Measurement of Temperature-Dependent Bound Water in Clays

Heating of clay under drained conditions results in thermal expansion of the soil minerals and pore water and can also cause a change in the soil fabric, producing irrecoverable deformations. These deformations are most pronounced for normally and lightly overconsolidated clays that undergo significant net compression and have been attributed to changes of the adsorbed or bound water layer with temperature. The current article quantifies the change in bound water content through laboratory measurements of temperature dependence in the specific gravity of solid particles while accounting for the thermal expansion of the soil constituents. The experiments compare changes for three representative clays of differing mineralogy at temperatures ranging from 20 degrees C to 38 degrees C. The laboratory experiments confirm that the specific gravity of clay decreases with temperature consistent with the conversion of bound water to free water, while accounting for the thermal expansion of the pore water. The density of oven-dried solid particles obtained from gas pycnometer measurements is also lower than the value obtained from typical water submersion methods because of the existence of the bound water layer. We interpret the experimental measurements assuming a mass density of bound water and an exponential decay of bound water content with temperature. Using these assumptions, we show that the thickness of the bound water layer decreases from 9 angstrom at 20 degrees C to 6 angstrom at 40 degrees C for three clays of widely differing mineralogy.