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Working reports


Posiva's working reports are used to publish research results related to the practical aspects of the final disposal of spent nuclear fuel. Approximately one hundred working reports are published every year, usually in Finnish or English. The reports are written by Posiva employees or specialists from third party organisations. The working reports are an important tool for distributing the results of research work. All working reports are published on the Posiva website.



Workreport 2011-84

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Name:

Quality Control and Characterization of Bentonite Materials

Writer:

Kiviranta, L. & Kumpulainen, S.

Language:

English

Page count:

98

Summary:

Before bentonite material is taken into use in performance testing, the quality of the material needs to be checked. Three high grade bentonite materials: two natural Na-bentonites from Wyoming, and one natural Ca-bentonite from Milos, were characterized. Each material was characterized using duplicate or triplicate samples in order to study variability in material quality in batches. The procedure consisted of basic acceptance testing (water ratio, CEC, swelling index, liquid limit, and granule size distribution), advanced acceptance testing (exchangeable cations, chemical and mineralogical composition, density, swelling pressure and hydraulic conductivity) and complementary testing (herein surface area, water absorption capacity, montmorillonite composition, grain size distribution and plastic limit).

All three materials qualified the requirements set for buffer bentonite for CEC, smectite content, swelling pressure, and hydraulic conductivity. Wyoming bentonites contained approximately 88 wt.% of smectite, and Milos bentonite 79 wt.% of smectite and 3 wt.% of illite. Precision of smectite analyses was ±2 %, and variances in composition of parallel samples within analytical errors, at least for Wyoming bentonites. Accuracy of quantitative analyses for trace minerals such as gypsum, pyrite or carbonates, was however low. As the concentrations of these trace minerals are important for Eh or pH buffering reactions or development of bentonite pore water composition, normative concentrations are recommended to be used instead of mineralogically determined concentrations.

The swelling pressures and hydraulic conductivities of different materials were compared using EMDD. Swelling pressure was relatively higher for studied Ca-bentonite than for the studied Na-bentonites and the difference could not be explained with different smectite contents. Hydraulic conductivities seemed to be similar for all materials.

The results of index tests correlated with the smectite content. Thus, in a certain extent, index tests can be used to determine the smectite content indicatively for quality control purposes. Previously set acceptance testing requirement limits for swelling index, liquid limit and CEC should be reconsidered, since Ca-bentonite tested in this study did not fulfill the requirement for swelling index, the previously set liquid limit requirement value was way below the values measured in this study, and because the previously set CEC requirement limits were based on a technique that needed different requirement limits for Na- an Ca-bentonites, on contrary to the method used in this study.

Keywords:

Quality control, characterization, bentonite, index test, composition, swelling pressure, hydraulic conductivity.

File(s):

Quality Control and Characterization of Bentonite Materials (pdf) (1.8 MB)


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