Ice-soil Mixing Method To Improve Homogeneity Of Bentonite Material For HLW Disposal

Compacted bentonite is considered as an ideal buffer/backfill material in engineering barrier for high level waste(HLW)disposal.In China,the buffer/backfill material research is currently in a transition stage from small-scale specimens to large-scale blocks,and scientists are facing the challenge of how to produce a homogeneous compacted bentonite in the initial sample preparation stage.There are two major problems encountered during the process of adding water for sample preparation,that is the soil agglomeration and uneven distribution of water content because of the high plasticity of bentonite.It is the difference between the agglomerates and the surrounding bentonite powder that results in the heterogeneity of compacted bentonite.Gaomiaozi(GMZ)bentonite powder is selected as the test material to prepare six different water content(5%,10%,15%,20%,25%,and 30%)in this study.Compared with traditional spray method,this experiment explored a new water adding technique,mixing <1mm ice powder with bentonite powder(namely the ice-soil mixing method).Soil sieves with the diameters of sieve mesh 10 mm,5 mm,2 mm,1 mm,0.50 mm,and 0.25 mm from the top to the bottom were used to separate macro-agglomerates(agglomerates with size upon 0.25mm)in bentonite-water mixture.The property differences in water content,density,and shrinkage of agglomerates in different size were tested to evaluate the heterogeneities of loose bentonite-water mixture.Besides,an index P was introduced to evaluate the dispersity of water content among agglomerates in the loose mixture quantitatively.The effects of uneven water content distribution on shrinkage cracks and strength of compacted bentonite specimens were also analyzed.Results showed that macro-agglomerates characterized by a higher water content,higher shrinkage ratio,and lower dry density than that of small bentonite powder in loose bentonite-water mixture.It was the uneven distribution of water content that resulted in the heterogeneous of loose mixture.By analogy with the variation coefficient in statistics,an index P,considered the effects of agglomeration,could be used to evaluate the dispersity of water content distribution.The low value P indicated that the ice-bentonite mixing method led to a homogeneous water content distribution of bentonite than the spray method.By the ice-bentonite mixing method,the humidity was more homogeneous in the vertical section of compacted bentonite compared to the spray method.After drying,there were less shrinkage crack and higher mechanical strength ofcompacted bentonite by replacing the spray method to the ice-bentonite mixing method.Compared with spray method,the ice-soil mixed method also had advantages such as less time consuming,less adhering to the container,and less water content deviate to targets.Furthermore,microwave-assisted thaw method could significantly accelerate the thawing rate without affecting the amount of agglomerates and water content of specimens.This research proves that adjusting the water content of bentonite with the ice-soil mixing method can obviously increase the uniformity of compacted bentonite with a high mixing efficiency,which provides a new technology innovation.