Role of cation mixing in the sol formation of Ca/Na-montmorillonite

Compound Ca/Na-montmorillonites were prepared by mixing given amounts of homoionic Ca-montmorillonite with given amounts of homoionic Na-montmorillonite. Colloidal sol formation in deionized water of these mixed Ca/Na-montmorillonites were investigated by means of turbidity measurements or through swelling pressure evolution in specially constructed swelling pressure test cells. It is concluded that the sol phase is absent in homoionic Ca-montmorillonite, which is interpreted as a manifestation of ion-ion correlations that are strong for divalent ions. Using the swelling pressure test cells it was found that Na-montmorillonite readily penetrates filters with pore size 2 μm, whereas in the case of Ca-montmorillonite no material was lost through the filters even at a pore size of 100 μm. However, mixed Ca/Na-montmorillonite with 20% or more sodium swells extensively and give rise to rapid increase in turbidity and behave qualitatively the same as homoionic Na-montmorillonite with respect to sol formation. This is also confirmed in erosion experiment using the swelling pressure test cells as well as in sedimentation tests monitored with a turbidimeter. Montmorillonite with 90% or more calcium in the interlayer behaves similar to homoionic Ca-montmorillonite. Thus it is concluded that 90% of the clay layer charge needs to be compensated by Ca2+ for correlation interactions to prevent sol formation.