Linking the diffusion of water in compacted clays at two different time scales: tracer through-diffusion and quasielastic neutron scattering.

Diffusion of water and solutes through compacted clays or claystones is important when assessing the barrier function of engineered or geological barriers in waste disposal. The shape and the connectivity of the pore network as well as electrostatic interactions between the diffusant and the charged clay surfaces or cations compensating negative surface charges affect the resistance of the porous medium to diffusion. Comparing diffusion measurements performed at different spatial or time scales allows identification and extraction of the different factors. We quantified the electrostatic constraint q for five different, highly compacted clays (rho(b) = 1.85 +/- 0.05 g/cm(3)) using quasielastic neutron scattering (DENS) data. We then compared the DENS data with macroscopic diffusion data for the same clays and could derive the true geometric tortuosities G of the samples. Knowing the geometric and electrostatic factors for the different clays is essential when trying to predict diffusion coefficients for other conditions. We furthermore compared the activation energies E-a for diffusion at the two measurement scales. Because E-a is mostly influenced by the local, pore scale surroundings of the water, we expected the results to be similar at bath scales. This was indeed the case for the nonswelling clays kaolinite and illite, which had E-a values lower than that of bulk water, but not for montmorillonite, which had values lower than that in bulk water at the microscopic scale, but larger at the macroscopic scale. The differences could be connected to the strongly temperature dependent mobility of the cations in the clays, which may act as local barriers in the narrow pores at low temperatures.