Steam treatment of smectites drastically changes their properties and contributes to a remarkable reduction in osmotic swelling. To understand the effects of steam on physical properties, studies of the rheological and microfabric features were made on a gel-forming montmorillonite (SWy-1), for which steam and sonication treatments were used to first eliminate and then restore the osmotic swelling properties, respectively.

Rheological experiments demonstrated that steam treatment greatly modified the properties so that a "floc" (non-gelling colloid) instead of a "gel" formed in suspensions of steam-treated smectite. Subsequent sonication of increasing strengths created stronger colloidal clay structures, with a "floc-to-gel" transition in concentrated samples and formation of non-gelling colloids of increasing viscosity and thixotropy in dilute suspensions.

Microfabric features were observed in SWy-1 at the quasi-crystalline and aggregate levels using atomic-force microscopy and environmental scanning electron microscopy, respectively. Steam treatment induced "dissociation" of different microfabric units. Consequently, quasi-crystals no longer closely packed together, aggregates of various sizes had no attachment to one another, and cracks prevailed on and among aggregates. Swelling features were restored to the steam-treated smectite by either sonication or aging via different mechanisms.

Aiming at a mechanistic understanding, further study of the steam-treated clay systems was made on a synthetic hectorite (laponite) and a kaolinite (KGa-1), in addition to SWy-1. Synthesis of results from humidity-controlled X-ray diffraction, thermal analysis, and non-polar (N₂ and CO ₂) and polar (H₂O) gas adsorption elucidated steam-induced property changes with regard to intracrystalline swelling, dehydration and dehydroxylation, external surface areas, and micropore and mesopore structures, especially in the smectite species. The properties and availability of surfaces were critical in determining the degree of steam-induced impacts on clay systems. At the molecular level, although the internal chemical structure of the clay minerals was not affected by steam treatment, as revealed by solid-state ²⁷Al and ²⁹Si nuclear magnetic resonance spectroscopy, a prominent steam-induced modification of the chemical structure on external clay surfaces was speculated to produce reduced-force interactions with probe molecules as well as among clay surfaces.