The Swelling of Clay: I. Basic Concepts and Empirical Equations

The primary objective of the present study was to elucidate the relative contributions of exchangeable cations and surface forces to the swelling pressure, Π, of montmorillonite. To achieve this objective, measurements were made of the amount of water imbibed by Na‐saturated montmorillonites having different cation exchange capacities, b dimensions, and specific surface areas under applied pressures of 0, 1, 3, and 5 atm. The resulting data were used to develop the empirical equation (Π + 1) = exp [α/( m w / m m )] in which m w / m m is the mass ratio of water to montmorillonite and α is a linear function of the specific surface area and cation exchange capacity. Analysis of double‐layer theory in the light of this equation led to the conclusion that Π is affected less by exchangeable cations than by surface forces. It was suggested that exchangeable cations have a secondary effect because few of them dissociate from the layer surfaces.