Experimental Study on Alteration of Highly Compacted Bentonite -Dissolution Rate of Montmorillonite and the Effect on Permeability

Document available in extended abstract form only. Highly alkaline environments induced by cementitious materials in radioactive waste repositories possibly alter bentonite buffer materials. The alteration for long time may change the physical and chemical properties of buffer materials. In this paper, in order to study the dissolution rate of montmorillonite in highly compacted bentonite and the hydraulic conductivity of altered highly compacted bentonite, the laboratory experiment was done that alkaline solution is injected to highly compacted bentonite and it is analyzed after the injection for designated periods. Three test pieces of compacted bentonite (KunigelV1, Kunimine Co. Ltd.) with the dry density of 1.60 g/cm3 were prepared using unconfined compression apparatus. Each of them was set in the laboratory apparatus as shown in Fig. 1. 0.30 mol/L sodium hydroxide solution was injected for 167 days for two of the three test pieces (Case 1, 2) and 350 days for one (Case 3). The concentrations of components of the outlet solution were measured about every 10 cm3. The tests temperature was 70±0.2 deg. C. After the injection, the test pieces were analyzed using solid analysis techniques: Electron Probe Micro Analyzer (EPMA), X-Ray Diffraction (XRD), and Atomic Force Microscope (AFM). The temporal changes of Si and Na concentration and pH of the outlet solution of Case 3 are shown in Fig. 2, for example. Case 1 and Case 2 showed the good consistency with Case 3 until 167 days. The vertical sections of the test pieces were exposed as shown in Fig.3. The sections were coated by resin and smoothed for EPMA analysis. The element color maps normalized by Al were made for Si, Na, Ca, and Mg. The Si/Al maps for Blank and Case 1-3 are shown in Fig. 4. The test pieces for Case 1 and 2 were altered more strongly for the lower part, closer to the inlet, than the upper part. The test pieces for Case 1 and Case 2 were divided into the upper and the lower part while that for Case 3 was not. After measuring the density, they were dried and loosened for the solid analyses. The XRD charts around the second strong peak of analcime are shown in Fig.5. Using AFM, the one-hundred crystal particles of montmorillonite in each part were randomly chosen and their Basal Surface Area and Edge Surface Area were measured. Based on statistical study using the AFM measurements, it is shown that the upper bound of dissolution rate of montmorillonite in the compacted bentonite is 9.7x10-13 mol/m2/sec in significance level of 95%. The value is less than one thirtieth of the Sato equation (Sato et al., 2004) as shown in Fig.6. That is because the pore solution of compacted bentonite is close to equilibrium for montmorillonite dissolution. The values of hydraulic conductivity of compacted bentonite in this study are plot against montmorillonite partial density as shown in Fig.7, which are approximately one tenth of the estimates by the equation of Ito and Mihara (2005). (authors)