Determining the bound water content of montmorillonite from molecular simulations

Abstract

The physical, chemical and engineering properties of clay, particularly for strength, deformation and permeability, largely depend on the characteristics of bound water. The bound water has been a hotspot research focus on the matter in soil mechanics, engineering geology and so on. Despite numerous studies on bound water of clay, a unified theory for the quantitative determination of bound water film thickness has not been proposed yet. This paper put forwards a point of view that the boundary of bound water in clay is located at the position of the water layer closest to the shear plane in the electrical double layer (EDL). Molecular dynamics (MD) was adopted to determine the thickness of bound water in Na-montmorillonite. The thickness of bound water in the pore containing two water layers was determined as about 6 Å regarded as tightly bound water after verifying by experimental data. The physical properties of bound water were also investigated showing that the bound water suffers compressive stress more than 103 kPa from the clay surface and has shear viscosity several times greater than bulk water. Additionally, an application for predicting the hydraulic conductivity of clay by considering the bound water was introduced. A modified Kozeny-Carman equation with the variable viscosity of the seepage fluid was proposed, satisfying accuracy compared with experimental data. The findings in this study are helpful for revealing the mechanism of bound water on the surface of soil particles.