Alteration of Isolating Properties of Dense Smectite Clay in Repository Environment as Exemplified by Seven Pre-Quaternary Clays

摘要

Seven pre-quaternary clays with a smectite content ranging between zero and about 25% were taken as possible reaction products resulting from chemical alteration of dense sodium bentonite. They were characterized with respect to the mineral composition and microstructural constitution and tested with reference to their hydraulic conductivity, swelling ability and creep properties. It was found that since they were all less permeable than a typical large granitic rock mass they would serve as flow barriers in a repository. Thus, even rather extreme chemical attack is not expected to eliminate the most important barrier function of Na bentonite in repository environment. However, slight mechanical disturbance of a heterogeneously altered smectite clay buffer or seal, may be critical. Thus, the investigated, less smectitic clays experienced a rather dramatic increase in hydraulic conductivity on expansion and remolding. This is explained by the inability of a microstructurally discontinuous smectite component - particularly in the Ca-form - to swell and fill voids. The minimum content of Na smectite to preserve the self-healing capacity is estimated at 15-25%. Slight or moderate cementation was indicated by two of the clays by the creep tests. At a smectite content of 15-25% it is probable that self-healing will take place after a mechanically induced breakage of the cementing bonds. The tests gave a good basis for future development of rational, routine tests as well as for a relevant characterization of buffer material candidates. (ERA citation 13:056327)