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

• We analyzed water-filled pore distribution in water-saturated compacted clay.
• The non-interlayer water population was estimated from 1H NMR relaxometry and freezing point depression.
• The trends between two methods agree for the population of non-interlayer water.
• T1–T2 correlation experiments gives information on water dynamics.

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 1.4 g/cm3 saturated by three salt concentrations (deionized water, 0.1 mol/L and 1.0 mol/L). The water-saturated compacted samples were prepared in PCTFE cylinder by immersed in the liquid under vacuum for 1 month. 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 4.7 ms, 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 4 nm 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 1.4 g/cm3, suggesting that high-mobility bulk-like water molecules existed in this sample.