ABSTRACT
Bentonite, a weathered volcanic ash, is predominantly smectite minerals (montmorillonite typically constituting 75-90% of the mass of the bentonite) with exchange sites primarily occupied with sodium or calcium ions (see Section 3.4). Typically, natural sodium bentonite is used although "sodium-activated" calcium bentonite (in which the exchangeable calcium cations have been replaced with sodium ions to increase the swell capability and decrease the hydraulic conductivity of the bentonite in the GCL) also have been used. The other constituents of the bentonite may include quartz, carbonates (calcite, dolomite, siderite), feldspars, mica, organic matter, illite, kaolinite, chlorite, zeolites, volcanic glass, apatite, haematite, limonite, and heavy minerals such as pyrite, magnetite and zircon. The typical distribution of cations in sodium bentonite used for GCLs is (Egloffstein, 2001): 50-90% Na+, 5-25% Ca2+, 3-15% Mg2+, 0.1-0.5% K+. Calcium bentonite has also been used in GCLs but since the hydraulic conductivity of calcium bentonite is about one order of magnitude higher than that for sodium bentonite it is not com-
monly specified. Where calcium bentonite is used, the resulting GCL will typically have a much larger mass per unit area than a GCL manufactured from sodium bentonite.
