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Table 10. Characteristics of bentonite and cement as annular seals <br />[Information from ASTM (1992), Aller and others (1989), Hardy and others (1989), Driscoll (1986), <br />Gillham and others (1983), and Claassen (1982)] <br />BENTONITE <br />(A hydrous aluminum silicate composed primarily of montmorillonite) <br />Advantages: <br />- Readily available and inexpensive. <br />- Pellets and granules are easy to use. <br />- Remains plastic and will not crack if it remains saturated. <br />- Expands from 10 to 15 times dry volume when hydrated. <br />- Low hydraulic conductivity (about 1 x 10-7 to 1 x 10-9 centimeters per second) <br />Disadvantages: <br />- Effectiveness of seal difficult to assess. <br />- Complete bond to casing not assured. <br />- Because of rapid hydration, bentonite can stick to walls of annulus and bridge annulus. <br />- May not be an effective seal in unsaturated zone because of desiccation. <br />- Can affect the chemistry of the surrounding ground water by cation exchange of Na, Al, <br />K, Mg, Ca, Fe, and Mn from the bentonite with other cations in the ground water. <br />- Sets up with a pH between 8.5 and 10.5, which can affect the chemistry of the <br />surrounding ground water. <br />- Most bentonites contain about 4-6 percent organic matter, which might affect the <br />concentration of some organic constituents in ground water. <br />CEMENT <br />(Composed of calcium carbonate, alumina, silica, magnesia, ferric oxide, and sulfur trioxide <br />with pH ranges from 10 to 12) <br />Advantages: <br />- Readily available and inexpensive. <br />- Can assess continuity of placement using temperature or acoustic-bond logs. <br />Disadvantages: <br />- Requires mixer, pump, and tremie pipe for placement. <br />- Generally more cleanup required than with bentonite. <br />- Contamination can be introduced to borehole by the pump. <br />- Failure of the grout to form a seal can occur because of premature and/or partial setting <br />of the cement, insufficient grout column length, voids and/or gaps in the grout <br />column, or excessive shrinkage of the cement. <br />- Pure cement will shrink during the curing process, resulting in a poor seal between the <br />cement and both the casing and the borehole wall. <br />- Additives to the cement to compensate for natural shrinkage can cause an increase in <br />pH, dissolved solids, and temperature of the ground water during the curing <br />process. The increased pH causes precipitation of calcium and bicarbonate ions <br />from the ground water. <br />- Soluble salts in the cement can be leached by the ground water, thereby increasing the <br />concentrations of calcium and bicarbonate in the ground water. <br />- Cement may cause unusually high values of pH in ground-water-quality samples. <br />- Heat of hydration during curing can deform or melt thermoplastic casing such as PVC. <br />38