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- Hatem, Mohammed, et al.
(författare)
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Performance of Cement-poor Concrete with Different Superplasticizers
- 2014
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Ingår i: International Journal of Research and Reviews in Applied Sciences. - 2076-734X .- 2076-7366. ; 18:2, s. 163-172
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Tidskriftsartikel (refereegranskat)abstract
- Concrete can be used for casting plugs in deep boreholes where fracture zones are intersected. They will be exposed to flowing groundwater and be in contact with very tight seals of smectite clay installed where the surrounding rock is tight. The cast concrete must be able to carry the clay segments placed over it after a few days. Its bearing capacity does not have to be very high after that since the clay soon adheres to the rock and carries itself. The concrete must be poor in cement for minimizing the risk of creation of voids caused by dissolution of the cement and it should have “inert” aggregate of quartz-rich material. Inorganic superplasticizers instead of conventional organic ones should be used for eliminating the risk of degradation and loss by formation of colloids that can carry radionuclides to the biosphere from holes bored in repository rock. The two concrete types discussed in the present study had Portland and Merit 5000 low pH cement as binders and crushed quartzite as aggregate. Talc mineral powder and ordinary organic Glenium 51 were used as superplasticizers for comparing their impact on the physical properties. The matrix of the cement-poor talc concrete gave ductile behaviour during initial hardening. The very dense matrix of either of the concretes would not lead to compression of the system even after complete loss of cement, which will happen over a longer period of time. The overall conclusion was that talc as superplasticizer and conditioner of the concrete can make the concrete sufficiently fluid for constructing seals at depth in boreholes, and react with cement to provide high strength with some delay. pH is much lower in Merit than in Portland concrete, which causes less impact on the clay seals. Portland concrete has five times higher strength than Merit concrete after a week but three times lower strength after 28 days.
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| 2. |
- Pusch, Roland, et al.
(författare)
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Roles of clay and concrete in isolating high-level radioactive waste in very long holes
- 2013
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Ingår i: International Journal of Research and Reviews in Applied Sciences. - 2076-734X .- 2076-7366. ; 16:2, s. 263-273
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Tidskriftsartikel (refereegranskat)abstract
- Groundwater flow transports possibly released radionuclides from underground repositories to the biosphere. It can also make construction difficult as is obvious from examining technical solutions for disposal of high-level radioactive waste (HLW) in long subhorizontally bored holes (KBS-3H) and in very deep boreholes (VDH). The presence of intersected, water-bearing fracture zones requires concrete for sealing these parts of the holes while the rest contains canisters surrounded and separated by dense, expandable clay. Casting of the concrete should be preceded by grouting of the fractured rock using cementitious materials composed so that mutual physical and chemical interaction do not degrade either of them. For the sake of rock stability the horizontal holes have to be located at very moderate depth, 400-500 m, where the rock has a high average hydraulic conductivity, while the slimmer, steep holes reaching down to 4 km are kept stable by using clay mud in the construction phase and dense clay for long term performance. The rock at this depth is much less permeable than higher up and the groundwater sufficiently salt to be maintained there, causing only local thermally induced circulation of possibly contaminated water. The KBS-3H concept involves practical difficulties and risks in the installation of the clay seals and waste canisters, for which the risk of shearing by slip of frequently intersected steep fractures is a major threat after closure of the repository. The VDH concept relies on effective sealing of the upper part of the deep holes and puts less demand on the seals in the lower, waste-bearing part, for which the buoyancy conditions of the groundwater make it a major barrier to upward migration of possibly released radionuclides.
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