Changes on the Mineralogical and Physical Properties of FEBEX Bentonite Due to Its Contact With Hyperalkaline Pore Fluid
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Changes on the Mineralogical and Physical Properties of FEBEX Bentonite Due to Its Contact With Hyperalkaline Pore Fluids in Infiltration Tests A. M. Fernández, A. Melón, D.M. Sánchez, M.P. Galán, R. Morante, L. Gutiérrez-Nebot, M.J. Turrero, A. Escribano CIEMAT, Dpto. Medio Ambiente. Avda. Complutense 22, 28040 Madrid (Spain) ABSTRACT Two infiltration tests with a synthetic hyperalkaline solution at pH 13.5 were performed. The aim was to analyse the interaction of concrete solutions with compacted smectitic bentonites. The experiment was performed inside an anoxic glove box at 30-35 ºC with FEBEX bentonite compacted at a dry density of 1.65 g/cm3 and a water content of 13.4%. Outflowing water coming from the bentonite was collected over time. A total of three times the bentonite pore volume was recovered and twelve chemical analyses of the resulting pore water were obtained. The ionic strength of the pore waters decreases with time from 0.20 M to 0.03 M and the pH increases from 7.9 to 9.0. After a test period of 595 days, the cells were dismantled, and the study of the state and alteration of the bentonite was undertaken. A modification in the adsorbed cation population, an increase in the Na- and K-smectites type, and the presence of a tri-octahedral smectite (saponite) and zeolites (phillipsite and chabazite) were observed. INTRODUCTION Concrete will be used as support of tunnels and galleries in the geological disposal of nuclear wastes in argillaceous host formations and also as waste containment material. The bentonite barrier will become saturated with the water resulting from the host-rock/concrete interaction. Different laboratory scale tests and geochemical modelling have been performed to study the interactions of clays with highly alkaline fluids [1-6]. The mineralogical alterations of smectites could lead to changes in different physico-chemical properties of the bentonite which could jeopardize its performance as a bentonite barrier. In this context, a concrete-bentonite interaction experiment has been performed at a high solid to liquid ratio with FEBEX bentonite. The aim of the experiment is to analyse the buffering capacity of the bentonite and the clay mineral stability in a high-pH environment over a long contact period. EXPERIMENT The tests were performed with FEBEX bentonite, extracted from the Cortijo de Archidona deposit (Almería, Spain). It consists of more than 90 percent montmorillonite, Ca and Mg being the main cations in the exchange complex, in which Na is also present. This bentonite originated from the alteration of pyroclastic volcanic rocks and contains numerous accessory and trace minerals [7,8,9].
Two infiltration tests in a small-scale hermetic cell- whose internal diameter is 50 mm and inner length 25 mm- have been running under anoxic conditions inside a glove box (
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