81    SODIUM BENTONITE GEOCOMPOSITE LINERS:
PROPERTIES AND APPLICATIONS
Robert Trauger, Technical Services Manager
Colloid Environmental Technologies Company
Arlington Heights, Illinois 60004
INTRODUCTION
Because of its unique chemical and physical properties, sodium bentonite has long been used as an
additive to natural site soils for the construction of low-permeability liner systems. More recently, the
explosive development and use of geotextile materials has led to unique geocomposites, wherein two
or more geotextiles are united into one product. One of the most exciting developments in this field
involves the incorporation of sodium bentonite within a thin geocomposite, thus forming versatile,
high-performance liner products. Four such products are currently available, yet each has very
different components and characteristics. These products have generated substantial interest in the
engineering and regulatory communities and have been accepted for use in a variety of lining projects.
However, additional experience and research are still required for a more complete evaluation of
bentonite geocomposite liners. This paper summarizes the available research results, discusses applications and installation techniques, and provides a general assessment of the current state of development of bentonite geocomposite liners.
ORIGIN OF BENTONITE
The physical and chemical properties of sodium bentonite result directly from the unique geological
conditions responsible for its formation millions of years ago. The most widely known and commercially exploited sodium bentonite deposits are found in what is now South Dakota, Wyoming, and
Montana. The bentonite in these areas originated through the aqueous deposition and subsequent
weathering of huge amounts of volcanic ash. Eons ago, much of the western United States was
covered by a salten sea upon which the ash landed and then sank in a broad, uniform layer. Later,
mammoth upheavals in the region pushed these deposits to the surface and exposed them to the
weathering action of wind and rain. In time, the ash particles were modified from their original
igneous structure into the unique clay structure of montmorillonite, or bentonite.1
Montmorillonite is a species of the smectite clay mineral group. Montmorillonites are generally
classified by their dominant exchangeable cation, either sodium or calcium, but the ratio of exchangeable sodium to calcium may vary considerably.2 Although the terms montmorillonite and bentonite
are often used interchangeably, the term bentonite is formally applied to montmorillonites of volcanic
origin. Small deposits of montmorillonite were also formed by hydrothermal activity or by groundwater alteration of igneous rock, but the commercial deposits in the United States were form from the
deposition of volcanic ash.3 The three U.S.-manufactured geocomposite liners all contain sodium
bentonite mined from these deposits. Outside the United States, the vast majority of montmorillonite
deposits are of the calcium variety, which has substantially different properties than the sodium
variety. Specially treated calcium bentonite is used in the European-manufactured bentonite
geocomposite.
DESIRABLE ENGINEERING PROPERTIES OF BENTONITE
Sodium bentonite has several properties which make it desirable for use in environmental engineering applications. Most of these properties are due to the presence of exchangeable cations between
individual bentonite crystals (Figure 1). Interlamellar sodium, calcium, and magnesium ions provide
weak electrostatic bonds to hold the negatively charged outer surfaces of the crystals together, and the
presence of these ions is also critical in determining the swell capacity of the bentonite. When
monovalent sodium predominates, several layers of water molecules can be adsorbed in the interlamellar space between the bentonite crystals without destroying the electrostatic bonds. Although not
46lh Purdue Industrial Waste Conference Proceedings, 1992 Lewis Publishers, Inc., Chelsea, Michigan 48118.
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