists in one of the three forms such as nanoplatelets (one dimensional at nanoscale), nanowires or nanotubes (fibrous particle and two dimension at nanoscale), and nanodots (all the three dimension at nanoscale) (TRS – Royal Society and Royal Academy of Engineering, 2004). Nanoplatelets are the clay minerals having platy shape such as kaolinite, illite, smectite, chlorite, and vermiculite. Nanowires or nanotubes are the fibrous type clay 17  mineral such as imogolite, halloysite, palygorskite, and sepiolite. On the other hand, nanodots consisting of organic matter, allophane, and metal oxides (mainly iron oxides). Exfoliated smectite nanoplatelets containing both negative permanent charge (isomorphous substitution) and variable charge (pH dependent)  are widely used for making clay-polymer nanocomposites as it has ability to exfloite in dilute aqueous solution to produce 1 nm thick and 2:1 layer of nanoparticles.  The properties of naturally occurring soil nanoparticles are summarized in Zhang, 2007. These nanoparticles in soil plays an important role as it has influence on soil solubility, transport, and degradation of the pollutants and also it makes soil behaviour complex and reactive (Calabi-Floody et al., 2011; Nowack and Bucheli, 2007; Pranzas et al., 2003). Nanoparticles are found naturally in soil, for example, smectite, imogolite, halloysite, palygorskite, sepiolite, allophane, hematite, goethite, aluminosilicate, oxides and hydroxides of Al, Fe, and Mn enzymes, humic substances, viruses and mobile colloids (Kretzschmar and Schafer, 2005). Majeed & Taha, 2013 presented two way of applying this technique in geotechnical engineering that studying soil structure in nanometer scale for gaining better understanding of soil nature and manipulation of nanoparticles with the existing soil as an external factor. Yalamanchili et al., 1998 also suggested some novel methods for investigating the soil nano-structure and particle analysis through SEM, atomic force microscopy (AFM) and transverse electron microscopy (TEM). The dimension, shape and morphology of the particles and its crystal in nano scale can be presented by these methods in an image form (Köllensperger et al., 1999). In addition, the porosity of the soil can be measured by analyzing the nanometer image after TEM and there is an improvement of shear behaviour of clay having nanostructure rather than clay sediments with random orientation as described in Shephard, Bryant, & Chiou, 1982. 18  (Hochella, 2002) stated that the surface morphology and shear strength parameters such as adhesive forces and friction angle between the soils particles can be measured using the image of AFM. The chemical and mineralogical composition of the nanoparticles can be determined by using energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD), respectively. The size of the bentonite nanoparticles and its crystal can be determined using particle size analyzer (PSA) and XRD, respectively.  Nanoparticles poses high specific surface area based on the principle that only one atom out of 1000 atoms is located at the boundary for the micro-clay particles. Whereas, 15 % atoms present in the boundary when particle size reduced from micron to 12nm and about 40 % atoms present on the interface when particle size reduce to 5 nm. Moreover, the specific surface area of outer side is differing from the inner one due to the intra void in between particles as described in Carrado et al., 2004. The intra-particle void or nanoporosity in soil is found in two forms: inter-aggregate and intra-aggregate causes higher liquid limit (LL) and plastic limit (PL) in the wetting stage and after being dried it shows opposite behaviour like shrinkage (Borchardt, 1989; Ladd et al., 2004). The increase in surficial atoms also leads to improve the catalytic, hydraulic and thermal conductivities, thixotropic property and shear strength, magnetic and elastic properties. In addition, the high surface to volume ratio improves the cation exchange capacity (CEC), decrease the gravity force on the particles, poses lower bulk density, show quantum behaviour for the random movement and energy of particles (Majeed and Taha, 2013). Therefore, the main reason for the changing of soil properties in nano form is due to surface charge and quantum effect (TRS – Royal Society and Royal Academy of Engineering, 2004). Taha & TAHA, 2012 observed that there is an improvement of maximum dry density (MDD), optimum moisture 19  content (OMC), LL, PL and unconfined compressive strength (UCS) at a nano-clay content of 0.2 %. 2.3 Materials and Methods 2.3.1 Materials In the current study, Wyoming bentonite (a type of sodium bentonite) was used to prepare the nanoparticles. Generally, the sodium bentonite is used in the clay barrier system due to its low hydraulic conductivity, self-sealing capacity and swelling characteristics. It is composed of about 70 % montmorillonite content along with quartz and illite. The montmorillonite is a flaky shaped particle with monoclinic crystal having lateral dimension ranging from 1000 Å to 5000 Å and thickness of 10 Å to 50 Å. Mineralogy of montmorillonite is described by combination of silica tetrahedrons sheet and aluminum or magnesium octahedron sheet (Das, 2013). The average particle size of this bentonite was found within the range of 15 to 20 μm.  Because of its crystalized structure and mineralogical composition, this bentonite showed higher specific surface area (750 m2/g), cation exchange capacity (76 meq/ 100g), absorption capacity (7 to 10 times of its own volume) and swelling potential (18 times of dry volume) (Dixon, 1994). Beside that the specific gravity and refractive index was found 1.74 and 1.48 to 1.53, respectively. In addition to attain wet grinding process isopropyl alcohol (IPA), which is a volatile solution, was used to maintain a safe pulverization temperature. 2.3.2 Mechanical Attrition Bentonite nanoparticles can be produced from both the bottom up process such as self-assemblage and template synthesis and top down process such as mechanical attrition process. In the current study, both the processes were synchronized to prepare bentonite nanoparticles. Based on the value of induced mechanical energy to mixture, mechanical attrition is classified as 20  low energy and high-energy systems. In the low energy ball mill, the ball is dropped directly from the top of the mill to the feed stock materials by controlling an optimum speed. Whereas in the high-energy ball mill, particle size of the powder can be reduced to nano-size by changing the chemical composition of predecessors at least cost (Koch, 1997). Different types of attrition mills such as attritor mills, vibratory mills, high speed blender and shakers, planetary mills and even large scale ball mills are widely used for pulverizing the materials to prepare nanoparticles (DeCastro and Mitchell, 2002). For example, nanoparticles of soils preparation with the pulverizer is mentioned in the study of Taha, 2009 using Fritsch Pulverisette 6. The grinding bowls and balls are generally made of agate, silicon nitride, sintered corundum, zirconia, chrome steel, Cr-Ni steel, tungsten carbide, and polyamide depending on the hardness of the material used. The zirconia ball have grater hardness and suitable for the flaky particles like bentonite.  In this study, Wyoming bentonite was pulverised using a planetary ball mill (Pulverisette 7) having 80 ml zirconia bowls which is advantageous for small scale project. The vial in this ball mill rotates like a planet along with a supporting rotating disk causing special movements around their own axis. The vial and supporting disk rotate in the opposite direction and therefore centrifugal force acting inside the vial causing friction between the grinding balls as they rundown inside the vial chamber and ground particles. In addition, the grinding balls lift off and moves freely through the inside chamber and impact on the inside vial wall causing grinding of materials. In the first stage of wet grinding, different parameters such as types and size of grinding balls, types of solvent, speed and duration of grinding were selected through continuous grinding and particle size analysis using zetasizer. Initially steel and zirconica balls having diameters of 10, 5 and 2 mm were used to observe the effect of ball size and types. At first, each of the bowls was filled with 100 g ball and 7 g bentonite. And the remaining volume of the bowls 21  was filled with water.  Then the bowls were kept into the rotating vial and it was pulverized for 30 minutes. After completion of the grinding the balls were separated using 4.75 mm sieve and the screened materials were sent to zetasizer for particle size analysis.  After confirming the ball types and diameters, the particles were ground using IPA instead of water with zirconia balls having diameter of 5 mm at a speed of 400 rpm for 30 minutes and the appropriate solvent for wet grinding process was selected. At the final stage of grinding the speed and the duration of grinding was confirmed in the similar way as described. Finally, all the samples were ground using the selected parameters and it was sent for the synthesis process.   2.3.3 Extraction of Nanoparticles The stability of nanoparticles in aqueous solution depends on the solution chemistry condition such as pH, ionic strength, dissolved organic carbon amount and composition (Boxall et al., 2007; Hochella et al., 2008; Ryan and Elimelech, 1996). Li et al., 2011 presented that the nanoparticles are stable in simple composition, higher pH and lower concentration. Before extracting the nanoparticles, ground particles were dispersed in an aqueous solution and the nano-sized particles were separated from the matrix using Sonic Dismembrator (Model 500) with a 19 mm probe tip. Energy control is very important for the extraction of nanoparticle as nanoparticle extraction increases with the energy and after reaching a certain energy level it decreases. In the first stage of extraction, a soil solution was prepared by mixing 950 ml distilled water and 10 g soil in a 1000 ml glass beaker. About half of the solution was then transferred to a 500 ml glass beaker and after that the beaker is placed on an ice bath to keep it cold. The ultrasonic probe tip was submerged in the soil-water suspension to a depth of 2cm above the bottom of the beaker. A specific energy of 8400 kJ/kg was applied to disperse the particles which were calculated by using the following equation (2.1): 22   ptE VTs ……………………….