Adsorbed cation effects on unfrozen water in fine-grained frozen soil measured using pulsed nuclear magnetic resonance

Unfrozen water within frozen soils is a key component that determines a soil's thermophysical response to changing physical and environmental conditions. The physicochemical nature of the clay component of fine-grained soils, including the adsorbed cations, strongly controls the amount and mobility of unfrozen water. Here we present the unfrozen water measurements of five standard clays and one heterogeneous soil. For each soil, we prepared a suite of the untreated soil and four cation treatments, which represents a comprehensive data set of clays and dominant cations typically found in soils. We measured unfrozen water content using a refined pulsed nuclear magnetic resonance (P-NMR) testing facility and the normalization method, which yielded accurate and repeatable results. Results indicated that cation treatments have negligible effect on the unfrozen water content of kaolinite, illite, and chlorite. Conversely, soils that are partially or completely composed of smectite demonstrated the largest unfrozen water content when treated with Na+ cations, and a marked reduction with the K+ treatment. From the results of the standard clay testing, we estimated the unfrozen water content for the natural, heterogeneous soil, producing curves that matched those measurements to within 4% for all cation treatments. This suggests that the unfrozen water content of a heterogeneous soil with a known mineralogy may be approximated from a database of measured unfrozen water contents of standard clays.