Paméla Faure

Proton NMR relaxation as a probe for setting cement pastes

We report a 20-MHz proton nuclear magnetic resonance T1 relaxation study of cement paste hydration in the early stages of setting, using different centimeter-sized samples of cements of various origins and different water-to-cement ratios. In every sample, during the first few minutes of hydration, it is found that inverse Laplace processing of inversion-recovery measurements systematically exhibits at least two T1 values: a long one, around 100 ms, whose value correlates well with water content and which may be attributed to bulk water surrounding cement grains; and a short one, around 2 ms, which is quite insensitive to water-to-cement ratio and which may be attributed to water embedded in floculated cement grains before setting occurs. The time evolution of the longest T1 value for several hours is also shown to exhibit a characteristic five-stage behavior that is well correlated with known stages of the hydration process: initial reaction, induction period, acceleration period, deceleration period and slow hydration reaction. These results are compared with calorimetric measurements and electrical conductivity literature.

Drying kinetics driven by the shape of the air/water interface in a capillary channel.

Abstract.We look at the drying process in a simple glass channel with dominant capillary effects as is the case in microfluidics. We find drying kinetics commonly observed for confined geometry, namely a constant period followed by a falling rate period. From visualization of the air/water interface with high resolution, we observe that the drying rate decreases without a drying front progression although this is the usually accepted mechanism for confined geometries. We show with FEM that in our specific geometry the falling rate period is due to changes in the shape of the air-water interface at the free surface where most evaporation occurs. Our simulations show that the sensitivity of the drying rate to the shape of the first air-water interface from the sample free surface implies that slight changes of the wetting or pinning conditions can significantly modify the drying rate.Graphical abstract

Water transfer and crack regimes in nanocolloidal gels.

Direct observations of the surface and shape of model nanocolloidal gels associated with measurements of the spatial distribution of water content during drying show that air starts to significantly penetrate the sample when the material stops shrinking. We show that whether the material fractures or not during desiccation, as air penetrates the porous body, the water saturation decreases but remains almost homogeneous throughout the sample. This air invasion is at the origin of another type of fracture due to capillary effects; these results provide insight into the liquid dynamics at the nanoscale.

NMR determination of sorption isotherms in earlywood and latewood of Douglas fir. Identification of bound water components related to their local environment

Abstract Earlywood (EW) and latewood (LW) have different hygromechanical behaviors, if subjected to relative humidity (RH) variations. To understand this effect better, the adsorption mechanisms of EW and LW of Douglas fir were studied by 2D 1H NMR relaxometry under conditions of equilibrium moisture content (EMC) at 20°C. Two bound water components were detected with relaxation times T1 and T2 indicating that they are located in distinct environments but these are similar in EW and LW. Sorption isotherms were calculated and analyzed based on the sorption model of Dent. A difference of sorption energy between the two water components is in agreement with their mobility difference observed on T1−T2 correlation spectra. Moreover, for the two bound water components, EW and LW exhibit different sorption isotherms at high RH. This may be attributed to a difference of adsorption capacity. Based on the macrofibril models provided by the literature, the following hypothesis is proposed: bound water components are located in lamellar and lenticular areas, both leading to possible deformations.

Magnetic resonance imaging evidences of the impact of water sorption on hardwood capillary imbibition dynamics

Imbibition in poplar wood is studied from macroscopic measurements (mass and deformation) along with magnetic resonance imaging experiments allowing to observe bound and free water dynamics. Additional experiments with silicone oil allow to compare the characteristics of water imbibition with those of a liquid not hygroscopically adsorbed in wood. It was shown that, in contrast to porous media with an impermeable solid structure, the water imbibition in the hydraulic conduits of hardwood is not simply driven by the standard capillary effects associated with a good wetting of the solid surface, but it is in fact strongly affected by the adsorption of bound water in cell walls. More precisely bound water appears to progress far beyond the front of free water, and the free water penetration along the sample axis apparently coincides with the development of a region saturated with bound water. This likely explains that water imbibition is about three orders of magnitude slower than expected from standard Washburn imbibition process.