(2.1) where, E= Specific energy (kJ/kg); p= power input (0.70 kW); β= net energy transmitted (100 %); t= ultrasonic time (600 s); V= sample volume (0.50 L); Ts= initial total solid concentration (0.01 kg/L). In the second stage, the solution was diluted up to 3 L glass beaker and dispersed in an ultrasonic device for 10 minutes at 250C. Then the larger particles were sorted out by centrifuged on the basis of stokes law (Tang et al., 2009). The centrifugation method was used as it is less time consuming and no chemical is required than other method of sedimentation (Akbulut et al., 2012). The centrifuging time (tc) was calculated for a rotational speed of 4000 rpm, temperature of 250C and maximum particle diameter of 100 nm using stokes law as follows: 212 2 2log3.81cRRtN r d     …………………………..(2.2) where   is the viscosity coefficient ( 0.089 Pa.s at 250C);  R2 is the distance from the particles in the centrifuge tube to the center of the axis of the centrifuge (20 cm); R1 is the distance from the surface of the liquid in the centrifuge tube to the center of the axis of the centrifuge (14 cm); N is the centrifuge speed (4000 rpm); r is the radius of the particle in the centrifuge (0.10 µm) tube and Δd is the difference in density between the particles(2.65 g/cm3) and water (2.5 g/cm3).  The theoretical time was found 30 min to settle all the particles larger than 100 nm. The centrifuging time required for a speed of 4000 rpm and temperature of 250C was also determined through trial and error method. In this method, centrifugation of the dispersed sample was 23  performed for different centrifuging time and at a fixed temperature and rotational speed of 250C and 4000 rpm, respectively. The particle size for each time interval was determined using Zetasizer nano series (Nano ZS, Marvern Instruments, Worcestershire, UK) and the optimum time required for settling particles having diameter greater than 100 nm was selected. After determining the optimum time required for centrifuging, the supernatant was then filtrated through 0.2 μm membrane filter paper attached with a vacuum filter. The filtrated supernatant was kept on hot stirrer plate to evaporate all the water. It was then kept oven at 1050C for 24 hr and ground through pulveriser to make it in a powder form. The particle size and chemical composition of the dry powder was confirmed through SEM and EDS, respectively. The mineralogical change of the bentonite samples after the grinding process was observed using XRD analysis. 2.4 Results and Discussions The Wyoming bentonite has an average particle size of 15 μm. To prepare the bentonite nanoparticle the ball milling process of bentonite started with selection of favourable ball material and ball size. The effect of types and diameter of ball is shown in Figure 2.1. Initially, steel ball was used to grind the particles but chemical intrusion was found in the grounded nanoparticles. Therefore, zirconia ball having different diameter was used to grind the particles because the ground particles are less affected with the zirconia ball. In addition, a shift towards the smaller particle size was observed with small diameter zirconia balls. The results encouraged to use small diameter (2mm) zirconia balls that introduce better particle size distribution than the 10 mm and 5 mm balls.  24   Figure 2.1: Effect of ball types and size on the grinding of nanoparticles at a speed of 400 rpm for 30 minutes. The effect of solvent on the grinding and particle size determination using zirconia ball having diameter of 5 mm at a rotational speed of 400 rpm for 30 minutes is shown in Figure 2.2. It is observed that the average particle size found using water as a solvent is 2467 nm, whereas in the case of IPA solvent it is about 934.4 nm. It is also observed that the particle size distribution curve obtained using IPA solvent showed smooth peak at a particle size of about 2000 nm but the water solvent showed two different peak at a particle size of about 800 nm and 500 nm.  In addition, the particles grounded using water solvents are being swelled due to the present of dipole water molecule and intercepted the pulverizing process. Therefore, IPA solvent in the wet grinding process showed better result than water solvent.  100 1000 1000001020304050Intensity (%)Particle Size (nm) 2 mm Zirconia ball 10 mm Zirconia Ball 5 mm Steel Ball 10 mm Steel Ball25   Figure 2.2: Effect of solvent on the grinding of nanoparticles using zirconia ball having diameter of 5mm at a speed of 400 rpm for 30 minutes.  To determine the grinding speed and duration another set of experiment were performed using 2mm zirconia balls at different speed level for 90 minutes as shown in Figure 2.3. The result depicts that higher speed resulted smaller particles. For example, the average particle size was found 2039 nm for a speed of 200rpm, whereas as it was reduced to about 934.4 nm at a speed of 800 rpm. Speed more than 800 rpm causes vigorous shaking and increase the temperature. Therefore, particles grinding with a speed of 800 rpm showed optimum value.   100 1000 100000510152025Intensity (%)Particle Size (nm) water IPA 26   Figure 2.3: Effect of rotational speed on the grinding process using zirconia ball having diameter of 2 mm for 90 minutes. In the final stage of grinding the optimum duration of grinding was determined using zirconia ball having diameter of 2 mm at a safest speed of 800 rpm for four different sets of durations as shown in Figure 2.4. It is observed that the size of the particles was decreased with the increase of the grinding duration for a particular grinding condition. For example, the average particles size was reduced about 24 % (1957 to 1492 nm) for the increase of grinding period from 30 to 60 minutes.  It can be further reduced about 37 % (1492 to 934.4 nm) by increasing the grinding period from 60 to 90 minutes. Owing to the previous result, a combined grinding scheme was applied to obtain better results i.e. initially bentonite particles were ground using zirconia balls having diameter of 2 mm at a speed of 800 rpm for 30 minute and then it was 100 1000 1000005101520253035Intensity (%)Particle Size (nm) 200 rpm 400 rpm 800 rpm27  ground using zirconia balls having diameter of 0.1 mm at a speed of 800 rpm for 60 minute. And resulting further reduction of particle size about 12 %.   Figure 2.4: Effect of grinding period on the particle size of ground particles using zirconia ball having diameter of 2 mm at a speed of 800 rpm.It is observed from the mechanical attrition that the average particle size was about 820 nm. Therefore, it is necessary to remove the larger particles from the ground materials.  In the second stage of improvement, the ground particles were dispersed through ultra-sonication and then particles having size more than 100 nm of the dispersed solution were separated using centrifuging techniques. At the prior to the centrifugation, the parameter such as speed, temperature, particle size and size of centrifuging tube were fixed to determine the centrifuging time using stokes law. The theoretical time required to settle all the particles larger than 100 nm was found 30 minutes. The time of centrifugation was then confirmed through continuous 100 1000 1000005101520Intensity (%)Particle Size (nm) 2 mm dia ball for 30 min 2 mm dia ball for 60 min 2 mm dia ball for 90 min 2 mm dia ball for 30 min & 0.1 mm dia ball for 60 min28  particle size analysis using the zetasizer for different time interval as shown in Figure 2.5. The other parameters such as the rotational speed, temperature and the capacity of centrifuging tube were fixed to 4000 rpm, 250C and 600 mm, respectively. The average particle size was found about 214.6, 207.3, 202.9, 193 and 189.8 nm for the centrifuging time of 15, 20, 25, 30 and 35 minutes, respectively. Therefore, it can be noted that the particle size was reduced about 75 % after centrifugation at 4000 rpm for 35 minutes. Tough theoretically particles present in the supernatant should be less than 100 nm at a centrifuging time of 30 minutes but due to the agglomeration of the clay particles in aqueous solution, the particle size was found after the zetasizer was higher than 100 nm.  Figure 2.5: Effect of centrifugation time on the particle size of ground particles at a speed of 4000 rpm.The samples were further studied in SEM image to confirm the particle size due to their agglomeration tendency. The SEM image was extracted at different locations to confirm the 10 100 1000 1000003691215Intensity (%)Particle Size (nm) without Centrifuging 15 min 20 min 25 min 30 min 35 min29  particle size determined by the zetasizer (Nano ZS). Omnimet image analysis software was found useful to determine the dimension of the particles. The problem of analyzing the particle size distribution with the SEM image was that images were particularly for specific locations rather than the bulk sample. However, sizes from the images were extracted for varied range of magnification to capture the whole sample. The SEM image of the original Wyoming bentonite and its nanoparticles after grinding and centrifugation is shown in Figure 2.6 at a magnification of 50000. It is observed that the ground and synthesis samples have greater portion of nano size particles in the range of 20 to 100 nm as shown in Figure 2.7. It is also observed that about 93 % of the sample was less than 200 nm and 61 % of the nanoparticles without any synthesis were less than 100 nm. Whereas, the synthesis nanoparticles shows about 92 % of the particles are less than 100 nm and all the particles are below 140 nm. Therefore it can be seen that the combination of both the bottom up and top down process improve the process of nanoparticles preparation to a large extent.     