Optimal Curing Humidity For Compacted Bentonite-sand Blocks As HLW Buffer Barrier

Compacted bentonite-sand blocks constitute buffer barriers and play essential roles in the safety of high-level radioactive waste（HLW）repositories.The bentonitesand blocks are manufactured to the desired dry density and moisture content by special compaction equipment.Then,the blocks undergo storage,transportation,and installation.The relative humidity（RH）in a block’s environment will likely change from very low at the production site to very high in tunnels with moisture inflow and stagnant air.As bentonite buffer blocks are very sensitive to RH,they will react to environmental air.In a dry environment,the blocks will lose moisture and shrink and crack,whereas in a wet environment,the blocks will absorb moisture and swell and crack.Either way,the blocks’ swelling and cracking may affect the blocks’ performance as a buffer barrier.Therefore,the blocks must be cured during all stages to ensure the quality of the physical properties of the blocks.In this study,Gaomiaozi bentonite and quartz sand were used,and the optimum curing RH of the bentonite-sand blocks were found by the combination of compacted bentonite-sand small samples and full-scale blocks.First,the bentonite-sand small samples with different initial moisture contents（11.23-21.63%）were compressed in the laboratory to simulate the production of buffer blocks.The small samples were cured at different RH of 33%,75%,85% and 100% in order to find the optimum compaction condition and curing RH.During the curing process,the periodical mass change of samples was weighed by a balance,the size change was measured by a vernier caliper,and the thermal conductivity was tested after the curing was balanced.The test results indicate that the moisture variation of small samples during curing was consistent with its soil-water characteristic curves（SWCCs）.When the compacted bentonite-sand small samples were cured at the RH of 33%,75% and 85%,the samples were dehydrated and became dried,resulting in desiccation shrinkage and cracking.When the RH was 100%,the samples with a lower initial moisture content of 11.23-14.99%tended to absorb moisture from the environment and swelled.While the samples with a higher moisture content of 17.22-21.63% were desiccated to shrink,no obvious cracks were observed on the surface.When the RH was 100%,the variation of water content and volume with an initial water content of 17.22% was the minimum,which was considered as the optimum curing RH for buffer blocks.Simultaneously,the optimum RH of industrial-scale buffer blocks used in the disposal repository can be estimated by the SWCCs of the small samples,and the development of drying shrinkage cracks can be quantitatively evaluated by the thermal conductivity.Then,according to the optimum compaction condition and curing RH of bentonite-sand small samples,the curing of full-scale compacted bentonite-sand blocks was carried out.Therefore,the blocks initial moisture content was 17%,and the curing RH were 75% and 100% according to the possible RH in the disposal repository during installation and the optimal RH of the small samples.During the curing process,the moisture content of these blocks was measured periodically by recording their weights and their volumetric shrinkage as measured by fixed dial indicators.The final moisture and dry density distributions were measured by sawing a block into 64 subsamples,and then the effect of the curing process on the homogeneity of the blocks’ dry density and moisture content distribution is discussed.The test results indicated that 100% is the optimum environmental RH for a block with 17% moisture content.After 3 months curing in the RH100% environment,no visible surface crack was developed on the block surface.The moisture content of the block was 15.15%,lower than its initial condition and the shrinkage strain of the block was lower than 1%.In addition,the curing of the block in the RH100% environment resulted in the moisture homogenization demonstrated by the decrease of the coefficient of variation（CV）of moisture content,that is,the CV of moisture content of the block decrease from 5.35%to 1.07% after 3 months curing.