Combined time-lapse magnetic resonance imaging and modeling to investigate colloid deposition and transport in porous media

Colloidal particles can act as vectors of adsorbed pollutants in the subsurface, or be themselves pollutants. They can reach the aquifer and impair groundwater quality. The mechanisms of colloid transport and deposition are often studied in columns filled with saturated porous media. Time-lapse profiles of colloid concentration inside the columns have occasionally been derived from magnetic resonance imaging (MRI) data recorded in transport experiments. These profiles are valuable, in addition to particle breakthrough curves (BTCs), for testing and improving colloid transport models. We show that concentrations could not be simply computed from MRI data when both deposited and suspended colloids contributed to the signal. We propose a generic method whereby these data can still be used to quantitatively appraise colloid transport models. It uses the modeled suspended and deposited particle concentrations to compute modeled MRI data that are compared to the experimental data. We tested this method by performing transport experiments with sorbing colloids in sand, and assessed for the first time the capacity of the model calibrated from BTCs to reproduce the MRI data. Interestingly, the dispersion coefficient and deposition rate calibrated from the BTC were respectively overestimated and underestimated compared with those calibrated from the MRI data, suggesting that these quantities, when determined from BTCs, need to be interpreted with care. In a broader perspective, we consider that combining MRI and modeling offers great potential for the quantitative analysis of complex MRI data recorded during transport experiments in complex environmentally relevant porous media, and can help improve our understanding of the fate of colloids and solutes, first in these media, and later in soils.

The mechanisms of plaster drying

We show that the drying rate of plaster pastes is significantly lower than that expected for a pure liquid evaporating from a simple homogeneous porous medium. This effect is enhanced by the air flow velocity and the initial solid/water ratio. Further tests under various conditions and with the help of additional techniques (MRI, ESEM, Micro-tomography) for measuring the drying rate and local characteristics (water content, porosity) prove that this effect is due to the crystallization of gypsum ions below the sample free surface, which creates a dry region and decreases the drying rate by increasing the length of the path that the vapour has to follow before reaching the free surface.

Transport and Adsorption of Nano-Colloids in Porous Media Observed by Magnetic Resonance Imaging

We use magnetic resonance imaging to follow the adsorption of colloids during their transport through a porous medium (grain packing). We injected successive pulses of a suspension of nanoparticles able to adsorb onto the grains. To get quantitative information we carry out 2D imaging and 1D measurements of the evolution in time of the distribution profile of all particles (suspended or adsorbed) in cross-sectional layers along the sample axis during the flow. For the first injections we observe the 1D profile amplitude progressively damping as particles advance through the sample, due to their adsorption. 2D imaging shows that successive injections finally result in a coverage of grains by adsorbed particles regularly progressing along the sample. The analysis of the results makes it possible to get a clear description of the adsorption process. In our specific case (particle charged oppositely to the adsorption sites) it appears that the particles rapidly explore the pores and adsorb as soon as they encounter available sites on grains, and the surplus of particles goes on advancing in the sample. A further analysis of the profiles makes it possible to distinguish the respective concentration distribution of suspended and adsorbed particles over time at each step of the process.

Magnetic Resonance Imaging and Relaxometry as Tools to Investigate Water Distribution in Soils

Relaxation times and two imaging sequences (spin echo and single point imaging) were performed onto repacked soil samples to study respectively water distribution within the porosity and to measure water content profiles, distinguishing water contained in large pores from water contained in the whole porosity. These methods were applied to 25 samples of the same soil that was prepared to obtain aggregates of three different size, then repacked to five bulk densities. Samples were then equilibrated with water at five matric potentials. We found that T1 and T2 measurements present similar time distributions with essentially four peaks. We attributed the two shortest times to textural pores, and the two longest times to structural pores. The water profile measured with spin echo sequence was attributed to water contained in structural pores.