Figure 2.6: Microstructure of (a) bentonite, (b) ground bentonite before synthesis, and (c) ground bentonite after synthesis   (a) (b) (c) 30   Figure 2.7: Particle size analysis of nanoparticles before and after synthesis. Figure 2.8 showed the results of elementary analysis using energy dispersive spectroscopy (EDS) on samples of bentonite and bentonite nanoparticles using steel and zirconia balls. It is observed that the element that appears in the bentonite samples is Oxygen (O), Sodium (Na), Magnesium (Mg), Aluminium (Al), Silicon (Si), Sulphur (S), Chlorine (Cl), Potassium (K), Calcium (Ca) and Iron (Fe). The chemical formula of sodium based bentonite such as smectite ((Na,Ca)0.33(Al,Mg)2(Si4O10)(OH)2·nH2O) confirms that the tested Wyoming bentonite is in the smectite group, however a very few percentage of impurities of Iron, Sulphur and Chlorine was found from the analysis. The ball milling process is an abrasive process to prepare the nanoparticles of bentonite because elements of the balls and grinding bowls may intrudes into the pulverized material. Therefore, suitable ball material was selected from the elementary analysis. Although, there is no significant change in the principal element of the bentonite was observed 10000 1000 100 10020406080100120Cumulative percentage (%)Particle size (nm) Original Bentonite Before synthesis After synthesis31  but some impurities were found after the grinding process. For example, the iron (Fe) content found in the nanoparticles ground with steel balls is much higher compared to ground with zirconia balls. In addition a significant amount of carbon (C) was also observed in the nanoparticles ground with steel balls. Therefore, it can be concluded that nanoparticles ground with zirconia balls showed better results than steel balls and the ground nanoparticles showed almost identical elementary results compared to the original bentonite.     Figure 2.8: Elementary analysis of the bentonite and its nanoparticle using EDS   32 The X-ray diffraction is used mainly to determine the crystal structure of the material. The original bentonite and its nanoparticles were used for the XRD analysis and the acquired patterns are shown in  Figure 2.9. Where, Figure 2.10 presents that the bentonite samples include 76.7 % montmorrilonite content along with 12.9 % Albite and 10.4 % Quartz. Therefore, this bentonite can be classified in a smectite group as it shows high amount of monmorrilonite content, which makes it highly prone to swelling after adding water. In addition, there was no significant reduction (only 4.4 %) in montmorrilonite content of nanoparticles due to the grinding process. The decrease in monmorriolinite content is due to the release of water molecules at high temperature generated during pulverization, which converts it to illite. The percentage of quartz was remained constant after the grinding process, but the albite content totally covert to anothite. The XRD analyses also confirmed the miller index of the montmorillonite content and the interlayer d spacing was found within a range of 2 Å to 12 Å using the X’Pert High score software from Pananalytical.   33  Figure 2.9: X-Ray diffraction patterns of Wyoming bentonite and its nanoparticles   Figure 2.10: Crystal composition of Wyoming bentonite and its nanoparticles  34 Chapter  3: Effect of Fluid Chemistry on the Microstructure of Light Backfill: An X-ray CT Investigation 3.1  Background X-ray computed tomography (X-ray CT) is a non-destructive and non-invasive technique used to investigate the microstructure of an object based on the attenuation coefficient of the electromagnetic wave such as an X-ray. This technique was first developed by Godfrey Hounsfield, 1973, and he received a Nobel Prize in 1979 for the invention of the X-ray CT equipment. X-ray CT was first used in the field of geotechnical engineering a few years after the invention. For example, X-ray CT has been used to determine the bulk density of soils (Petrovic et al., 1982) and the water content and water movement through soils (Crestana et al., 1984; Hainsworth and Aylmore, 1983). The exploration of synchrotron based X-ray micro-tomography by Flannery et al., 1987, has led to the development of elemental map and dynamic flow profiles at a resolution of 2.8 μm. Thereafter, this technique was applied in different fields of geotechnical studies such as the spatial distribution of soil properties (Heeraman et al., 1997; Nunan et al., 2006; Pierret et al., 2002; Rogasik et al., 2003; Young et al., 2001), pore network structures (Al-Raoush and Willson, 2005) porosity (Grevers et al., 1989; Heijs et al., 1995), permeability (Ketcham and Carlson, 2001; Mooney, 2002), and the characterization of pore space geometry and fractures with respect to different variables such as density (Anderson et al., 1990; Petrovic et al., 1982) and layer detection (Lipiec and Hatano, 2003; Macedo et al., 1998).  Industrial, medical, and synchrotron X-ray systems have various applications, dependent on the differing features of each system. In the medical scanner, the low energy X-ray source (<125kV) and high efficiency detector rotate around a static object whereas in the  35 industrial scanner, the object rotates in a cone-shaped beam of polychromatic X-ray. The medical scanner is also used for scanning an object at a meter-scale while in the industrial X-ray system an object can be scanned from a meter-scale to a micro-scale. Similar to the industrial scanner, the synchrotron scanner can scan an object to micrometers, where high intensity and highly collimated electromagnetic radiation emitted from the magnetic field of a particle accelerator is used during beam focusing. The advantage of the synchrotron scanner over others is its rapid scan speed and acquisition of low-noise data with few artefacts, which create an excellent environment of visualizing soil structure as described by Lehmann et al., 2006. In this study, the effect of fluid chemistry on the hydraulic behaviour and microstructure of the LBF prepared with 50% Na-bentonite and 50% sand at a maximum dry density of 1.24 Mg/m3 was observed. The particle size distribution of Wyoming bentonite shows that about 90% of the particles are within the range of 2 to 15 μm. Various researchers have studied the microstructure of compacted bentonite-sand mixtures using mercury intrusion porosimetry (MIP) and scanning electron microscopy (SEM) (Delage et al., 2006; Lloret et al., 2003; Villar and Lloret, 2001). The main disadvantages of these techniques are that they require some preparation, such as preliminarily dehydration of the samplesand local observation of thin samples. These disadvantages can be resolved by using micro-focus X-ray CT. The use of X-ray CT on the investigation of microstructure of compacted bentonite based clay barriers (Kawaragi et al., 2009; Kozaki et al., 2001; Saba et al., 2014; Van Geet et al., 2005) is limited and most research is focused on the observation of macro-porosity (Baveye et al., 2002; Gantzer and Anderson, 2002; Nunan et al., 2006; Rogasik et al., 2003). Recently, Feeney et al., 2006, indicated that with advanced scanning technologies, it is  36 possible to scan an object having a voxel less than 15 μm, which contributes to the interpretation the fraction finer than sand. However, the phase isolation of a soil mass is still a challenge as there is no standardized approach for the interpretation of a particle’s size distribution and orientation, air filler pore spaces, and water volumes (Helliwell et al., 2013). Literature regarding the application of X-ray CT for compacted bentonite-sand mixtures mainly consist of the observation of microstructure and anisotropic swelling behaviour (Kozaki et al., 2001; Saba et al., 2014; Tomioka et al., 2010), and properties of cracks, such as porosity and pore networks (Gebrenegus et al., 2011; Kawaragi et al., 2009). According to the author’s knowledge, there are no studies regarding the application of X-ray CT to detect the effect of fluid chemistry on the microstructure of bentonite-sand mixtures. Therefore, this study will be a first attempt to investigate the 3D-microstructure, porosity, pore connectivity, and permeability of compacted bentonite-sand mixtures in relation to variable pore fluid chemistries. This paper will also present a comparison of X-ray CT data with respect to the experimental data.  3.2 Materials and Methods 3.2.1 Materials Na-bentonite with a montmorillonite content of 70% and silica sand were used to prepare the LBF samples. The particle size analysis showed that about 90% of the particles are within the range of 2 to 15 μm and the sand particles are within the range of 200 to 600 μm. Initially, three dry samples, (i) dry sand, (ii) dry bentonite and (iii) dry bentonite-sand mixture, were prepared at a dry density of 1.65, 1.5 and 1.5 Mg/m3, respectively, to understand the microstructure of the materials. This observation allowed for a comparison between the microstructure of the dry and saturated compacted bentonite-sand mixtures. Samples were saturated with distilled water (DW) and two concentrations (50 g/L and 100  37 g/L) of two salt solutions (NaCl and CaCl2), for a total of five samples. Polypropylene tubes with a volume of 0.2 mL and inner diameter of 5.5 mm were used for containing the samples during scanning.  3.2.2 Swelling Pressure and Consolidation Testing In this study swelling pressure was measured using an incremental stress method using 1D consolidation apparatus. Initially, 50-50 bentonite-sand was mixed in the dry state as this yields the highest homogeneity and repeatability for both swelling and saturated hydraulic conductivity measurements (Gebrehawariat, 2005; Gebrenegus et al., 2011). The carefully mixed bentonite-sand materials were then mixed with DW or salt solution. A total of five samples such as S1, S2, S3, S4 and S5 were prepared by using DW, 50 g/L NaCl, 100 g/L NaCl, 50 g/L CaCl2 and 100 g/L CaCl2, respectively. The initial water content of all of the samples was 19 %.A compacted bentonite-sand sample having a diameter of 63.5 mm and height 10 mm was prepared at a dry density of 1.24 Mg/m3 to compute the hydraulic behaviour of the LBF. A computer-controlled automated oedometer (GDS) was used to conduct both the swelling pressure and hydraulic conductivity test. The maximum swelling pressure was determined by applying an incremental stress approach. In this approach, each incremental load was applied and the corresponding deformation was recorded until the sample returned to its original height. The maximum applied load at which the sample could rebound to its original height was considered the maximum swelling pressure. After completing the swelling stage of the sample, the loading for the consolidation test was applied. The applied loads for the loading stage for the consolidation was 250, 500, 1000, 1500 kPa and for the unloading stage was 750, 250, 100, 50 and 25 kPa.  38 3.2.3 X-ray CT Observation X-ray CT primarily depends on the imaging techniques rather than the paramagnetic elements of a soil (Macedo et al., 1998). The process consists of a synchrotron light or conventional X-ray tube as the source, a sample manipulation stage and a detector.  