“…Visual observation of the tablets during the dissolution test (Fig. 6) indicated that bentonite forms a gel structure in dissolution media at pH 7.2, whereas the tablets disintegrated at less acidic pH values, which could be related to the dissolution of montmorillonite at low pH and itself was responsible for gel-building in bentonite (Bendou & Amrani, 2014; Mouzon et al , 2016; Alkrad et al , 2017; González-Santamaría et al , 2020). Fig.…”
“…Visual observation of the tablets during the dissolution test (Fig. 6) indicated that bentonite forms a gel structure in dissolution media at pH 7.2, whereas the tablets disintegrated at less acidic pH values, which could be related to the dissolution of montmorillonite at low pH and itself was responsible for gel-building in bentonite (Bendou & Amrani, 2014; Mouzon et al , 2016; Alkrad et al , 2017; González-Santamaría et al , 2020). Fig.…”
“…Secondly, the saturated bentonite can support and suspend the large particle aggregate, which is precipitated and separated downward in the slurry, so that the particle distribution of the slurry is more uniform, and the strength and permeability of the cured backfill will be improved. In addition, we believe that the microbead effect of bentonite and fly ash fine particles can improve the bonding force between slurry particles [44]. When bentonite is moistened and expanded, its particles will attract one another and form a binding water film when they contact the surface of gangue/tailings particles [45].…”
Backfill mining has significant advantages in safe mining, solid waste utilization and ecological environmental protection, but solid waste materials (tailings, gangue and coal gasification slag, etc.), as derivative residues of the chemical and metallurgical industries, contain a large number of heavy metal elements, which is posing great challenges to the underground environment after backfill. In order to study the feasibility of bentonite for reducing the permeability of gangue/tailing sand cemented backfill body, relevant tests were carried out from the basic performance index, flow performance and mechanical properties of paste backfill materials. The test results show that bentonite has a significant effect on the water secretion rate of cemented fillers, and also promotes the improvement of slump and diffusion diameter of backfill slurry. The enhancement effect of mechanical properties in the early stage is not obvious, mainly concentrated in the middle and late stages of specimen curing. With the increase of bentonite content, the 28-day uniaxial compressive strength increased from 7.1 MPa and 7.9 MPa to 8.7 MPa and 9.0 MPa, respectively. Bentonite is filled between the pores of the cemented backfill with its fine particles and water swelling, which can reduce the porosity and permeability of the gangue and tailings cemented backfill. Therefore, on the premise of satisfying the flow and mechanical properties of paste backfill, bentonite can be used to improve the permeability of cemented backfill and reduce the leaching and migration of heavy metal ions.
“…Although Ordinary Portland cement (OPC) is the most common cementitious material, its pore solution has a pH > 12.5, potentially forming a hyper-alkaline plume due to groundwater percolation. The contact of a hyper-alkaline plume with buffer materials, such as compacted bentonite, that surround cementitious materials may lead to the alkaline alteration of these materials, which may cause the dissolution of minerals [2][3][4][5][6] and/or secondary mineral formation to occur [4][5][6][7][8][9]. These phenomena may change the permeability of barrier materials and affect the long-term performance of the repository.…”
In facilities for the geological disposal of radioactive waste in coastal areas, the long-term alteration of cementitious materials in engineered barriers is expected to occur due to the ingress of groundwater derived from seawater. Although the reaction between cement and seawater has been studied, the alteration behavior caused by the reaction between seawater and low-pH cement, which is expected to be used in a disposal facility, has not yet been clarified. In this study, the effects of cement type on cement–seawater interactions were investigated, and the chemical stability and mineral evolution of cement pastes caused by reactions with seawater were determined. The dissolution of cement hydrates occurred upon increased contact with seawater, and the formation of secondary minerals, including carbonate and Mg-containing minerals, was observed. The progress of dissolution depended on the mineral composition of the initially formed cement hydrates, and low-pH cement containing pozzolanic materials showed less resistance to seawater. Differences in pH and Si concentration that are due to the type of cement used had a strong influence on the evolution of minerals (especially Mg-containing minerals), implying that the formed mineral species possibly affect the migration characteristics of radionuclide.
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