NMR 1D-Imaging of Water Infiltration into Porous Bitumen-Salt Matrices: The Effect of Salt Solubility

Abstract

We investigate water infiltration in porous matrices made of bitumen and salts with different solubilities. Dispersion, inside bitumen, of either SrSO4 or MgSO4 at 40% mass fraction was achieved, as a way to model materials used in the context of nuclear waste conditioning. Pulsed field gradient nuclear magnetic resonance (PFG-NMR) and environmental scanning electron microscopy (ESEM) measurements allow us to characterize the evolution of the porous structure as a consequence of water infiltration (due to a leaching phenomenon) for durations up to 1.5 year. PFG-NMR enables performing 1D-imaging of water at different times to monitor its slow seeping inside the material. Profiles observed at the maximum time interval (1.5 year) demonstrate that different leaching behaviors exist depending on whether the salts dispersed inside bitumen are of the soluble or insoluble type. NMR relaxation and diffusion measurements were also taken. Remarkably, when performed in combination with 1D-imaging, such measurements yield information on the surface-to-volume ratio of the water-filled porous network, at different times, as a function of depth. In the case of the matrix-containing insoluble salts, relaxation measurements lead to discrimination between two different water populations differing by their T 2 or T 1 values. These values are described in the framework of surface-driven relaxation. A two-step model of leaching is proposed that globally accounts for the different observations (NMR 1D-imaging, relaxation/diffusion, as well as ESEM). The present work gives insight into the leaching behavior of porous bitumen-salt matrices and could be used as an input for modeling their evolution on longer timescales.

Investigation of bituminized waste products swelling behavior due to water uptake under free leaching conditions: Experiments and modeling

“…$$D_w^{mat}$$ and $$D_s^{mat}$$ calibrated on M5_NaNO3_M35_BaSO4 are 275 and 143 times larger than those in the mono‐salt NaNO 3 matrix, respectively. This significant increase of diffusion coefficients in the presence of poorly soluble salts is consistent with the existing results in the literature, 15–17 which reported that the insoluble salts finely dispersed in a bitumen matrix enhance the water progression in the material. The experimentally observed acceleration effect in the presence of BaSO 4 crystals (cf.…”

“…This observation indicates that the presence of the poorly soluble salt BaSO 4 accelerates the leaching process and the approach toward the stabilization phase. This observation is consistent with the existing nuclear magnetic resonance (NMR) results in the literature, 15,16 which suggests that insoluble salts dispersed in the bitumen matrix enhance the water progression in the material and thereby increase its water uptake rate and the resulting swelling.…”

“…After only about 1000 days, almost all sodium nitrate was leached out, and the leaching front had already crossed the full height of the sample, yielding a fast swelling of the BWP up to its maximum swelling ability. The presence of barium sulfate, which could be suggested as being an inert component at first glance, seems to play a critical role in transport kinetics, as already observed in previous studies 15,16 . The restructuration of barium sulfate particles into aggregates has to be further investigated to identify its trigger and associated phenomena.…”

“…The presence of barium sulfate, which could be suggested as being an inert component at first glance, seems to play a critical role in transport kinetics, as already observed in previous studies. 15,16 The restructuration of barium sulfate particles into aggregates has to be further investigated to identify its trigger and associated phenomena.…”

“…Still working on French BWPs, B. Gwinner 12–14 investigated the evolution during leaching of transport properties and BWPs porosity. Nuclear magnetic resonance (NMR) was used to monitor the water progression inside the material during the free leaching tests carried out on simplified French BWPs 15,16 . Results show that the insoluble salts dispersed in the bitumen matrix enhance the water progression in the material and thereby increase its water uptake rate.…”