The basic principle of X-ray CT is the progressive attenuation of electromagnetic waves by absorption and scattering of the X-rays emitted from the source. When the X-rays pass through the sample it creates a series of radiograph images of the sample which are acquired at an incremental angle over 360°. The attenuation coefficient is the amount of absorbed or scattered photons by an object depending on the density of materials, electron density of the voxel of interest, and the incident energy (Helliwell et al., 2013). The attenuation coefficient of radiograph images are integrated based on the mathematical filtered back propagation algorithms to generate 2D image slices, termed tomographic reconstruction (Stock, 2008a; Taina et al., 2008; Wildenschild et al., 2002). Each reconstructed tomographic slice is comprised of a distinct unit known as voxels (3D pixels) which demonstrate the spatial resolution of the scans.   39      Figure 3.1: (a) Sample mounted on the rotation stage of the Xradia Micro XCT-400; (b) Sample Holder In the current investigation, the Xradia Micro XCT-400 X-ray tomographic microscope (Figure 3.1a) located at the University of British Columbia Okanagan (UBCO) campus was used to acquire the 3D radiographs of both dry and saturated samples. A specially designed sample holder (Figure 3.1b) was used to support the samples and to minimize the loss of water. After placing the sample holder on the rotating disk, an energy of 70 kV and current of 125 μm were used to acquire the radiographs over a rotation from 180° to 180°. Although it was challenging to acquire good quality images with a voxel size less than 2 μm, the particle size analysis of Wyoming bentonite showed that about 90% of particles were within the range of 2 to 15 μm. The spatial resolution of a CT image depends on the focal spot size, performance of detector, binning number, and the distance between the source and detector from the sample. For example, Dhondt et al., 2010, indicated that high resolution images can be acquired by placing the sample closer to the source and further from X-ray Source Detector Sample stage  Sample Holder a b  40 the detector, although the larger distance of detector from the sample results blur images. Higher resolution can also be attained by reducing the size of the sample. However, for a better understanding of the pore distribution and pore networks in field conditions, larger samples are required for adequate representation of the field conditions (Young et al., 2001). The application of the ‘region-of-interest’ technique has significant benefits to ensure high resolution for large soil samples (Carminati et al., 2009; Stock, 2008b) but tends to blur the image quality. After several trails, it was found that good quality images could be acquired with a resolution of 1.0695 μm by placing the sample at a distance of 37 mm from the source and 8 mm from the detector. Another important parameter for acquiring good quality images is the exposure time. The exposure time (time required for taking an individual radiographs) was carefully selected as it determines the number of photons counted at the detector and affects the level of noise in the image. An average of 3200 radiographic projections was acquired with an exposure time of 25 sec. The acquired images were then reconstructed to 8 bit gray scale 3D volume (voxels) using XMReconstructor-Cone Beam software developed by Xradia.  In order to investigate the effect of water on the properties of bentonite-sand mixtures, the dry samples of each material and their mixture were scanned along with the DW (S1) and salt solution saturated LBF (S2 to S5). A polypropylene tubes tube was used for sampling the LBF specimens after the consolidation test. After the consolidation test the LBF samples were in the swelling stage and thus the porosity of the samples showed maximum values. The two dimensional scanned images of dry bentonite-sand at a dry density of 1.60 Mg/m3 and water saturated bentonite-sand LBF at a dry density of 1.24 Mg/m3 are shown in Figure 3.2. The scanned images from the dry samples showed better  41 quality than the wet samples due to the development of high swelling pressure in the saturated sample. This swelling pressure changes the structure of bentonite particles causing them to spread out due to the formation of a strong bentonite-water gel which reduces the ability of fluid flow, according to the diffuse double layer theory.   Figure 3.2: Two-dimensional X-ray CT images of (a) dry bentonite-sand mixtures; (b) DW saturated bentonite-sand LBF sample after one dimensional consolidation test.   3.2.4 Image Analysis 3.2.4.1 Preprocessing of Scanned Images Figure 3 presents the image preprocessing steps of dry sand, bentonite, bentonite-sand mixtures and DW saturated LBF. Initially, scanned images were cropped to nearly square in size and the brightness of the images was adjusted to improve the visualization. The images contained speckled noise and were de-noised using a median filter with a radius of 1.35 pixels. In the first stage of segmentation, the de-noised images were segmented to classify the individual voxels with a common mean gray scale value termed the threshold value. The (a) (b) Bentonite-water gel Sand grain Bentonite Air Void Sand grain Air Void  42 threshold value in the segmented images is ordered in terms of attenuation density where the densest part is represented by bright voxels and the least dense portion is represented by darker voxels. Global thresholding, based on the histogram of the image, was used to segment materials of interest in the soil (Figure 3.3b). In the second stage, the interconnected particles were segmented using the watershed algorithm presented by Vincent 1991 (Figure 3.3b). This algorithm first calculates the gradient of the image and then searches the region of highest density that divides the neighboring local minima. The highest density region, local minima and their connecting edges are considered as the catchment basin, surface hole, and shed or dam. The segmentation is done on the immersion basis so that water successively fills surface holes enclosed by the shed of the catchment basin. The key advantage of this technique is that it firstly detects the main edges of the object and then computes the watershed of the identified gradient. Prior to applying the watershed algorithm, the images were processed using a Gaussian filter with a radius of 1.5 pixels to reduce the initial number of regional minima. The segmented images were then used to observe the 3D microstructure, porosity, pore size distribution, pore connectivity and permeability of LBF samples prepared with both the DW and salt solutions.   43 Figure 3.3: Image preprocessing steps for the dry sand, bentonite, bentonite-sand mixtures and DW saturated LBF. 3.2.4.2 Analysis of Porosity and Pore Size Distribution  A 3D visualization of the microstructure improves the reflection of the actual arrangement of the particles in a bentonite-sand-water matrix and quantifies the continuous pore network and pore connectivity (Mooney, 2002). However most of the research regarding pore networks and pore connectivity focus on the macro level because of the scale limitation associated with X-ray CT imaging (Helliwell et al., 2013). The 3D microstructures     (a) Scanned image     (b) Cropping of volume of interest and thresholding     (c) Application of Gaussian filter and watershed segmentation  44 of the compacted specimen were composed with a resolution of 1.2 μm (voxel size: 1.0695 x1.0695 x1.0695). An algorithm was used to determine the volume porosity of the 3D oriented samples. This algorithm is a counting program which counts all the black and white voxels from the 3D binary stack; the porosity was then determined by dividing the black voxels with the total voxels. An algorithm was developed and coded in Matlab by the authors in order to determine the pore size distribution of the 3D oriented samples. All of the image slices were converted into binary, where ‘0’ denoted a black pixel (i.e. pore) and ‘1’ denoted a white pixel (i.e. particles). The binary images were cropped to 300 x 300 μm to reduce the simulation time. In the first stage of the algorithm, a function was developed to pick a black pixel (X) from the binary image (Figure 3.4a) and then moved along the entire neighbouring pixels according to Figure 3.4b. When the function reached a white pixel the movement stopped and the white pixel was marked as a boundary for that pore. However if the function found any neighboring black pixels during each movement, it would send the black pixel to a type of data structure known as a queue. A queue is a sequential data collection system, in which the entities are added and removed according to the first-in-first-out (FIFO) basis and for a single queue structure all the removed data are marked with the same label number (Figure 3.4c). In a similar way, ‘X’ move the three global axes to determine the total voxel occupied by the first pore and label them with the same number. The total voxel found for a particular set of labels represents the volume of that pore. After labeling the first pore, the function randomly picks another black pixel and labels it accordingly. This process of counting pore volumes continued until all of the black pixels was labelled. The volume of  45 each pore present in a 3D pore structure was determined accordingly to make a pore volume distribution plot.    (a) (b) (c) Figure 3.4: Determination of pore size distribution (a) picking a pore from binary image; (b) possible movement along neighboring pixels; (c) labeling the connected black pixel.   3.2.4.3 Analysis of Pore Connectivity  Investigation of pore space geometry and pore connectivity within the soil mass is important to characterize fluid flow. The pore connectivity depends on the size and number of pore throats surrounding a single pore. The 3D pore space geometry was visualized and connected porosity was computed using an Avizo built-in function named connected component analysis. The interconnected pores were analysed in such a way that the volume porosity is considered as connected if the pores share at least one common voxel face as mentioned in Hemes et al., 2015. In the current investigation, the connected porosity volumes were divided into the following four groups according to their volume: 1 to 10 μm, 10 to 100 μm, 100 to 1000 μm and greater than 100 μm.  3.2.4.4 Analysis of Hydraulic Conductivity The ability of a porous material to transmit a single phase fluid, known as absolute permeability (m2), was measured from the 3D microstructure of the LBF samples using X(0)1 011 10 000000 00000111 1111X011 10 000000 00000111 1111111 133111 1122 22 2222222 2 46 Avizo-Xlabsimulation Software. The absolute permeability was computed by solving a single phase flow problem for a unit drop of applied pressure in all of the axial directions of the samples (Krotkiewski et al., 2011). Darcy’s law was used to determine the permeability of porous material only on the macro-scale using  K Pv L  ………………………………………… (3.1) where, μ is the dynamic viscosity of fluid (0.000891 kg/m·s for water at a temperature of 25°C), L is the length of porous media in the flow direction (m), v is the velocity through the porous media (m/s) and ΔP is the pressure difference applied to the sample (Pa). However on the micro-scale, where free flow occurred, it was necessary to understand the flow through individual pores and pore throats. This problem was solved by using both Darcy’s Law and Stokes’ theorem for the incompressible and Newtonian fluid flowing in a steady and laminar manner. To make the calculation easier, a simplified Stokes’ equation was used, which are 2. 