THE RELAT ION BETWEEN COMPOSIT ION AND SWELL ING IN CLAYS*

The ' phenomenon of swelling is associated with the hydration of clay; however, all clays do not swell when hydrated. Steps in the mechanism of hydration and swelling of different ypes of clays as observed and intei'preted by several investigators and some theories proposed as to the cause of hydration and swelling are reviewed. The concept of clays as colloidal electrolytes that dissociate to a greater or less ex-tent when dispersed in water seems to explain most satisfactorily the significant rela-tion between the degree of swelling on hydration and the composition of the 'clay minerals. In the kaolinite group, in which there "are generally no replacements, the small number of exchangeable cations associated with the clay structure are presumed to be held by broken bonds on the edges of the sheets. Even though kaolinite, as shown by Marshall, is more highly ionized than montmorillonite, this greater ionization, be-cause of the small number of cations present and their location on the edges of the sheets, cannot pry the units apart or leave the sheets sufficiently charge d to cause the mineral to exhibit the phenomenon of swelling