An NMR-Based Procedure for Evaluating the Apparent Surface Relaxivity and Pore Size Distribution of Soils

摘要

A nuclear magnetic resonance-based procedure is proposed to evaluate the surface relaxivity parameters pk (k = 1 or 2 for spin-lattice or spin-spin relaxation) and pore size distributions (PSDs) of six different soils, including four expansive clays, a silt, and a loess. It is shown that both p1 and p2 first increases with specific water content (w) until w increases up to about 10 %, and then p1 approaches a stable value, whereas p2 slightly decreases. The underlying mechanisms for the surface relaxation of water protons are evaluated. It is shown that the water proton relaxation in expansive clays is controlled by the partially fixed ions confined in diffuse double layers, whereas that in silt and loess is dictated by the proton exchange between water molecules and surface hydroxyls and the dipole-dipole interaction between water molecules and hydrogen-bonding molecules. As a consequence, the pk values of silt and loess are larger than those of expansive clays. A new method is proposed to determine the apparent surface relaxivity parameter that is used to determine the PSD of soil based on the surface relaxation time distribution. The proposed procedure is validated by comparing the estimated PSDs with the measurements of mercury intrusion porosimetry.

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