0 .................................................(3.2)0VV P      where,  . is the divergence operator,   is the gradient operator, V is the velocity of the fluid in the fluid phase of the material, 2   is the laplacian operator and P  is the pressure of the fluid (Pa)  in the fluid phase of the material. The boundary conditions associated with the computation of permeability are that (i) no slip occurred in the fluid-solid interface, (ii) the flow is isolated within the system, and (iii) the fluid can freely spread on the face of the sample. Permeability (K in m2) computed from both the Darcy’s and Stokes’ equations was converted to hydraulic conductivity (k in m/s) using the Bear, 1972.  47 K gk  ………………………………………………(3.3) where, ρ is the density of fluid ( 1000 kg/m3 for water) and g is the acceleration due to gravity (9.81 m/s2). 3.3 Results and Discussion 3.3.1 Swelling Pressure  The maximum swelling pressures of the compacted LBF at different pore fluid conditions obtained from the laboratory tests are presented in Table 1. The results showed that the swelling pressure decreased while the hydraulic conductivity increased in the samples prepared with the salt solution. It was observed that the variation in swelling pressure was more prominent to S2 and S3 compared to the S4 and S5. For example, samples S4 and S5 showed that the decrease in swelling pressure is about 80 % that of S1. However, samples S2 and S3 showed a decrease in swelling pressure is only 20 % of S1. Table 3.1: Experimental results of the samples Sample No. LBF samples prepared with Maximum Dry density (Mg/m3) Maximum Swelling pressure (kPa) Porosity (%) Experimental X-ray-CT -- Dry sand 1.65 -- -- 30.47 -- Dry bentonite 1.50 -- -- 68.52 -- Dry bentonite-sand mixture 1.60 -- -- 29.02 S1 DW  1.22 225 61.97 0.17 S2 50 g/L NaCl  1.23 175 61.65 0.19 S3 100 g/L NaCl  1.27 150 60.39 0.22 S4 50 g/L CaCl2  1.33 50 57.59 0.46 S5 100 g/L CaCl2  1.34 25 49.59 0.41     48 3.3.2 Porosity The porosities of both the DW and salt-solution saturated LBF were calculated from the one dimensional consolidation test and X-ray CT image analysis (Table 3.1). The final porosity value found for the salt solutions was adjusted according to (Siddiqua et al., 2011). It was observed that the porosity value found at the end of the consolidation test showed significantly higher values compared to the X-ray CT analysis. These higher values of experimental porosity are attributed to the swelling behaviour of Wyoming bentonite by constructing a thicker diffuse double layer (DDL) after being saturated with water. This DDL spreads the solid particles present in the saturated LBF to a wider range by creating a bentonite-water gel around the particle’s periphery. Although, this bentonite-water gel does not allow fluid flow through it, in the calculation technique used in the experimental porosity investigation (i.e. experimental porosity is the change in volume of solids compared to the initial volume), the bentonite–water gel was considered as void space, resulting in higher porosity values. Dixon et al., 1985 also showed that the value of effective porosity is lower compared to the total pore space per unit volume due to the presence of highly viscous bound water in the pore space. In the case of image analysis, the bentonite-water gel was considered as a compound mass resulting in low porosity estimations. Moreover, the value of experimental porosity decreased with the addition of salt solution due to the reduced thickness of DDL by the action of cations present in the pore fluid. However, the estimated porosities from the image analysis tended to increase with the addition of salt solution, similar to findings of Studds et al., 1998. Therefore, the porosity values calculated from the X-ray CT investigation showed better results compared to the experimental ones.    49 3.3.3 Pore Size Distribution Figure 3.5 depicts the volumetric pore size distribution of compacted dry materials (sand, bentonite and bentonite-sand mixtures) and both the DW and salt-solution saturated LBF. It was observed that almost 70% of the pores present in the compacted sand are on the macro-size, whereas the dry bentonite resulted with about 50% of the pores within the 1 to 10 μm3 range. Additionally, the 50-50 bentonite-sand mixtures reduced the pore size, where about 75% of the pores were within the range of 1 to 10μm3. This is due to the micro-sized bentonite particles filling the macro pore of the sand particles. The pore size distribution of samples S1, S2 and S3 had about 50% of the total pores in the range of 1-10 μm3 and had about 90% of the total pores in the range of 10-100 μm3. While, the percentage of pores found within the range of 1 to 10 μm3 decreased in the LBF samples saturated with CaCl2 solution from 60% to 40%.  Figure 3.5: Particle size distribution of dry materials (bentonite, sand, bentonite-sand mixture) and DW and salt solutions saturated LBF. 100000 10000 1000 100 10 1050100Cumulative frequency (%)Pore volume (bin center) (m) Dry sand Dry bentonite Dry bentonite-Sand mixture DW saturated LBF 50 NaCl saturated LBF 100 NaCl saturated LBF 50 CaCl2 saturated LBF 100 CaCl2 saturated LBF 50 3.3.4 Pore Connectivity The number of pore volumes interconnected on different faces of the pore by pore throat were analysed to establish a relationship between the pore throats in the microstructure of the LBF and the hydraulic conductivity. Although there are different calculation techniques to determine the number and volume of connected components (Vervoort and Cattle, 2003; Vogel, 1997), in this investigation a built in function in Avizo (Avizo 9.0) ‘connected component analysis’ was used to label and compute the separate components in a binary image. The connected components were divided into four size ranges according to their volume. The percentage of connected components in each size range was determined to investigate the ability of samples to transport fluid, as the ability of fluid transfer increases with the increasing percentage of larger sized connected pores. The result of the connected component analysis for the representative samples S1 revealed a total volume of 3.31 x 107μm3 (Figure 3.6a). While the maximum number of interconnected pores were within the range of 1 to 10 μm3 and contributed to 23% of the total resolved porosity (Figure 3.6b), the maximum porosity (about 50 % of the total resolved volume porosity) was governed by the interconnected pores which had a volume in the range of 100 to 1000 μm3 (Figure 3.6c). Few interconnected pores were found within the range of 100 to 1000 μm3 (Figure 3.6d), which contributed to 22 % of the total resolved porosity and only two pores are found having a volume larger than 1000 μm3 (contributed only 5 % of total resolved porosity) (Figure 3.6e). The enlarged view of the interconnected pores in Figure 3.6e shows how the pore bodies are connected with pore throats which create a larger volume of pores.  51     (d) (c) (a) (b)  Zoom in view  Zoom in view  Zoom in view   52   Figure 3.6: Results of the connected component analysis of DW saturated LBF samples (a) 3D Microstructure of the representative samples; (b) interconnected pore with a volume ranging from 1 to 10 μm3; (c) connected pore with a volume ranging from 10 to 100 μm3; (d) interconnected pore with a volume ranging from 100 to 1000 μm3; (e) interconnected pore with a volume larger than 1000 μm3; (f) interconnected pore space showing different sizes of pore volumes. The effect of pore fluid on the interconnected pore volumes in terms of number fraction and percentage of total porosity is shown in Figure 3.7. It was found that the total porosity volume contributions of interconnected pores of the LBF samples were shifted, to a large extent, with the presence of salt solutions. The majority of the numbers of pore fractions were found below the volume of 100 μm3 for the LBF samples. Sample S1 contained the maximum number of pore fractions within the range of 1 to 10 μm3, but the percentage contribution to the total volume porosity was found within the range of 10 to 100 μm3. The peak contribution to the total porosity of samples S3, S4 andS5 was found for the pore volume ranging from 100 to 1000 μm3, while sample S2 showed the peak value within the range of 10 to 100 μm3. Moreover, the effect of the samples S4 and S5 was more (f) (e)  Zoom in view Pore throat Pore bodies  53 pronounced in increasing the larger volume interconnected pores. Therefore, it was concluded that the LBF samples prepared with a salt solution increased the volume of the micro-pores resulting in higher permeability within the sample.  Figure 3.7: The effect of pore fluid on the number fractions and total porosity volume contributions of connected pores.  