Experimental Study On Sealing And Erosion Processes And Colloids Stability Of GMZ Bentonite

Buffer material is the last engineered barrier in the multi-barrier system of geological disposal repository,and one of its key functions is to seal off fractures in crystalline host rock.At the bentonite-rock interface,the swelling capability enables the installed compacted bentonite to gradually swell and fill engineering gaps and voids as well as to further enter into the fractures when in contact with aqueous solutions.However,bentonite particles in the fractures is susceptible to be eroded by flowing groundwater under favorable conditions due to its very low density,as a consequence the long-term safety function of buffer/backfill material may be jeopardized.In this thesis,the GMZ bentonite,which had been selected as potential buffer/backfill material in Chinese HLW disposal repository,was investigated using various erosion test devices and approaches on the basis of a thoroughly understanding of its fundamental properties including compressibility,swelling pressure,hydraulic conductivity and thermal conductivity,to obtain parameters and mechanism of erosion.The follows are the main findings.1.A series of tests were conducted to investigate the compressibility,swelling capacity,hydraulic conductivity etc.of GMZ bentonite.Test results showed that the swelling pressure of GMZ bentonite has negligible difference when saturated by deionized water and BS28 groundwater.The maximum swelling pressure increases exponentially with the finally dry density while the hydraulic conductivity decreases exponentially with the finally dry density.The engineering gaps and voids have obvious effect on the density of bentonite and further cause swelling pressure to decrease exponentially.Nevertheless,the swelling pressure of samples with the identical final dry density is basically the same with or without gaps and voids.Thus,the exponential correlation between the swelling pressure and the final dry density is still valid to predict the swelling pressure when the engineering gaps and voids exist.The thermal conductivity of dry GMZ bentonite is equally high with Beishan granite by the addition of 30%～35%graphite powder.Temperature has a negligible influence on the thermal conductivity of bentonite and bentonite/graphite mixtures.2.The swelling and hydraulic test cell was redesigned to saturate highly compacted bentonite samples followed by water circulated over the filters which in close contact with sample surfaces.The swelling pressure and turbidity of circulating water was measured.Parameters that may influence bentonite erosion,including flow rate,groundwater salinity,filter pore size were investigated.Based on test results,deionized water will results in significant particle release from the compacted samples and the swelling pressure typically drop as much as more than 20%within a few hours after initiation of water circulation even under very low flow rate,e.g.1m L/min.Within the range set by this experiment（max.10m L/min）,seemingly mass loss and swelling pressure drop of bentonite increase with flowrate.Generally the erosion process will gradually mitigate and the swelling pressure will maintain constant after a certain time instead of continuously droping to 0.The released particles are mainly montmorillonite.No sign of erosion was observed when GMZ bentonite was flushed by BS28groundwater or Na Cl slutions of concentrations higher than1g/L.However,when the BS28 groundwater was diluted to one-eighth of its original TDS value,or the concentration of circulating Na Cl solutions droped below 0.5g/L,significant particle release and noticeable swelling pressure drop was monitored.No swelling pressure drop was recored when the GMZ bentonite contacted with flowing Ca Cl₂of different concentrations although some particle could be detected in the effluent.Therefore,the ionic type and concentration of solutions are the key factors influcing bentonite erosion.Bentonite is less susceptible to erosion when subjected to aqueous solutions of high ionic concentration,and divalent counter-ions are more effective to diminish erosion.This is probably due to the fact that the generation and stability of bentonite colloidal particles is controlled by the ionic type and concentration of the clay-water system.3.Artificial fracture cells made by acrylic plates with 0.1～1.5 aperture through which an even flow of deionized water,BS28 groundwater or saline solution was provided,were employed to simulate the potential extrusion and erosion behavior of bentonite buffer at a transmissive fracture interface.The effect of solution chemistry,flowrate,fracture aperture and fracture slope on erosion were analyzed.The extrusion distance is proportional to montmorillonite content and initial dry density of emplaced bentonite,and decreases with increasing salinity of groundwater.Larger fracture apertures result in more pronounced mass loss and extended extrusion.In the tests with high ionic concentrations where erosion did not occur,the influence of fracture aperture and fracture slop on the stability of extruded clay was unconspicuous.On the other hand,in tests where erosion did occur,gravity played a very important role and the mass loss were accelerated strikingly,and the extruded clay gel exhibited structural instability.The effect of fracture slope on extrusion and erosion depends on fracture aperture,and the impact of fracture solpe on speeding up the process of bentonite particle release is signifcently enhanced with increasing fracture aperture.The layer of accessory minerals is the limiting mechanism of attenuated erosion observed in horizontal fracture.However,in the sloped fracture condition where the slope angles are large than 45°,seemingly the accessory minerals could not form an integrated and effective layer to diminish particle settling.4.The stability of GMZ bentonite colloids was closely connected to the ionic strength of aqueous solutions and dispersed bentonite colloids were prone to aggregate as the ionic strength rising.Divalent counter-ions such as Ca²⁺were more effective to cause coagulation of bentonite colloid than monovalent counter-ions.The alkaline and neutral conditions were favorable for bentonite colloidal stability.Colloid aggregation resulted from p H and electrolyte concentration was reversible.The migration of GMZ bentonite colloids in Beishan granite powder column was effectively facilitated at high p H and low electrolyte concentration conditions.5.Systematical experimental study by this thesis confirmed that the GMZ bentonite–BS28 groundwater system do not generate colloids,thus none of the performed experiment observed chemical erosion when GMZ bentonite was exposed to BS28 groundwater.