Abstract
AbstractMany safety functions required of the compacted bentonite buffer in the KBS-3 concept rely on processes influenced by the composition of the pore water. Important safety-relevant processes are related to the bentonite buffer,e.g.swelling, precipitation and dissolution reactions, and transport of water, colloids and ions. One of the methods used in analysing pore water in compacted bentonite is the ‘squeezing technique’. Various possible artefacts which can occur during squeezing, such as mixing of different pore-water types, dissolution of accessory minerals and cation exchange, need special attention.The present work describes the methodology for studying the composition of the non-interlamellar pore water by combining squeezing methods, chemical analyses, microstructure measurements and geochemical modelling. Four different maximum pressures were used to squeeze the compacted bentonite pore water. The origin of the pore water was studied by analysing the bentonite microstructure both before and after squeezing using SAXS and NMR, the cation exchange and dissolution reactions were studied by chemical analyses and geochemical modelling.The pore-water yield increased from 32 to 48 wt.% from the initial amount of porewater in the samples when the maximum squeezing pressure was increased from 60 MPa to 120 MPa. About 35 wt.% of the water collected originated from the interlamellar (IL) pores. The ratio between IL and non-IL pore waters as well as the composition of the squeezed porewater was constant in the squeezing-pressure range used. The results of microstructural measurements by SAXS were in perfect agreement with previous studies (e.g.Muurinen & Carlsson, 2013). The dissolving accessory minerals have an effect on the ratio of the cations in the squeezed solution while the migration of anions in bentonite seems to be diffusion limited. According to geochemical modelling the chloride concentration of the non-IL pore water in compacted bentonite before squeezing was 0.34 Mgreater than in the squeezed pore water due to the mixing of two main water types.
Highlights
Background of the samplesTo study the behaviour of the alternative buffer materials (ABM) in field conditions, three packages were assembled in the Äspö Hard Rock Laboratory (HRL) at the end of 2006
The aim of the present study is to develop a squeezing methodology to analyse the chemical composition of non-IL water from compacted bentonite by combining chemical analyses, measurements of microstructure and geochemical modelling
The part of the block to be used for squeezing studies was enclosed in metal transportation vessels in inert gas and transported to VTT’s laboratory where it was further cut into smaller subsamples with a band saw under anoxic conditions: (1) A 1 cm-thick slab was cut from one side of the block to prepare 1 cm3 (1 cm × 1 cm × 1 cm) sub-samples for water content measurements
Summary
To study the behaviour of the alternative buffer materials (ABM) in field conditions, three packages were assembled in the Äspö Hard Rock Laboratory (HRL) at the end of 2006. The part of the block to be used for squeezing studies was enclosed in metal transportation vessels in inert gas and transported to VTT’s laboratory where it was further cut into smaller subsamples with a band saw under anoxic conditions: (1) A 1 cm-thick slab was cut from one side of the block to prepare 1 cm (1 cm × 1 cm × 1 cm) sub-samples for water content measurements. (2) 3 cm-thick slabs were cut from the block to prepare ∼27 cm (3 cm × 3 cm × 3 cm) blanks for the squeezing studies. Four parallel sub-samples were chosen for the squeezing experiments and one for a reference sample for microstructural and water content measurements. The wet clay samples were cut into pieces, and microstructure and water-content measurements were performed. The amounts of bentonite and water that remained in the sinters were determined by gravimetric measurement of weight loss at 105°C, before and after ultrasonic cleaning
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