A study on the influence of inorganic salts on the behaviour of compacted bentonites

Abstract

Clay liners are frequently installed at waste disposal sites as a means of preventing pollutant migration and minimizing or eliminating the potential for ground water contamination. Because of their low hydraulic conductivity and high contaminant adsorption capacity, bentonite is used as a liner material to prevent subsurface contamination. However, presence of various chemicals in waste could affect the hydraulic and contamination adsorption capacity of bentonite and in turn reduce its usefulness as barrier material. In addition to the salt solution, a change in the mineralogical composition such as montmorillonite content, cation exchange capacity, specific surface area, exchangeable sodium percentage of the bentonite also significantly influences its swelling and hydraulic behaviour. This study was carried out to study the effect of salt solution and mineralogical composition on the behaviour of compacted bentonite. Two bentonites with varying mineralogical composition, which was reflected in their different liquid limit and free swelling value, were evaluated for their free swelling, Atterberg limits, swelling potential, swelling pressure, and hydraulic conductivity in the presence of various concentrations of NaCl and CaCl2 solution. To study the effect of initial compaction conditions on swelling and hydraulic behaviour in presence of salt solution, studies were also carried out on samples compacted at optimum moisture content (OMC)–maximum dry density (MDD) and 5% dry of OMC–MDD. The result shows that the liquid limit, swelling volume, swelling pressure of the compacted bentonites decreased, whereas, plastic limit and hydraulic conductivity increased with increase in the salt concentration. The results also show that the effect of the salt on the properties of the bentonites depends on the salt type, salt concentration and initial compaction condition of the bentonite. The effects due to salt concentration were found to be more pronounced for the bentonite of higher quality which is marked by a higher swelling capacity, liquid limit, cation exchange capacity, exchangeable sodium percentage and specific surface area.