D 3.3.5 Hydraulic Conductivity The experimental results showed that the hydraulic conductivity of the LBF samples prepared with CaCl2 solution at the unloading stage increased by about 27 times compared to samples prepared with DW, whereas for the NaCl solution treated LBF, the hydraulic conductivity increased only about 2 times that of the samples prepared with DW. This change in hydraulic behaviour is attributed to the thinning of the DDL, which varies inversely with the square root of the concentration of pore fluid (Mitchell and Soga, 1976; Yong and Warkentin, 1975). However, to inspect the hydraulic behaviour of the LBF 1~10 10~100 100~1000 >100001020304050607080Number fraction pores (%)Pore volume (bin center) (m3)Number fraction pores DW water 50 g/L NaCl 100 g/L NaCl 50 g/L CaCl2 100 g/L CaCl20102030405060 Contribution to total porosity volume (%)Contribution to volume porosity DW water 50 g/L NaCl 100 g/L NaCl 50 g/L CaCl2 100 g/L CaCl2 54 samples through X-ray CT, a small representative sample (about 400 x 400 x 350 μm) was cropped to analyze. Though it is difficult to determine the true behaviour from this small sample, the results contribute to the understanding of fluid flow through individual and connected pores and gives insight to the hydraulic conductivity of the dry materials. For example, the hydraulic conductivity of the dry sand was 3.9 x 10-2 m/s, similar to the typical values for sand mentioned in (Das, 2013). However, the hydraulic conductivities found for both the dry bentonite (1.7 x 10-4 m/s) and bentonite-sand mixture (1.7 x 10-4 m/s) were significantly different from the typical values (10-10 to 10-12). This is because the hydraulic conductivity of saturated fine grained soil largely depends on the swelling pressure and the bentonite-water gel formation according to the DDL theory (Siddiqua et al., 2011). The hydraulic conductivity of the pore fluid treated LBF determined from the X-ray CT investigation showed similar patterns where they increased with increasing salt concentration (Figure 3.8). The hydraulic conductivities determined from the scanned images of CaCl2 solution treated LBF showed a better correlation with the experimental data compared to samples prepared with the NaCl solution. However, hydraulic conductivities computed from the scanned images of the DW treated samples showed significant variation with the experimental measures.   55  Figure 3.8: Performance study of the X-ray CT analysis on the variation of hydraulic conductivity with the pore fluid type and concentration.  0 50 1001E-121E-111E-101E-091E-081E-07Experimented NaCl CaCl2X-ray CT observation NaCl CaCl2Hydraulic conductivity, k (m/s)TDS (g/L) 56 Chapter  4: Performance Study on the Hydraulic Behaviour of Nanoparticle-based LBF Exposed to Pore Fluid 4.1 Background Bentonite based clay barrier is widely used for sealing the high level radioactive waste to protect the bio-geosphere from the contamination of buried radioactive waste for at least 100,000 years (Madsen, 1998).  Bentonite is a natural clay mineral derived from the weathering of feldspar and principally containing potassium (K), Sodium (Na) and Calcium (Ca) montmorillonite. It is grouped as semectite having permanent layer charge with a family of expansible 2:1 phyllosilicate minerals (Bailey, 1979), and its properties mainly depend on the structure of montmorillonite as well as smectite (Grim and Guven, 2011). Negative charge is produced on the surface of the 2:1 layer from the substitution of the basic crystalline units, such as the tetrahedral sheet and the octahedral sheet (Odom, 1984). Montmorillonite has very thin flat crystalline negatively charged platelets stacked together by positively charged Na+, K+ and Ca2+. These negative charges are neutralized by the positively charged cation on the surface of the 2:1 layer, which results hydration and swelling of the surface layer due to the addition of water. The swelling pressure of the bentonite depends on the bonding between the clay sheets and the positively charged ions. The bonding of clay sheets with K+ is much higher than Ca2+ and Na+ and is treated as non-swelling clay because of its appropriate diameter, which minimizes the gap between the sheets. The Na-montmorillonite has high swelling capacity compared to Ca2+ because of its weaker electrical interaction between the monovalent Na+ and negatively charged Platelets by entering more water into the interlayer molecules with larger hydration cell (Reeves, Sims, & Cripps, 2006). For example,  Koch, 2008 showed that the swelling volume and water absorption of Na-bentonite is about 4 and 5 times greater than Ca-bentonite. However, the swelling  57 pressure of the buffer material decreased with the increase of salt concentration of the ground water (Dixon et al., 1996; Jönsson et al., 2009; Karnland, 1998; Karnland et al., 2006; Pusch, 2013; Warkentin and Schofield, 1961; Xie et al., 2007). The loss of swelling pressure of Ca-bentonite in salt solutions is less compared to Na-bentonite. This beneficial propeties of Ca-bentonite in salt solution, especially in NaCl solution, is attributed to the exchange of Ca2+ with the Na+ ions. Karnland et al., 2005 showed that the loss of soil pressure in saline water can be overcome using Ca-bentonite with high montmorillonite content (>75%). However, Na-bentonite works as a good buffer material in deep geological repositories for sealing high level radioactive waste because of its thermo-hydro-mechanical properties with water (Reschke and Haug, 1991). Moreover, both the Na and Ca-bentonite shows similar swelling pressures at higher density because the microstructural patterns of both types of bentonite are similar at a higher density (Lee et al., 2012; Pusch, 2002). Koch, 2008 also suggested the following criteria for using bentonite buffer material:  (i) high montmorillonite content (75-90%), (ii) high cation exchange capacity (0.80-0.95 mmol(eq)/g), (iii) medium water absorption (150-200% Enslin/Neff value), (iv) low hydraulic conductivity (10-11 to 10-14 m/s), (v) low sulphur content and (vi) low organic content. In addition, for backfill in saline water should have sufficient mechanical and chemically stability against the upward movement of buffer material and flowing saline water.   The type, quantity and composition of the pore fluid that interrupt the deep geological barrier mainly depends on the type of host rock and interaction of different elements in the repository (Castellanos et al., 2008). However, crystalline rock found in the Canadian Shield is being considered of potential repository location in Canada  by NWMO., 2005 because of its beneficial  hydraulic behaviour. In addition, it contained total dissolved solid (TDS),  58 primarily Ca-Na-Cl type, within the range from 8 to >100 g/L at a depth of greater than 500 m (Baumgartner et al., 2008). Therefore, pore fluid present in the Canadian Shield has significant impact on the barrier system. Although,  several studies have shown that the pore fluid decrease the swelling pressure and increase the hydraulic conductivity of the geological barrier (Dixon et al., 2002; Karnland et al., 1992; Lloret et al., 2003; Mata and Ledesma, 2005; Villar, 2005). However, no reports were found that investigate the improvement of hydraulic behaviour, when salt solution is present in the geological barrier. Out of the different clay barriers proposed by Gierszewski et al., (2004), this study selected 50-50 bentonite-sand light backfill (LBF) at a dry density of 1.24 Mg/m3 to observe the effect of pore fluid on the hydraulic behaviour of LBF. Siddiqua et al., 2011 also demonstrated that the effect of pore fluid on the swelling pressure and hydraulic conductivity of LBF is most prominent compared to dense backfill (DBF). Moreover, nanoparticles were used to investigate the performance of nanoparticle based LBF to enhance the swelling pressure and reduce the hydraulic conductivity that increased due to solution chemistry. Particles having size less than 100 nm are considered as nanoparticles. Bentonite nanoparticles were prepared using both the mechanical attrition and synthesis. The main advantage of mixing nanoparticles with the traditional LBF is that it poses high specific surface area, which reflects higher surface surge. This additional surfaces charge contributing to increase the thickness of diffuse double layer (DDL) by resolving the cations present in the pore fluid and therefore resulting high swelling capacity and lower hydraulic conductivity.  Therefore, nanoparticle based LBF could be used in the deep geological repositories, where salt solution affect the permeability.    59 4.2 Materials and Methods 4.2.1 Bentonite and Bentonite-nanoparticles Wyoming bentonite (a type of Na-bentonite) used in the current investigation, were collected from the Wyoming, Montana and South Dakota in USA. The mineralogical composition (Table 4.1) of bentonite showed that it contained about 76 % montmorrilonite content with some Albite (12.9 %) and Quartz (10.4 %), which specified high swelling capacity of the used material. However, nanoparticles, made from the same bentonite exhibited a little variation from its parent properties. It is seen that the Albite content and a very little portion of montmorrilonite was converted to Illite and Anorthinte due to mechanical grinding but the Quratz content remained unchanged. The existence of cations in soil has a significant influence on the swelling pressure and mass transport properties (hydraulic conductivity). The chemical composition of bentonite indicates the presence of a large amount of Oxygen (47.2 %) and Silica (33.5 %) along with cations is shown in  Table 4.2. It is observed that percentage of sodium content is higher than other cation valance present in the both bentonite and bentonite-nanoparticles. Moreover, the particle size analysis results showed that about 90% of the particles in Wyoming bentonites are within the range of 2 to 15 μm. However, about 92 % particles in the bentonite-nanoparticles are less than 100 nm.   Table 4.1: Mineralogical composition of bentonite and bentonite nanoparticles.   Montmorrilonite Albite Quartz Anorthite Illite Bentonite 76.3 12.9 10.4 0 0 Bentonite-nanoparticles 72.3 0 10.4 6.9 10.4   60 Table 4.2: Chemical composition of bentonite and bentonite nanoparticles.  Sodium (Na+) Potassium (K+) Calcium (Ca2+) Magnesium (Mg2+) Aluminium (Al+) Silica (Si2+) Oxygen (O2-) Chlorine (Cl-) Bentonite 1.58 1.52 0.92 1.49 11.4 33.5 47.2 0.63 Bentonite-nanoparticles 1.62 0.12 0.62 1.56 10.5 25.8 56.5 0 4.2.2 Sand Compacted bentonite is a very good buffer material because of its high swelling potential and low hydraulic conductivity. But the main problem associated with the use of highly compacted bentonite materials is development of desiccation crack because of its high compressibility, which can be reduced by using coarse sand or silt. In the current investigation silica sand were mixed with bentonite to prepare the light backfill. The particle sizes of sand were found within the range of 200 to 400 μm. 4.2.3 Sample Preparation According to Gierszewski et al., (2004), LBF is the mixture of 50% bentonite and 50% sand compacted at a dry density of 1.24 Mg/m3. In this investigation dry sand and bentonite were mixed thoroughly to make a uniform mixture. The pore fluid type and concentration were selected according to the TDS found in the Canadian Shield mainly composed of NaCl and CaCl2 (Baumgartner et al., 2008). Four salt solutions (50 g/L NaCl, 100 g/L NaCl, 50 g/L CaCl2 and 100 g/L CaCl2) were prepared by dissolving anhydrous salt of NaCl and CaCl2 in distilled water (DW). Afterward, the uniform mix of bentonite-sand was saturated with a target moisture content of 19 % using both DW and salt solutions. The saturated mixture was compacted into a consolidation ring by hand to obtain the target dry density of 1.24 Mg/m3. In order to observe the effect of pore fluid on the swelling pressure and hydraulic conductivity, five samples (S1 to S5) were prepared according to the test scheme (Table 4.3). Subsequently, two different percentages (1 and 2% of bentonite by dry  61 weight) of nanoparticles were selected to investigate the performance of nanoparticle based LBF (S6 to S15). The nanoparticle based LBF was prepared in the similar technique that used for existing LBF preparation concept. The only exception is that the nanoparticles were first mixed with the dry bentonite to make a well graded mix and afterward it was mixed with sand. A series of test samples of 63.5 mm diameter and 10 mm target height were molded in a GDS consolidation ring (diameter 63.5 mm and height 20 mm) for the oedometer test on the LBF.  