Compressibility Behavior of Bentonites by Stern Theory based on Constant Surface Charge Conditions

Abstract

The compressibility behavior of clays is governed by the electrical double layer formed around the clay particles. The Gouy-Chapman diffuse double layer theory is often utilized to predict the compressibility behavior of clay minerals. The theory does not consider the effect of the size of the cations, however, and thus predicts unrealistically small void ratios for compacted bentonites under large mechanical pressures expected in high-level nuclear waste-repository applications. In this study, the Stern layer was introduced to incorporate the cation size effect in the prediction of the compressibility behavior of bentonites. The overall diffuse double-layer thickness at large pressures was much smaller than the initially assumed Stern layer thickness based on the exchangeable cation size for all the bentonites studied. A compressible Stern layer was, therefore, considered for the first time in the prediction of the compressibility behavior of bentonites. The compression behavior of the Stern layer under the applied loading is influenced by the ratio of the mid-plane to the Stern potential, which is dependent on the type and composition of the exchangeable cations on the clay surface. Stern layer compression was initiated when the potential ratio was in the range 0.65–0.75 for bentonites with various surface cation characteristics. The incorporation of cation size and a compressible Stern layer provided significant improvements over the existing models in predicting the compressibility behavior of bentonites over a wide pressure range. The compressibility data predicted by the proposed model showed very good agreement with the data measured for five bentonites from the literature in the pressure range 0.1–42 MPa.