Effect of salt solution on the optimum lime contents of bentonite and silt

In practice, prior to treating a soil with lime, the optimum lime content required for the cation exchange and for providing sufficient Ca2+ and high pH for pozzolanic reaction with the minimum dosage of lime, should be determined. The optimum lime content is water chemistry and clay mineralogy dependent. In this study, the different lime dosages were added in MX-80 bentonite and silt suspensions, which were prepared with deionised water (DW) and synthetic seawater (SSW), respectively. Then, the pH measurement was conducted to determine the optimum lime contents of MX-80 bentonite and silt. The continuous base (1-mol/L NaOH solution) titration test was conducted on silt/MX80 suspensions as well, in order to highlight the lime (Ca2+) role. Then, the impacts of salt solution and lime on the material behaviour such as dissolution and/or precipitation of new compounds were discussed, based on the measurement of pH followed by the quantification of the major species in the supernatants of MX80-lime/NaOH suspensions. Results showed that the optimum lime content increased with the increase of salt concentration, which could be attributed to the consumption of OH− ions by Mg2+ and Ca2+ ions in the salt solution, producing the precipitations of Mg(OH)2 and CaCO3. Due to the higher cation exchange capacity and higher bentonite solubility of MX-80 compared to silt, a higher lime addition was required to reach the optimum lime dosage pH threshold for MX-80 bentonite. The pH of the tested suspensions was found to be lower than that of blank deionised water and synthetic seawater, as OH− ions could be consumed by the material adsorption, dissolution of clay minerals and pozzolanic reaction. However, the pH of MX80-SSW-NaOH suspensions was higher than that of blank synthetic seawater in case of titration of NaOH solution from 2.5 mL to 6 mL. This is because some Mg2+ and Ca2+ ions were adsorbed on the clay mineral surface through cation exchange process and less OH− ions were consumed by the production of Mg(OH)2 and CaCO3 in the MX80-SSW-NaOH suspensions as compared to blank synthetic seawater.