Fig. 1. Bipolar sequence used for diffusion measurements.

Fig. 2. Example of measurements of diffusivity. Top: echo attenuation versus b in which the gradient amplitude g is varied. Bottom: diffusivity for five diffusion times. Clay sample CaCl₂ 0.1 mol/L, T₂=11.3 ms, w=30%.

Fig. 3. Examples of bimodal signatures observed on CaCl₂ mixtures.

Fig. 4. Textural diagrams inferred from T₂ distribution patterns: top for CaCl₂; bottom for NaCl solutions. The salt concentration is plotted as a function of the clay weight fraction. Flocculated systems (shaded area) are associated to bimodal T₂ distributions.

Fig. 5. Relaxation rate 1/T₂ for the different clay fractions and salinity concentrations. For flocculated systems (bimodal distributions), T₂ characterising the distance between flocs is used.

Fig. 6. Effective diffusivity vs clay weight fraction. For stable mixtures, the decrease of diffusivity can be fitted with an exponential decay. For flocculated systems (with bimodal distribution), the diffusivity increases as a function of salinity. The shaded area shows the domain of allowed values of the water diffusivity, which is delimited by D₀ (the bulk water diffusivity) at 30 °C, and water diffusivities for a clay gel prepared with deionised water.

Fig. 7. Effect of salinity at a constant clay fraction w (w=20 wt%).

Fig. 8. The evolution of the spacing between clay particles as a function of clay fraction for different salinities.

Fig. 9. Modelling of D as a function of the volume fraction of clay according to the model proposed by Sorrentino et al. [29]. Several couples (α, θ) are in agreement with the measurements.

Fig. 10. Normalised water diffusivity versus the ratio R_l of the two characteristic lengths of the suspension, i.e. the apparent spacing between clay particles divided by the Debye length, allows us to distinguish two regimes: (1) for R_l ranging from 2.5 to 15, the normalised water self-diffusion coefficient D/D₀ increases linearly with R_l from 0.3 to 0.8, and (2) for R_l ranging from 15 to 50, D/D₀ is constant about 0.85 ± 0.1. D₀ is the bulk water diffusivity. For suspensions prepared with deionised water, the ionic strength is calculated assuming that calcite and dolomite impurities equilibrate with deionised water (see text for details).

Kinds of smectite coagulation expected for different geochemical conditions and clay fractions (results reported by Lagaly and Ziesmer [5])

Summary of experimental conditions, of water diffusivity measurements and of T₂ relaxation time measurements in smectite gel samples. The conversion of w to _s was performed according to Eq. (2). (§) T₂ associated to relaxations mechanisms within clay aggregates

The surface relaxivities are listed in for given NaCl and CaCl₂ concentrations