Density of water adsorbed on bentonites: Determination and effect on microstructural void ratio modelling

Abstract

The ability of helium gas displacement pycnometry to characterise the reduction of adsorbed water density with increasing bentonite water content was tested. For this purpose, after obtaining the grain density (which was assumed to be constant), an intensive campaign of 45 experiments was carried out with two bentonites for a total of 90 determinations, obtaining very consistent water density results. Furthermore, the water retention curve of one of the two bentonites was fully characterised by 95 tests with a chilled-mirror dew-point psychrometer. The water content results were used to determine the microstructural void ratio, both assuming a constant water density and considering the variation of the adsorbed water density obtained with the helium pycnometer. It was verified that with the constant value, by assuming a lower water density (1 g/cm3) than the experimental (values of 1.20 g/cm3 were obtained under hygroscopic conditions), higher values of microstructural void ratios are predicted, up to 15% greater for a relative humidity of 70% (0.46 instead of 0.40). For higher relative humidities the comparison loses its consistency, since the presence of capillary water cannot be disregarded, and the calculation of the microstructural void ratio is based on the assumption that the water present in bentonite is mainly in the form of adsorbed water. However, this does not compromise the analysis performed, as the results obtained for lower relative humidity values provide enough information to demonstrate the relevance of the errors that can occur when deriving models of the microstructural void ratio without taking into account the variation of the density of adsorbed water.