Abstract
Compacted bentonite is regarded as a suitable buffer/backfill material in a high‐level radioactive waste disposal repository due to its low permeability, high swelling, and strong adsorption capacity, but the bentonite powder is difficult to compact to a high density only by increasing the compaction energy. Hence, a proposal is made to improve the compactness of bentonite by changing its grain size distribution. To obtain bentonite granules with different sizes, a granulation method is proposed in this paper, in which bentonite powders experience the processes of wetting and drying into plate bentonites, which are then crushed into granules. Furthermore, in this paper, the hydromechanical behavior of granular bentonite is evaluated to verify the feasibility of the method for preparing granules. For this purpose, the granular and original bentonite powder with similar grain sizes were prepared; then, various laboratory tests, including measurements of free swelling ratio, swelling pressure, water retention capacity, and compactness, were carried out. The test results show that the free swelling ratio, swelling pressure, permeability coefficient, and water retention capacity of granular bentonite are almost similar to those of original bentonite, and after compaction, the maximum dry densities of granular and original bentonites are 1.72 and 1.64 g/cm3 at the optimum moisture content (20%) and the energy consumption was reduced by 38% with the void ratio decreased from 1.30 to 0.8 compared with the original bentonite. It indicates that, compared with the original bentonite, the hydromechanical behavior of granular bentonites changed a little, but its compaction performance has been significantly improved. To investigate the differences in pore size distributions of granular bentonite and original bentonite, MIP and NA tests were performed on samples produced with the wetting‐drying agglomerate method, and the results show that the pores with a size of 10.0 μm almost disappear and the pores mainly exist with a size of approximately 1.0 μm. It can be verified that preparing granules by the wetting‐drying agglomeration method is feasible, the granulation process has little effect on the hydromechanical properties of bentonite, and after granulation, not only the compactness is improved but also the energy consumption is saved.
Highlights
Bentonite is a kind of montmorillonite-rich clay, and it is selected as the preferred matrix of buffer/backfill materials in a deep geologic repository for high-level radioactive waste disposal, due to its low permeability, high swelling, and strong adsorption capacity [1,2,3,4,5,6]
Torbjorn et al [8] carried out the compaction tests on the full-scale bentonite blocks with the block size of 500 × 571 × 400 mm, the bulk density of the compacted block reached about 2.0 g/cm3, and the compaction stress was as high as 50–100 MPa. is result reveals that high-powder compaction equipment that can allow vast compaction energy is required if the compacted bentonite needs to reach a high-density state, and the Advances in Civil Engineering method for improving the compactness of bentonite by increasing the compaction pressure is uneconomical, and the energy consumption is very high
The granular and original bentonite powder with similar grain sizes were prepared; the indexes measured from the free swelling ratio, swelling pressure, water retention capacity, and compaction tests were chosen to compare the hydromechanical performance of bentonites before and after being granulated
Summary
Bentonite is a kind of montmorillonite-rich clay, and it is selected as the preferred matrix of buffer/backfill materials in a deep geologic repository for high-level radioactive waste disposal, due to its low permeability, high swelling, and strong adsorption capacity [1,2,3,4,5,6]. Three types of large-scale bentonite (bentonite pellets) preparation methods have been reported: (1) rollercompacted pellets with almond or pillow shapes; (2) extrusion-produced pellets shaped as rods of varying lengths [11]; (3) pressure solid clods or granulated particles of protoliths [12,13,14,15], and the grain size of the crushed compacted pellets was about 1–5 mm. The granular and original bentonite powder with similar grain sizes were prepared; the indexes measured from the free swelling ratio, swelling pressure, water retention capacity, and compaction tests were chosen to compare the hydromechanical performance of bentonites before and after being granulated.
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