Table 4.3: Test scheme for swelling pressure and one dimensional consolidation test of LBF. Sample No. Nanoparticles mixed (%) Mixing fluid Mixing fluid TDS (g/L) Boundary condition Final water content (%) Dry density (Mg/m3) EMDD S1 0 DW 0 ICS 67 1.21 0.60 S2 0 NaCl 50 ICS 73 1.22 0.61 S3 0 NaCl 100 ICS 70 1.30 0.67 S4 0 CaCl2 50 ICS 61 1.23 0.62 S5 0 CaCl2 100 ICS 65 1.23 0.62 S6 1 DW 0 ICS 65 1.25 0.63 S7 1 NaCl 50 ICS 58 1.18 0.58 S8 1 NaCl 100 ICS 58 1.21 0.60 S9 1 CaCl2 50 CV 59 1.21 0.60 S10 1 CaCl2 100 CV 51 1.29 0.66 S11 2 DW 0 CV 56 1.26 0.64 S12 2 NaCl 50 CV 56 1.31 0.67 S13 2 NaCl 100 CV 37 1.38 0.73 S14 2 CaCl2 50 CV 38 1.30 0.68 S15 2 CaCl2 100 CV 39 1.32 0.68 4.2.4 Experimental Study Swelling pressure can be determined from the following methods such as (i) swelling load (SL) test, (ii) swell under load (SUL) test and (iii) constant volume (CV) test. These three tests give three different values for a particular soil.  Higher value of swelling pressure  62 is found in SL test, whereas SUL and CV test provide lowest and intermediate values, respectively. In this investigation, computer aided automated GDS Oedometer was used to measure the maximum swelling pressure and hydraulic conductivity, consequently. The swelling pressure for the samples S1 to S8 was determined for different incremental constant stress (ICS). Initially a sitting load was applied and when it back to its original position next incremental load was applied. The maximum applied stress for which it can back to its original sample height was considered swelling pressure. However, the swelling pressures of the samples S9 to S15 were determined using CV method, where the height of the samples was restricted to its original position. Subsequently, one dimensional consolidation test was conducted for the same sample after the completion of swelling test. The deformation reading corresponding to different incremental stress was recorded in every 30 sec. The applied stress was designed in two stages: loading stage (50, 100, 250, 500, 1000 and 1500 kPa) and unloading stage (750, 250, 100, 50, 25 and 5 kPa). Although the estimated hydraulic conductivity computed from consolidation test is slightly lower than the permeability test(Constant head and / falling head method), however consolidation test is advantageous because of its flexibility in application of wide range of pressure  (Budhu et al., 1991; Mitchell and Madsen, 1987; Sivapullaiah et al., 2000).  4.3 Results and Discussions The results of final moisture content, dry density and effective montmorillonite dry density (EMDD) are shown in Table 4.1. LBF samples prepared with or without nanoparticles were saturated with initial moisture content of 19 % to attain a target dry density of 1.24 Mg/m3. But it was difficult to maintain the dry density exactly same for all  63 the samples because of the hand compaction techniques. Moreover, the dry density of LBF samples saturated with salt solution was affected by the salt content, which was corrected using the method described by Siddiqua et al., 2011. For the similar reason the calculated EMDD using equation described in Siddiqua et al., 2011 showed different value for the LBF samples. However the deviation of both the dry density and EMDD value was not very significant. It is also observed that the final moisture content of the LBF samples saturated with salt solution was decreased compared to the DW saturated LBF. Although, the decrease in water content is not very significant for the LBF samples saturated with NaCl solutions but the decrease in final moisture content is very significant for the samples saturated with CaCl2 solutions.  4.3.1 Void Ratio The variation of void ratio with respect to applied vertical stress for LBF samples saturated with DW and salt solutions is shown in Figure 4.1 (a). It is found that the initial void ratio was constant up to the maximum swelling pressure and afterward for each loading increment it was decreased. The unloading curve for LBF samples saturated with both the DW (S1) and NaCl solution (S2 and S3) showed hysteresis behaviour at a applied stress of about 100 kPa, whereas LBF samples saturated with CaCl2 solution (S4 and S5) do not show any hysteresis behaviour even up to stress of 50 kPa. It is observed that the void ratio found for the LBF samples S2 and S3 was increased compared to the sample S1, whereas the void ratio was decreased significantly for the LBF samples S4 and S5. This decrease in void ratio of samples S4 and S5 is attributed by the reduction of the thickness of DDL as well as the swelling of LBF due to the replacement of Na+ bonding present in between the clay platelets with the strong Ca2+ bonding. The nanoparticles introduced into the LBF samples increased  64 the void ratio of the LBF samples saturated with DW and salt solutions (S6 to S15) as shown in Fig. 1(b) and 1(c).   Figure 4.1: Variation of void ratio with respect to applied axial stress for the LBF samples prepared according to Table 4.1.  4.3.2 Swelling Pressure The maximum swelling pressure determined from both the ICS and CV methods are plotted in Figure 4.2. It is observed that the LBF saturated with DW exhibited higher swelling capacity compared to saline solutions. However, the reduction of swelling capacity due to the NaCl solution is not very significant compared to the CaCl2 solutions. For example, the swelling pressure of sample S2 and S3 was reduced only three-fourth of sample S1, whereas the swelling pressure of samples S4 and S5 was reduced to one fourth of S1. The decrease in swelling pressure is mainly owing to the reduction of DDL thickness. It is also observed that the swelling pressure was increased with the increase of percentage of 0.91.01.11.21.31.41.01.11.21.31.41 10 100 1000 100001.01.11.21.3(c) 2% nanoparticles(a) 0% nanoparticles Void ratio DW 50 g/L NaCl 100 g/L NaCl 50 g/L CaCl2 100 g/L CaCl2(b) 1% nanoparticles Void ratioVoid ratioStress (kPa) 65 nanoparticles. However, the enhancement of swelling pressure due to the inclusion of nanoparticles was not very significant for both the DW and saline solution treated soil.  Figure 4.2: Change in swelling pressure due to the inclusion of bentonite-nanoparticles for LBF samples prepared with DW and saline water. 4.3.3 One Dimensional Constrained Modulus The one dimensional (1D) constrained modulus of elasticity from the void ratio - log vertical stress was calculated for the loading stage according to Bardet, 1997 with the consideration of small stress increments. The 1D constrained modulus of LBF samples prepared with both the DW and salt solutions is shown in Figure 4.3. This figure showed that there is no significant change in the constrained modulus due to the salt solution. Similar results also found by Siddiqua et al., 2011 and they also mentioned that the constrained modulus are independent of the type of materials. The constrained modulus found for the 0.0 0.5 1.0 1.5 2.0050100150200250300 DW 50 g/L NaCl 100 g/L NaCl 50 g/L CaCl2 100 g/L CaCl2Swelling pressure (kPa)Nanoparticles (%) 66 nanoparticle based LBF samples also showed similar results that found for the LBF samples without nanoparticles.  Figure 4.3: Change in 1D constrained modulus due to the inclusion of bentonite-nanoparticles for LBF samples prepared with DW and saline water. 4.3.4 Compression and Swelling Indices The slope of the virgin consolidation line and rebound line of the void ratio versus log-vertical stress plot known as the compression and swelling index were calculated, which are used to determine the consolidation settlement of over consolidated fine-grained soil (Işık, 2009).  Figure 4.3 and Figure 4.4 presents the enhancement of compression and swelling indices (CC and CS) due to the addition of bentonite-nanoparticles for both the DW and salt solution treated LBF. It is observed that the value of CC of the LBF samples saturated with saline solution was decreased significantly (Figure 4.3). For example, the average value of CC of LBF saturated with NaCl and CaCl2 solution was decreased to 34 % 0.0 0.5 1.0 1.5 2.0010203040 DW 50 g/L NaCl 100 g/L NaCl 50 g/L CaCl2 100 g/L CaCl21D constrained modulus (MPa)Nanoparticles (%) 67 and 54 %, respectively compared to the DW saturated LBF.  This decrease in compression index is attributed by the reduction of the thickness of DDL as well as repulsive force among the clay particles due to the action of salt solution (Priyanto et al., 2008; Siddiqua et al., 2011; Weimin et al., 2014). To minimize the loss of CC that occurred due to the electrolytic action of salt solutions were minimized using nanoparticles. The results of LBF samples prepared with nanoparticles present higher value of CC compared to the LBF samples prepared without nanoparticles. However, 1% nanoparticle based LBF gives higher value of CC compared to the 2% nanoparticle based LBF saturated with salt solutions and DW.   