Pore distribution of water-saturated compacted clay using NMR relaxometry and freezing temperature depression; effects of density and salt concentration

Abstract

The estimation of water population in water-saturated compacted clay was studied with 1H NMR relaxometry and freezing point depression. The clay samples and saturated condition were Na-montmorillonite at 0.8 and 1.4g/cm3 saturated by three salt concentrations (deionized water, 0.1mol/L and 1.0mol/L). The water-saturated compacted samples were prepared in PCTFE cylinder by immersed in the liquid under vacuum for 1month. All NMR measurements were conducted using intact PCTFE vessel, which was possible to keep confinement condition during experiment.In order to distinguish interlayer with four hydrated state and non-interlayer water, we assumed four T1 thresholds corresponding to 1-, 2-, 3, and 4 hydrated layers, which were 1.2, 2.3, 3.5, and 4.7ms, respectively. The populations of hydrated state and non-interlayer water were calculated from these thresholds at 30°C. The sample with lower density exhibits higher population of non-interlayer water up to 55%.Low-temperature 1H NMR experiments were also conducted to support these results in view of freezing point depression against pore size. Mesopore water in approximately 4nm space observed in the calorimetric study was considered as non-interlayer water and the threshold temperature, which was equivalent to a freezing point of −12.5°C from Gibbs–Thomson equation. The result showed that population of non-interlayer water by expected from freezing point depression agreed with 1H NMR relaxometry within 10%. Correlation experiments were also conducted between longitudinal (T1) and transverse relation times (T2) at −10°C to obtain dynamical information on water. A small peak was obtained close to a T1/T2 line of 1.0 at a clay density of 1.4g/cm3, suggesting that high-mobility bulk-like water molecules existed in this sample.