Water Sorption Characteristics of Aluminum‐ and Calcium‐saturated Soil Clays

Abstract

AbstractWater sorption by Al‐ and Ca‐saturated bentonites and soil clays with montmorillonitic, mixed, illitic, and kaolinitic mineralogy were quantitatively evaluated at water vapor pressure levels (P/P0) of 0.50, 0.90, and 0.98 by thermogravimetry (TG) and differential scanning calorimetry techniques. After a 14‐d equilibration time, Al‐saturated clays adsorbed less water than Ca‐clays at low water vapor pressures but 10 to 12% more at P/P0 = 0.98. Heats of dehydration (ΔH) followed similar trends but the enthalpy required to remove 1 g of H2O from either Al‐ or Ca‐clays was approximately equal for clays with similar mineralogy. Enthalpy per unit weight of H2O decreased with decreasing interlayer space availability reaching minima in kaolinitic clays. Moisture retention differences between Al‐ and Ca‐clays at different P/P0 levels are attributed to structural organization differences of the H2O molecule—exchangeable cation network in the interlayer space as well as differential expansion at high hydration states. Practical implications of these relationships may be higher field moisture capacities, higher moisture retention during drought, and poorer field expression of cracks in acid soils with relatively high degree of Al‐saturation, especially when the soil contains significant montmorillonite quantities. Thirty‐five acid soil samples on which these relationships were tested indicated significant dependence of soil moisture characteristics on the degree of Al saturation of the clay surface.