Self-Diffusion of Water and Ions in Clay Barriers

Low-pH cement mortar-bentonite perturbations in a small-scale pilot laboratory experiment

“…The hydraulic conductivity of FEBEXbentonite at room temperature (23°C) and at a dry density of 1.6 g/cm 3 has been reported previously to be on the order of 10 -13 m/s (Villar, 2000(Villar, , 2002Villar & Lloret, 2004), in agreement with the value of hydraulic conductivity calculated in the present study (1.6×10 -13 m/s). This might support the validity of the pilot experimental setup because the advective transport regime was very low and the diffusion was relevant (Muhammad, 2004;Bourg & Tournassat, 2015), as expected in a real repository scenario. Three main chemical modifications have been observed around the reactive interfaces in the experiment: (1) decalcification of C-S-H at the C-W and C-B interfaces; (2) carbonation at the C-B interface (on the low-pH cement mortar side); and (3) Mg-perturbation in bentonite next to the C-B interface.…”

Ion adsorption and diffusion in smectite: Molecular, pore, and continuum scale views

“…Most countries with HLRW storage programs are currently investigating clayey media, such as 33 bentonite and shale, for use as engineered barriers and/or host rocks of geologic repositories 34 (Andra, 2005 2011; Altmann et al, 2012). At the conditions that would exist in proposed HLRW repositories, 36 clay barriers display very low hydraulic conductivities, the ability to self-heal when fractured, 37 3 and water and solute mass fluxes that are dominated by molecular diffusion on time-scales of 1 millions of years (Neuzil, 1986(Neuzil, , 1994(Neuzil, , 2013 2014; Bourg and Tournassat, 2015). It is important to note that an understanding of diffusion 8 rates is needed for both relevant radionuclides and predominant earth metals, because the latter 9 elements can affect actinide solution speciation and compete with radionuclides for mineral 10 surface sites.…”

Modeling the transport of water and ionic tracers in a micrometric clay sample

“…where ↵ is the equilibrium ratio between the total concentration of the species of interest liquid-like pore fluid (C b ). When no adsorption occurs, ↵ = is the porosity and D a only depends on the di↵usion of the tracer in the pores and the geometry of the sample (Bourg and Tournassat, 2015).…”

“…were taken from a recent compilation ( Bourg and Tournassat, 2015 increased to B = 0.5 in order to reflect the expected lower accuracy of these studies. is indeed not isotropic and therefore the experimental di↵usion tensor displays asymmetry (Bourg and Tournassat, 2015).…”

“…were taken from a recent compilation ( Bourg and Tournassat, 2015 increased to B = 0.5 in order to reflect the expected lower accuracy of these studies. is indeed not isotropic and therefore the experimental di↵usion tensor displays asymmetry (Bourg and Tournassat, 2015). However, given the nature of the MD simulations performed here, this aspect of the di↵usion will not be further investigated and only he average D a value in all three directions will be discussed from now on.…”