Figure 4.4: Change in compression index (CC) due to the inclusion of bentonite-nanoparticles for LBF samples prepared with DW and saline water. The CS value found from the rebound curve of the void ratio versus log-vertical stress plot for both the salt solutions and DW saturated LBF present the similar behaviour like CC (Figure 4.5). The value of Cs was found 0.09 for the sample S1, whereas it was decreased to 0.0 0.5 1.0 1.5 2.00.040.060.080.100.120.14 DW 50 g/L NaCl 100 g/L NaCl 50 CaCl2 100 CaCl2Compression index (Cc)Nanoparticles (%) 68 about 40 % and 67 % for the samples S2 and S3 and S4 and S5, respectively. The reduction in CS value is also attributed by the thinning of DDL layer due to the electrolytic action of pore fluid. Because, pore fluid increases the composition of cations on the surface of clay platelets by replacing the H+ with the cations present in the salt solutions. This replaced cations increase the bonding between the clay platelets because of their high cation valance. Thus the repulsive force between the clay particles decreased and consequently reduced the swelling. The nanoparticle based LBF did not show significant increase in CS, although the LBF samples prepared with 1% nanoparticles gives higher value of CS compared to 2%.   Figure 4.5: Change in swelling index (CS) due to the inclusion of bentonite-nanoparticles for LBF samples prepared with DW and saline water. 4.3.5 Hydraulic Behaviour of LBF The coefficient of consolidation (CV) of LBF for each of the load increments was determined using the square-root-of-time method from the deformation versus the square 0.0 0.5 1.0 1.5 2.00.010.020.030.040.050.060.070.080.090.10 DW 50 g/L NaCl 100 g/L NaCl 50 CaCl2 100 CaCl2Swelling index (CS)Nanoparticles (%) 69 root of time plot. Figure 4.6 presents the variation of CV with the percentage of nanoparticle inclusions for LBF samples prepared with both the DW and salt solutions. It is observed that for each loading conditions, the value of CV was increased after being saturated the LBF samples with salt solutions. Although the increase in CV for the LBF samples saturated with NaCl was not very significant but the CaCl2 solution increased the value of CV to a great extent. For example, the average value of CV for the samples S2 and S3 was found 2.5 times greater than sample S1, whereas in the case of samples S4 and S5 it was about 18 times greater than sample S1 at a pressure of 1500 kPa (Figure 4.6d). Therefore, the increase in CV is more prominent for the CaCl2 solutions compared to NaCl solutions. Because the adsorbed Ca2+ makes the bonding between the clay platelets stronger than adsorbed Na+ on the clay surface. And resulting change in void ratio, which is responsible for the increase in CV value (Siddiqua et al., 2011). Moreover, it is also seen that the value of CV decreased for higher pressure. For example, CV of sample S1 at a pressure of 250 kPa was found about 17 times greater than that of a pressure of 1500 kPa. The increase in CV due to the action of salt solution present in the deep geological repositories can be minimized by using bentonite-nanoparticles. For example, the value of CV of samples S4 and S5 was found 1.16 x 10-5 m2/s at a pressure of 1500 kPa, which reduced to 1.16 x 10-6 m2/s (about one-fifth of LBF prepared without nanoparticles) after being addition of 2 % nanoparticles (Samples S14 and S15).    70  Figure 4.6: Change in coefficient of consolidation (CV) due to the inclusion of bentonite-nanoparticles for LBF samples prepared with DW and saline water. Figure 4.7 presents the variation of hydraulic conductivity with the percentage of nanoparticle inclusions for the LBF samples prepared with both the DW and salt solutions. The hydraulic conductivity of sample S1 was found 5.12 x 10-12 m/s at an applied pressure of 1500 kPa. At the same pressure, it was increased due to the addition of salt solutions. For example, the average value of k for the samples S2 and S3 was found 1.5 times greater than that of S1 and for the samples S4 and S5, it was found about 18 times greater than that of S1 (Figure 4.7d). Therefore, CaCl2 solution is more prominent to increase the value of k of LBF samples compared to NaCl solutions because Ca2+ adsorbed by the clay surface due to electrolytic action makes the bonding between the clay platelets stronger than Na+ adsorbed 0.0 0.5 1.0 1.5 2.00.01.0E-52.0E-53.0E-54.0E-55.0E-56.0E-57.0E-50.0 0.5 1.0 1.5 2.00.05.0E-61.0E-51.5E-52.0E-52.5E-53.0E-53.5E-50.0 0.5 1.0 1.5 2.00.05.0E-61.0E-51.5E-52.0E-52.5E-53.0E-53.5E-54.0E-50.0 0.5 1.0 1.5 2.00.02.0E-64.0E-66.0E-68.0E-61.0E-51.2E-5(c) Applied Pressure: 1000 kPa(b) Applied Pressure: 500 kPa DW 50 g/L NaCl 100 g/L NaCl 50 g/L CaCl2 100 g/L CaCl2Coefficient of consolidation, cv(m2/s)Nanoparticles (%)(a) Applied Pressure: 250 kPa(d) Applied Pressure: 1500 kPa DW 50 g/L NaCl 100 g/L NaCl 50 g/L CaCl2 100 g/L CaCl2Coefficient of consolidation, cv(m2/s)Nanoparticles (%) DW 50 g/L NaCl 100 g/L NaCl 50 g/L CaCl2 100 g/L CaCl2Coefficient of consolidation, cv(m2/s)Nanoparticles (%) DW 50 g/L NaCl 100 g/L NaCl 50 g/L CaCl2 100 g/L CaCl2Coefficient of consolidation, cv(m2/s)Nanoparticles (%) 71 by the clay surface. This strong bond between the clay platelets reduces the repulsive forces between the clay particles, especially the thickness of DDL and resulting neat increase of void ratio. The high void ratio allows water to follow and increase the hydraulic conductivity of LBF samples. It is also observed that the value of k also depends on the amount of applied pressure because it decreased with the increases of applied pressure. Similar to CV, the enhancement of hydraulic conductivity due to the action of pore fluid can be minimized by using nanoparticles. For example, the average value of k of LBF saturated with CaCl2 (S4 and S5) was found 9.37 x 10-11 m2/s at a pressure of 1500 kPa, which reduced to 1.92 x 10-11 m2/s (about one-fifth of LBF prepared without nanoparticles) after being addition of 2 % nanoparticles (S14 and S15). Although, there is no significant difference in k value obtained from the 1% and 2% nanoparticles based LBF.   72  Figure 4.7: Change in hydraulic conductivity (k) due to the inclusion of bentonite-nanoparticles for LBF samples prepared with DW and saline water. 0.0 0.5 1.0 1.5 2.00.02.0E-104.0E-106.0E-108.0E-101.0E-91.2E-91.4E-90.0 0.5 1.0 1.5 2.00.05.0E-111.0E-101.5E-102.0E-102.5E-103.0E-103.5E-104.0E-100.0 0.5 1.0 1.5 2.00.02.0E-114.0E-116.0E-118.0E-111.0E-101.2E-101.4E-101.6E-100.0 0.5 1.0 1.5 2.00.01.0E-102.0E-103.0E-104.0E-105.0E-106.0E-10(d) Applied Pressure= 1500 kPa(c) Applied Pressure= 1000 kPa(b) Applied Pressure= 500 kPa DW 50 g/L NaCl 100 g/L NaCl 50 g/L CaCl2 100 g/L CaCl2Hydraulic conductivity, k (m/s)Nanoparticles (%)(a) Applied Pressure= 250 kPa DW 50 g/L NaCl 100 g/L NaCl 50 g/L CaCl2 100 g/L CaCl2Hydraulic conductivity, k (m/s)Nanoparticles (%) DW 50 g/L NaCl 100 g/L NaCl 50 g/L CaCl2 100 g/L CaCl2Hydraulic conductivity, k (m/s)Nanoparticles (%) DW 50 g/L NaCl 100 g/L NaCl 50 g/L CaCl2 100 g/L CaCl2Hydraulic conductivity, k (m/s)Nanoparticles (%) 73 Chapter  5: Conclusions and Further Studies This Chapter comprises of a precise collusions with limitations of the study and recommendation for future studies 5.1 Conclusions Disposal of high-level radioactive waste in the deep geological repositories poses challenge due to the potential hazardous nature of the wastes. Although, the concept of engineered barrier systems have solved the problem of waste disposal to a great extent, but due to the diverse effect of pore fluid can diminish  the performance of sealing systems. This research aims is to observe the impact of pore fluid on the currently used light backfill (LBF) in the Canadian repository concept and to minimize the losses of hydraulic properties by incorporating nanoparticles of bentonite in the matrix of LBF. To meet the goals of the research, the whole thesis was divided into three parts. Firstly,, nanoparticles of bentonite were prepared using both the mechanical attrition and synthesis process and the detailed characterization of the nanoparticles were performed. Next the effect of pore fluid on the hydraulic responses was examined using one-dimensional consolidation tests and microstructural analysis. The microstructure of the samples from end of consolidation tests was analyzed in using X-ray CT techniques. In the final stage,, nanoparticles of bentonites were mixed to the LBF to observe the performance of nanoparticle-based LBF exposed in salt solutions. Based on the findings of this thesis, the following conclusions are summarized.   1. The average particle size was found about 820 nm from mechanical attrition using zirconia balls (having diameter of 2 mm for 30 minutes grinding and 0.1 mm for 60 minutes grinding) at a rotational speed of 800 rpm.   74 2. The synthesis process reduced the average particle size to a great extent (92 % of the particles are less than 100 nm and all the particles are below 140 nm) by separating the larger particles after centrifugation at 4000 rpm for 35 minutes.   3. The hydraulic conductivity and rate of consolidation was increased significantly for the LBF saturated with CaCl2 solution while the swelling pressure, compression index, and swelling index decreased. Although, CaCl2 solution changed these properties of LBF to a greater extent, however, the impact of NaCl solution is not very major.   4. An algorithm was developed to compute the porosity and pore size distribution of LBF. The results of X-ray CT analysis presents that the porosity, pore size and number of interconnected pores of the LBF samples increased with the salt solutions. As a result the samples become more permeable.  5. Although the increase of swelling pressure of nanoparticle-based LBF exposed in the salt solutions was not very significant, however, nanoparticles reduce the hydraulic conductivity of LBF samples to a great extent. Therefore, nanoparticle-based LBF is a good option to use as sealing materials in deep geological repositories, where pore fluid increase the hydraulic conductivity. 5.2 Limitations of this Study The imitations of this study are summarized below: 1. Characterization of bentonite nanoparticles was limited to SEM and Zeta sizer techniques. to .  2. Although the bentonite and its nanoparticles are in flaky shape but for the simplicity of the calculation it was considered as spherical shape.   3. The microstructure of LBF was completed using an X-ray CT observation which was limited to micro scale as the facility cannot use nano size samples. 5.3 Recommendations for Future Study Nanoparticles developed in this research are approximately within the range of 20 nm to 140 nm while target was to limit all the particles below 100 nm. Therefore, further studies are needed to limit all the particles below 100 nm by changing the centrifuging time and  75 speed. Also, Transmission Electron Microscope (TEM) analysis is recommended to characterize nano size particles. In this research, a first attempt was made to investigate microstructure of LBF samples after the consolidation tests using micro scale X-ray CT. 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