Solution Retention Capacity as an Alternative to the Swell Index Test for Sodium Bentonite

A new test, referred to as the solution retention capacity (SRC), is proposed as a potentially less tedious and quicker alternative to the swell index (SI) test (ASTM D 5890) for bentonite. The SRC represents the amount of a 50-mL solution retained in one gram of oven-dried bentonite after centrifugation at 2750 g, and the SRC values are reported in units of mL/g. The SRC for two sodium bentonites from geosynthetic clay liners measured using deionized water and solutions containing calcium chloride (CaCl₂) at concentrations ranging from 5 mM to 500 mM is compared with the SI for both bentonites and the same test liquids. In general, both SI and SRC decrease with increasing CaCl₂ concentration due to a decrease in the thickness of adsorbed layer of the bentonite particles. However, the SI is greater than the SRC, with SI falling in the range of 7.5 mL/2 g ≤ SI ≤ 30 mL/2 g and SRC falling in the range of 1.7 mL/g ≤ SRC ≤ 7.2 mL/g. The difference in the magnitudes of SI relative to SRC is attributed, in part, to the differences in the units (i.e., mL/2 g for SI versus mL/g for SRC) and the accelerations (i.e., 1 g in SI versus 2750 g in SRC) for the two procedures. Also, the SI includes the volume of the solid bentonite, whereas the SRC does not.