Water adsorption, microstructure, and volume change behavior of clay minerals and soil

Swelling attributable to intracrystalline water adsorption by montmorillonite (MT) and vermiculite (VR) saturated with cations which limit interlayer expansion were calculated as the product of one-half the change in interlayer spacing determined by x-ray diffraction and the difference between total surface area taken as 800 m²/g and external crystal surface area measured by N₂ gas adsorption. Swelling directly attributable to osmotic adsorption was calculated as the product of external surface area and theoretical double layer thickness. In homoionic, monomineralic systems of Ca- and Al-MT and Na-, Ca- and Al-VR, intracrystalline expansion accounted for 60, 70, 21, 56 and 44%, respectively, of total expansion during saturated vapor-wetting and 15, 50, 4, 12 and 13% during submersion in 0.01 N electrolyte solutions. Osmotic adsorption of these systems accounted for < 10% of the expansion on submersion in all cases.

Dry, static compaction of Ca-MT resulted in an increase in expansion with increasing initial density which was not explained by changes in predicted osmotic or intracrystalline expansion. This was attributed to expansion caused by gaseous pressures developing ahead of advancing wetting fronts. An inverse relationship was found between pore size and swelling caused by entrapped air pressures. Slow wetting ameliorated this expansion by allowing dissipation of entrapped air.

Relaxation of crystal strains during wetting was also implicated as contributing to expansion. Electron microscopy revealed crystal strain decreased in the order: Na-MT > Ca-HT > Al-MT > Na-VR ≃ Ca-VR ≃ Al-VR. Greater expansion by freeze-dried than oven-dried Ca-MT was explained by greater crystal strain observed in the former clay. Greater strain broadening of the (060) x-ray diffraction peak was observed for the freeze-dried clay and its mean b-dimension was slightly smaller than the oven-dried clay. Crystal strain relaxation is suggested to explain the relationship between b-dimension and swelling reported in the literature.

Predicted osmotic expansion for two soils was greater than in the pure clay systems and intracrystalline expansion smaller, due to a greater ratio of external to internal surface area in the soils. Osmotic adsorption accounted for 30-70% of the soil swelling during submersion in 0.0lM electrolyte solutions, while intracrystalline expansion accounted for < 10%. The magnitude of swelling due to entrapped air pressures was evaluated from the difference between expansion of atmospheric pressure-wet and vacuum-wet clods. This component accounted for 10-60% of the expansion of undisturbed and compacted samples during the second cycle of wetting from the air-dry state, but did not appear to be directly related to pore size. Changes in structure accompanying air-drying appeared to enhance expansion, especially for the high MT Iredell soil. It was suggested that this may be the result of increases in crystal strain during drying.