Bentonite Interaction with Saline High-pH Solutions
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Bentonite Interaction with Saline High-pH Solutions
Heikola Tiina1 and Vuorinen Ulla1 1 VTT Technical Research Centre of Finland, Nuclear Energy, Otakaari 3K Espoo, P.O. Box 1000, FI-02044 VTT, FINLAND ABSTRACT Degradation of cementitious materials produces leachates of high pH. Such an alkaline plume, if reaching the bentonite buffer, is likely to induce mineralogical and chemical changes in bentonite over long times and may jeopardise the set safety function of the buffer.The objective of this ongoing research is to study the possible alterations of two bentonites, MX-80 and Deponit CA-N, in alkaline leachates at two different temperatures. Also the buffering capacity of the bentonites against high pH will be evaluated. The ongoing batch experiments are carried out in an anaerobic glove-box (Ar atmosphere, low CO2) at two temperatures (25/60 °C) with three types of simulated cement waters (pH 9.7/9,3, 11.3/10.2 and 12.0/10.9) at 25/0 °C) and one saline groundwater simulate (pH 8.3/7.9) as reference. The solid to liquid ratio used is 1/10. For each set of experiments there are three parallels so that bentonite alteration can be analysed after three different time periods. In the experiment each bentonite sample is leached with several batches of leaching solution. For each renewal of the leaching solution the phases are separated by centrifugation, the reacted solution withdrawn and the chemical composition analysed. The high-pH experiments (11.3 and 12.0, at 25°C) have continuously shown an initial decrease in the pH-values after each leachate renewal, albeit less dramatic than in the beginning, indicating remaining buffering capacity of the bentonites. The other two experiments (pH 8.3 and 9.7 at 25°C) have shown rather unaltered pH-values. In general, slightly lower pH-values were observed in the Deponit CA-N samples than in those of MX-80. The main cations (Na and Ca) analysed in the leachates have shown a rather expected trends as a result of ion-exchange occurring in the bentonites. The analysed Si concentrations indicate possible dissolution of smectite. More conclusions are possible after the bentonites have been characterized. One experimental set of the 25 °C experiments has been finished and the bentonite phases are being characterized. Other experiment sets are still continued. INTRODUCTION According to current concept, bentonite is the choice to be used as one barrier material in the final repository of spent nuclear fuel. However, a repository cannot be constructed without using cement materials and therefore it can not be ruled out that a hyper-alkaline plume, originating from a cementitious source could reach the bentonite buffer. It is also known that temperature and alkaline cement pore water contributes to the dissolution of smectites in bentonite and causes changes in its physical and chemical behaviour [1, 2, 3, 4, 5]. Consequently, in the long term disposal the containment properties of bentonite may be at a risk. In contact with water the degradation of cement materials follows a known three
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