Development of cation exchange processes in geosynthetic clay liners 10 15 20 25 30 and the maximum increase in the relative fraction of Ca2+ was 10 percentage points and occurred for 8-hour and 1-day tests. For the 50 mM CaCl2 solution, the maximum decrease in the relative fraction of Na+ was also 14 percentage points and occurred for 4- and 8-hour tests, and the maximum increase in the relative fraction of Ca2+ was 12 percentage points and occurred for 2- and 4-day tests. The variation in BC reduced significantly after 4 to 8 days for the two relatively high CaCl2 solutions used in the test program. Correlations between swell index and 5 10 20 30 40 Bound Cations (cmol+/kg) FIGURE 13 Variation of swell index with concentration of bound cations (Na+ and Ca2+) 50 60 20 30 40 50 60 10 0.1 1 10 Elapsed Time (days) FIGURE 14 Combined variation of bound cations (Na+ and Ca2+) and swell index with time 18 Geosynthetics | October November 2023 15 20 25 30 35 10 5 100 amount of bound Na+ and Ca2+ in the conditioned bentonite are presented in Figures 13 and 14. Swell index increased with increasing Na+ and decreasing Ca2+ concentrations (Figure 13). Swell index has been demonstrated to be inversely proportional to CaCl2 solution concentration used for testing in previous studies (Scalia et al. 2014). Results of this investigation demonstrate the permanence of the modified GCL structure upon cation exchange by providing similar trends for conditioned specimens when DI water is used for the swell index tests. To attain swell index higher than 19 mL/2 g, more than 35 cmol+/kg Na+ and less than 35 cmol+/kg of Ca2+ were required in the bound cations in this test program. The variation of swell index with time was in line and character with the time-dependent variation of the BCs as presented in a double y-axis plot in Figure 14. For the two solutions with high calcium concentration (50 and 200 mM CaCl2 solutions), the timings for the significant changes in swell index and bound Na+ and Ca2+ concentrations were generally consistent, yet the timings for maximum Ca2+ and minimum Na+ fractions were not entirely consistent as individual sacrificial specimens were used in the test program. Variation of electrical conductivity with time is presented in Figure 15. Swell Index (mL/2g) Bound Cations (cmol+ /kg) Swell Index (mL/2g)