Reaction of bentonite in low alkali cement leachates : preliminary results from the Cyprus Natural Analogue Project (CNAP)

Bentonite is a key component in many designs for radioactive waste repositories. The plasticity, swelling capacity, colloid filtration, low hydraulic conductivity, high retardation of key radionuclides and stability in relevant geological environments all make bentonite an ideal barrier/buffer material. However, bentonite is chemically unstable under higher pH conditions and this is a potential problem for repository designs which mix cement and concrete with bentonite barriers. The hyperalkaline (pH~13) leachates from the cement are expected to cause alteration of the bentonite. Low alkali cements produce lower pH (around 10-11) leachates and it is expected that this will slow bentonite reaction (or even stop it altogether) over the timespan of relevance to repository safety. Unfortunately, it has proven extremely difficult to study these very slow reactions in the laboratory so an alternative approach, that of studying natural analogues of the reaction process, has begun in Cyprus. In this paper, preliminary details of this new investigation of long-term bentonite reaction in the natural hyperalkaline groundwaters of the Troodos ophiolite in Cyprus are presented. Here, groundwater pH values of 10.0 to 11.9 have been reported, falling into the range typical of lowalkali cements that are presently being developed for use in radioactive waste disposal. The aims of this stage of the project were to identify likely sites of hyperalkaline groundwater/bentonite reaction and assess the relevance of the current site conceptual model. Preliminary groundwater and petrographic data for one group of related sites where hyperalkaline groundwaters are present are also discussed.