Ion concentration caused by an external solution into the porewater of compacted bentonite

Arto Muurinen

Research output: Book/ReportReport


This study was part of a research project aimed at improving the understanding of swelling pressure in compacted bentonite as a function of density and ionic strength in the solution. In this part of the work, the concentration caused by an external solution into the porewater was studied. The tests were carried out with clay, from which easily dissolving impurities had been removed and the clay ion-exchanged to Na-form. NaCl solutions of different concentrations and deionized water were used to saturate the compacted bentonite samples through filter plates. The filter between the bentonite and saturating solution forms a semi-permeable membrane of the Donnan system. At test termination, the bentonite samples and saturation solutions were characterized. Modelling of the chloride concentrations in the porewater was done with the Donnan model assuming that chloride does not form neutral or cationic ion pairs or sorb in the bentonite. The first modelling of the porewater was based on the traditional Donnan model assuming a homogenous pore structure. At high NaCl concentrations, the model can predict the concentrations in the porewater rather well but at low concentrations, where the exclusion is stronger, the measured concentrations are clearly higher than the modelled values. In the second porewater modelling, it was assumed that there are two pore types, interlamellar and external (large) pores in bentonite. In this case, fitting is excellent with reasonable parameter selection. The pore structure and exclusion may, however, depend on the preparation of the bentonite sample. Effective homogenization of bentonite means smaller particles, smaller external pores and stronger anion exclusion.On the basis of modelling, the behaviour of different porewater types during the squeezing was evaluated. The evaluation concerned cases where the starting densities were 700 kg/m3 and 1 500 kg/m3 and the squeezing ended with a dry density of 1 900 kg/m3. In the sample of the lower density, about 20 % was evaluated to come from the interlamellar water, while in the sample of the higher density about equal volumes came from the interlamellar and large pores.
Original languageEnglish
Place of PublicationOlkiluoto
Number of pages36
Publication statusPublished - 2006
MoE publication typeD4 Published development or research report or study

Publication series

SeriesPosiva Working Report


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