Factors influencing mass diffusion in bentonite and mixtures of bentonite and sand

A model, based on the relative volumes of surface and interstitial water in soil, has been developed to study the effective mass diffusion coefficient in bentonite and mixtures of bentonite and sand. The results show that, when the surface water layer is fully developed to a thickness of 0.25 nm, the theoretical effective mass diffusion coefficient decreases with increasing dry density and bentonite content in a mixture of bentonite and sand. The development of surface water depends on the initial water content, the conversion of interstitial water to surface water, and the dry density. When the surface water layer is not fully developed, the effective mass diffusion coefficient does not necessarily decrease as the dry density increases. Considering the magnitude of measured mass diffusion coefficients in bentonite compacted to a dry density of 1.75 Mg/m3, the model indicates that in this system there is significant conversion of interstitial to surface water. This suggests that mass diffusion coefficients in compacted bentonite are mainly influenced by the relative volumes of interstitial and surface water. 8 references, 4 figures, 1 table