Index testing and assessment of the suitability of bentonite from the Imerys Bentonite Mine, for use in geosynthetic clay liners.

Abstract

Geosynthetic Clay Liners (GCLs) have become a suitable substitute for compacted clay liners. Their use centre on liquid impoundments, such as to prevent leachates from landfills seeping into the groundwater system and as sealant in tailings dam. GCLs are thin sheets of bentonite encased between two geotextiles. The main component of GCLs is bentonite, which formed as an alteration product of volcanic ash, comprised mainly of montmorillonite. Bentonite exists as either sodium bentonite or calcium bentonite, depending on the type of montmorillonite present within it. To perform as an effective hydraulic barrier, sodium bentonite is the preferred type due to its high-water retention characteristics and swelling potential. Often times, the bentonite does not meet the desired swell index. As such, a process known as activation is undertaken, whereby soda ash (Na2CO3) is mixed with borderline quality bentonite. This study investigated the suitability of bentonites from the Imerys mine in the Western Cape Province of South Africa for use in GCLs. In this research, both activated and non-activated bentonites were investigated. X-Ray diffraction analysis was conducted on the bentonites in order to determine their bulk mineralogical composition. Swell index test, fluid loss test, plate water absorption tests and Atterberg limits test were also conducted on samples of activated and non-activated bentonites. Furthermore, swell index tests were conducted to investigate the extent of beneficiation over time. The XRD results reveal that activated and non-activated bentonite have a smectite content of approximately 58 % and 67 % respectively with the major impurity being quartz. The swell index of non-activated bentonite was significantly lower than the activated bentonite. However, the activated bentonite did not swell to the required minimum of 24 ml/2g as it did not achieve full activation. Activated samples of bentonite tested at different times subsequent to activation reveal that the activation requires at least 4 weeks for the ideal ratio of 1:50, soda ash to bentonite, to fully activate. The fluid loss results also displayed results slightly above the required minimum, of 18 ml, as a result of the low swell index. Activated and non-activated bentonite has an absorption capacity of approximately 133 % and 121 %. The plasticity index is 101 % for activated bentonite, 15 % higher than non-activated bentonite. Moreover, a moderate correlation between plate water absorption and liquid limit was found for activated bentonite. The activated bentonite from Western Cape is suitable for use in GCLs provided the blend of bentonite is not of very low quality and sufficient time is given for activation to reach completion. Imerys bentonite is a medium quality bentonite with borderline index properties that requires beneficiation and time to achieve complete activation.