Technical Information » What is Bentonite?
RAWMAT® HDB pre-hydrated sodium bentonite Geosynthetic Clay Liners (GCLs) are made from pure Wyoming Sodium Bentonite, for the highest swelling ability and lowest permeability values possible.
Bentonite is a natural inert clay that was formed from volcanic activity during the Cretaceous period approximately 100 million years ago. Long periods of repeated eruptions laid ash into the sea where it was chemically altered and consolidated into layers of clay. Over time the ground folded and lifted thrusting up the clay and silt to form the Black Hills and Big Horn mountains of Wyoming, USA. The term bentonite is generally applied to colloidal clay associated with the Cretaceous Benton shale found near Fort Benton. Bentonite mining first began in 1888.
- Figure 1. A typical seam of bentonite in Wyoming.
The first recorded use of the mineral known as “the clay of a thousand uses” was for making cosmetics, it then became used as a foundry sand bond in the 1920s followed soon after as a drilling mud. The uses have continued to expand into the fields of bleaching clay, animal feeds, pharmaceuticals, colloidal fillers for paints and inks, ceramics and in the motor industry for spark plugs and catalytic converters.
Bentonite was formed all over the world during the Cretaceous periods, these bentonites are categorised into sodium and calcium bentonites (calcium varieties are commonly found in many areas of all continents). When bentonite comes into contact with water, the atoms and molecules dissolve and ions with negative charges develop. These negative charges cause a mutual repulsion and thus a swelling. Calcium bentonites are known as low swelling bentonite due to their swell capacity of only 3 times their own dry volume.
- Figure 2. The three sheet structure of a particle of bentonite.
These minerals are divalent in their formulation and as such they are stable. In sodium based bentonites the clay absorbs nearly 5 times its weight of water. At full saturation it can therefore swell to 16 times its own dry volume. The sodium bentonites are monovalent and as such unstable.
Sodium Bentonite consists of at least 90% montmorillonite and 10% other minerals, the most abundant being feldspar, quartz, calcite and gypsum. The montmorillonite is a three sheet structure mineral formed of several layers of tetrahedron and octahedron sheets, these sheet are parallel orientated, on the surface of the sheets there are atoms and molecules. When the sodium bentonite swells the charges cause the bentonite platelets to bond together and form a seal that acts as a very high quality waterproof barrier. The swelling occurs as the water enters the platelets, if the water contains "impurities" these are taken into the inter layers between the platelets where they can create a change in the formulation of the clay causing it to convert or exchange.
This cation exchange causes the high swelling unstable sodium bentonite to convert to a low swelling stable calcium bentonite. The effect this has is to cause the bonds between the clay particles to break as the clay shrinks back when it exchanges causing fissures or cracks to appear in the surface of the clay, these cracks allow the passage of water through clay and causing the barrier to leak.
Cracking and shrinking can be induced by ground contaminates and are well known results of failure in dry bentonite systems. An explanation of the term "contaminants" is required - this does not necessarily mean conditions requiring health and safety issues, this actually means naturally occurring divalent or trivalent ions examples of these include calcium, potassium, magnesium, hydrogen and iron. These can be found in hard waters, saline waters, calciferous soils and organic compounds. In addition these can be introduced in the form of fertilizers, hydrocarbons and brownfield sites. An informative paper by Professor Chitral Wijeyesekera which looks at the Cation Exchange Capacity (CEC) of bentonites is available from Rawell's Technical Department.
To prevent this ionic exchange with its products occurring, Rawell has developed a revolutionary waterproofing material by using its own patented process to polymerise and pre-hydrate the bentonite. The sodium bentonite in RAWMAT® HDB membranes and RAWSEAL® Waterstops is pre-hydrated with a patented polymer solution which provides a permanent protection to the ionic fingerprint of the clay protecting it from exchange and subsequent breakdown in ground contaminants, a major problem associated with other bentonite systems. To confirm the performance of the pre-hydrated RAWMAT® HDB membrane Rawell has undertaken an extensive testing regime to confirm the product can withstand a wide variety of liquids and contaminants. These tests use typical testing protocol to comply with worldwide standards, however Rawell have developed a method of testing which allows the tests to be continued beyond the 14 day period which is adopted in the standard testing procedure and have tests which have been running successfully for in excess of 10 years. In addition to these permeability tests the pre-hydrated bentonite membrane has been tested as a gas membrane with testing reports on Radon and Nitrogen.
| Permeability of RAWMAT® HDB using various permeants using ASTM D-5887
(the tests are modified to use the permeant as the hydrating fluid) |
||
|---|---|---|
| Chemical | Concentration | Result (K - m/s) |
| Tap Water | - | 5.0 x 10e-11 |
| Synthetic Saline Water | 100,000ppm | 2.9 x 10e-12 |
| Synthetic Leachate | * | 9.0 x 10e-12 |
| Potassium Acetate | 5% | 3.4 x 10e-13 |
| Ethylene Glycol | 30g/l | 4.6 x 10e-13 |
| Diesel | - | 2.8 x 10e-13 |
| Unleaded Petrol | - | 2.1 x 10e-12 |
| Hydrochloric Acid (HCI) | 10% | 1.1 x 10e-11 |
| River Mersey Sea Water | 20,000ppm | 4.7 x 10e-12 |
| Spent Transformer Oil | - | 7.9 x 10e-13 |
Using the testing cells developed by Rawell we have been able to run non-destructive long term testing to predict the expected life of the membrane with various permeants, using the results gained from the tests carried out we have been able to extrapolate the results to predict the performance of the material after 100 years. Based on these results it is anticipated that the permeability of RAWMAT® HDB using Municipal Solid Waste Landfill Leachate after 100 years exposure will be 5.2 x 10-10 m/s.
- Figure 3. Long Term Hydraulic Conductivity Test Results on RAWMAT® HDB - Municipal Solid Waste Leachate
Site Search
Advertisement
All information contained within this site is given in good faith and is correct to the best of Rawell Environmental Ltd.'s knowledge. However, this information is not legally binding and may be changed at any time without notice. Rawell offer a free advisory service, please Contact Us for more details. RAWMAT® and RAWSEAL® are registered trademarks of Rawell Environmental Ltd.