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

Highly alkaline environments induced by cementitious materials 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 quantitative data to determine the effect of alteration on the permeability of a 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. Dissolution of montmorillonite was described by the linear dependence on time, W(t)=W(0)-R(A)t, under the employed experimental conditions of pH 13 to 14 and temperatures of 90 to 170 degreesC, where W(t) denotes the density of montmorillonite (mg of montmorillonite/m(3) of sand-bentonite mixture), W(0) the initial density, RA (Mg m(-3) s(-1)) the rate of density decrease and t(s) the time after the contact with the simulated groundwater. RA is a function of pH and temperature (K), and expressed as R-A=(0.013+/-0.007)exp[-(3.7+/-0.2) x 10(4)/RT for the "pH 14.0" simulated groundwater, where R is the gas constant. 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 50 degreesC to minimize the effect of alteration of bentonite. The effective diffusivity of hydroxide ions was in the order of 10(-10) to 10(-11) m(2)/s, and is in the same order as those of Cl-, I- and tritiated water. (C) 2004 Elsevier B.V. All rights reserved.