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Summary:
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The purpose of this study was to become familiar with the isostatic compression technique and to manufacture specimens to study various aspects of the manufacturing process. These included for example the effect of moisture, maximum compressive pressure, vibration, vacuum, specimen size, coating, multiple compressions and duration of load cycle on the density and other properties of bentonite specimens. Also the amount of volumetric contraction was of interest in this study together with the used mould technology.
This work summarizes the tests done with isostatic compression technique during 2008. Tests were mainly carried out with MX-80 bentonite, which is a commercial product and currently the reference bentonite in the repository reference plan. Tests were made from June to November 2008 both in Finland and in Sweden. VTT made four test series in Finland. MABU Consulting Ab made two test series in Sweden. Also Posiva Oy carried out one preliminary series before this study in Finland.
The test results show that there is a clear relationship between density and moisture content at all pressure levels. The calculated degree of saturation of more moist samples remained at the level of 95 -to 98 % of full saturation. It should be possible to manufacture buffer blocks with high accuracy (density, water content, degree of saturation), if similar preliminary tests are done.
Tests did not support the assumption that vacuum (partial or full) in the specimen during compression increases the final density. Tests showed that pre-vibrated specimens had a slightly higher density but the difference was insignificant. Coarse raw bentonite produced the highest dry density of all sodium bentonites used. The highest dry density values were received with Minelco's Ca-bentonite, but the average water content was not extremely accurate.
The following recommendations were derived from the results of this project: additional tests should be carried out to determine the relationship between density, moisture, degree of saturation and strength of the specimen. Also tests should be carried out to produce information on homogeneity of large specimens and information on the effect of the isostatic pressure and duration of the compression. Tests should be done to show that also granulated material follows the found water content density relationship and to study the material loss due to machining to the final block dimensions. This must be done with large samples.
Techniques like vibration or vacuum - separately or combined - may be used to pre-compact the material in the mould to reduce both mould size and contraction of the sample and tool material during pressing. It may also be possible to reshape the mould with a vacuum inside. Tests should also be carried out to study the swelling capacity of materials compressed to different densities under different water contents – both swelling pressure and volume increase capacity due to swelling.
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