Computational study of the roles of alkali cation species in the swelling pressure of smectites

Interlayer cations largely determine the swellability of expansive clays. An atomic level study was carried out to determine the effect of monovalent interlayer cations on the swelling behavior of two different smectite minerals to establish their swelling pressure trends. Montmorillonite and beidellite clay minerals were simulated with five different alkaline metals as interlayer cations: Li+, Na+, K+, Rb+, and Cs+. A molecular dynamics simulation method was employed with varying charge balancing cations, layer charges and charge locations. The swelling pressure of homoionic smectite decreased as the interlayer cation progressed down the group from Li+ to Cs+ yielding the following trend: Li+ > Na+ > K+ > Rb+ > Cs+. The effects of layer charge and charge location were also evident. The outcome of this study is useful for the selection of smectite buffer materials for use in high-level radioactive waste disposal repositories.