Study On The Mineral Reaction Mechanism Of Gaomiaozi Bentonite And Simulated Cement Solution

Countries around the world are vigorously developing nuclear energy with the goal of energy conservation and emission reduction.During the operation of nuclear power plants,a large amount of spent fuel will be generated after use.Spent fuel becomes nuclear waste after extracting plutonium and uranium.Nuclear waste can be classified into five categories based on different levels of radioactivity:extremely short lifespan,extremely low level,low level,medium level,and high level radioactive waste.At present,the most widely accepted solution for high-level radioactive waste in various countries is the deep geological disposal method,which requires a"multiple barrier system"design,including host rock,buffer materials,canister,and high-level radioactive waste vitrification.As the most important artificial barrier in the disposal repository,buffer materials need to meet multiple buffer barrier performances requirements such as long-term stability and expandability.Cement is widely used for the concrete lining in high-level radioactive waste repositories which will be immersed in groundwater for a long history,releasing a strong alkaline solution referred to cement solution in chemistry.Cement solution can penetrate into the bentonite buffer material gradually,promote chemical reactions with montmorillonite,form a secondary mineral of illite-smectite mixed layer,or degrade the buffer material for a long time.In order to give full play to the excellent expansion characteristics and cation exchange capacity of the bentonite,GMZ bentonite must maintain sufficient chemical stability under the conditions of the repository.The chemical composition of the cement solution undergoes a four-stage evolution process.The main alkali metal ions in the solution of stage I are Na and K,and the p H value is 13.0-13.5.The main alkali metal ion in the solution of stage II is Ca,and the p H value is buffered at 12.5.The main component of stage III is hydrated calcium silicate gel,and the p H value is 10.4-12.5.In stage IV,the alkaline components in the solution are basically diluted,and the chemical properties tend to be in equilibrium with groundwater,and the p H value is 7-9.Cement leachate at different stages will react differently with bentonite buffer barriers,resulting in varying degrees of impact on the buffer barrier.The research object of this article is Gaomiaozi（GMZ）bentonite,a buffer backfill material for disposal repository in China.The research method is indoor static batch test of bentonite mixed with simulated cement solution.Fresh cement solution（YCS）and evolved cement solution（ECS）are standing for I-stage and II-stage cement solutions,respectively.YCS is a mixed solution of Na OH and KOH and ECS is a saturated Ca（OH）₂ solution with initial p H of 13.4 and 12.5,respectively.Simulated cement solutions are mixed with GMZ bentonite powder as suspension systems to prompt batch tests of static solid-liquid reaction.The reaction time is t=16,23,32,45,64,91,128,181,256 days.During the static reaction period,the suspension collected in stages is centrifuged into solid and liquid phases.The liquid phase were analyzed for p H,electrical conductivity and cation concentration.The solid phase was characterized by means of X-ray diffraction,Fourier transform infrared spectra and scanning electron microscope–energy spectrum.Analyze the mineralogy changes of GMZ bentonite at different times and under different test solution conditions to evaluate the long-term chemical stability of bentonite buffer materials in China’s repository.Experimental research was conducted on the modification and purification of calcium based GMZ bentonite from an engineering perspective.The experiment used four inorganic salts,sodium chloride,sodium carbonate,sodium pyrophosphate,and sodium hexametaphosphate,to modify GMZ bentonite with sodium.The mass fractions of the sodium agent were 2%,5%,10%,and 20%of the bentonite mass,respectively.The sodium reaction temperature was 20℃,60℃,and 80℃,and the sodium reaction time was 1h,2h,and 3h.After modification,use natural sedimentation method to extract particle size<5μm Bentonite particles of bentonite.The obtained bentonite was subjected to expansion capacity test and XRD test,and the optimal modification conditions of bentonite were analyzed based on the test results.The results of static batch test show that in YCS group,the p H value of the system during the test is more than 12.9,which meets the p H threshold condition of montmorillonite dissolution.The reaction rate of the whole system decreases with the extension of the test time.Montmorillonite minerals in bentonite are continuously dissolved in YCS,and montmorillonite content decreases from 56%to 10%,and illite/smectite mixed layer secondary minerals is formed.The wing-colloid formed on the surface of montmorillonite particles after the hydration of bentonite was dissolved by YCS,and the aggregate structure appeared denudation and fragmentation.The p H value of the ECS suspension system quickly dropped below 11.5 within 64 days.The ion exchange effect was basically completed within 64 days of the experiment,and sodium based bentonite was modified to calcium based bentonite due to the ion exchange effect.In ECS group,part of montmorillonite can be dissolved when the initial p H value of the solution is>11.5,but the dissolved part is relatively small.Montmorillonite will not be dissolved after the system p H is less than 11.5,and no secondary minerals appear in the solid-liquid reaction.In the modification and purification experiment,sodium chloride was first used as the modifier.Through the expansion capacity test and X-ray diffraction results,the optimal conditions for bentonite modification were selected as t=3h,T=60℃.Under the conditions of optimal modification temperature and time,by comparing the modification effects of four kinds of sodium reagents on montmorillonite,it can be seen that the modification effect of sodium chloride on montmorillonite increases with the increase of the content of sodium chloride,but the montmorillonite interplanar spacing does not decrease,and the ion exchange is not much;The sodium carbonate group has the best modification effect when the addition amount is 2%;The modification effect of sodium pyrophosphate and sodium hexametaphosphate groups improves with increasing addition.However,due to the mechanism of action of these two sodium agents,it is difficult to separate them from bentonite,thereby affecting the quality of bentonite.In summary,it is believed that the best modification effect can be achieved by using Na₂CO₃ with a mass fraction of 2%for 3 hours when the sodium reaction temperature of GMZ bentonite is T=60℃.