Thermal and saline effect on mineral-water interactions in compacted clays: A NMR-based study

The behavior of the confined pore water in three compacted clays, which are saturated with different fluids at temperature ranging from 22 to 75 degrees C, is investigated via the NMR technique. It is shown that the pore water is mainly retained by surface forces and effective surface diffusion coefficient is estimated to be equal to the same in all the water-saturated clays, approximately two-order lower than that of bulk water. The optimal pore size in clays is roughly estimated to equal to or smaller than several micrometers based on the effective surface diffusion coefficient, which has been validated by pore size distributions obtained with mercury intrusion porosimetry. The measured effective activation energy are different for these three clays, and the water molecules in soil B with the largest specific surface and cation exchange capacity have the largest effective activation energy. Effective activation energy is also obtained for soil C (natural bentonite) saturated with NaCl, CaCl2 and KCl solutions with a concentration of 1 mol/L and distilled water. It is shown that salt solution increases the effective activation energy for soil C when saturated with KCl or NaCl (monovalent ion), whereas decreases the effective activation energy when saturated with CaCl2 (divalent ion). The influence of cationic type on effective activation energy is discussed based on the diffuse double layer theory and the cation hydration mechanism. It is also shown that permeability coefficient of the clays increases with temperature. A larger effective activation energy means that the permeability coefficient is more sensitive to temperature.