Dissolution of montmorillonite in compacted bentonite by highly alkaline aqueous solutions and diffusivity of hydroxide ions

Nakayama, Shinichi ; Sakamoto, Yoshifumi; Yamaguchi, Tetsuji ; Akai, Masanobu; Tanaka, Tadao ; Sato, Tsutomu*; Iida, Yoshihisa 

Alkaline environments induced by cement in radioactive waste repositories are likely to alter montmorillonite, the main constituent of bentonite buffer materials. Over long time periods, the alteration may cause the physical and/or chemical properties of the buffer to deteriorate. For the purpose of acquiring numerical data to quantify the effect of alteration on permeability of bentonite buffer, dissolution rates of montmorillonite and diffusivity of hydroxide ions in compacted sand-bentonite mixture specimens have been measured under highly alkaline, simulated groundwater conditions. The dissolution rate of montmorillonite was given by the linear dependence on time under the employed experimental conditions of pH 13 to 14 and temperatures of 90 to 170C. The diffusivity of hydroxide ions was obtained in through-diffusion experiments combined with a pore diffusion model. The experiments were performed under relatively low temperatures of 10 to 50C to minimize the effect of alteration of bentonite. The effective diffusivity was on the order of 10 to 10 m/s.