Thank you for your question regarding the use of back-fill materials to reduce the resistance-to-ground of an earthing electrode, it is our pleasure to help.
The two types of back-fill material you mentioned are excellent examples of the disparity between the current available ground enhancement back-fills.
The first material you mentioned was GEM, while we have no specific knowledge of this particular brand of back-fill, it does fall into the category of carbon-based back-fills. Carbon-based ground enhancement back-fills have both a good and a bad side to them. The good side is that they are relatively inexpensive and they do have very low resistivities. Typically, these materials have resistivities that are well below 1-ohm meters, sometimes as low as 0.1 ohm meters. The bad side is that the metallurgical nobility of carbon is such that it destroys copper over time. Typically, after 5 years copper electrodes have been corroded and destroyed by the carbon. There are also potential environmental concerns with the use of carbon-based back-fills. As such, E&S Grounding Solutions does not currently recommend the use of carbon-based ground enhancement materials.
The other back-fill material you mentioned is bentonite clay. Bentonite clay is a natural earth soil (clay) that is simply mined from areas like Wyoming. It has almost no environmental concerns and will not corrode the copper. In fact, bentonite clay is protective of the copper. The down-side of bentonite clay is that it is has a resistivity of around 2-ohm meters, and needs water to stay conductive. Very dry soils may require the use of watering devices when using bentonite clay. However, the upsides of bentonite clay far out way its downsides, and as such it is the only back-fill material that E&S Grounding Solutions currently recommends for grounding systems.
Now in regards to your question about reducing the earth resistance of your electrode from 18 to under 5 ohms, it is highly unlikely that any ground enhancement material will achieve such a result. Ground enhancement materials will typically only improve your electrodes resistance-to-ground by a few percentage points, maybe 10% to 20% at best.
Generally speaking, if your electrode is currently measuring 18-ohms, than an additional electrode of the same specifications will also measure 18-ohms. Assuming you install the second 18-ohm electrode at least 2x the diagonal length away from the first electrode, you should have an electrode system that will measure 9 ohms. If you install a total of three (3) more of these 18-ohms electrodes, each at least 2x the diagonal length away from each other, for a total of four (4) electrodes, you should have a system that is just under 5-ohms.
Obviously, four (4) electrode systems can be very expensive and you may not have the physical room to install them at the spacing you need. We would recommend that you get an electrode system designed. E&S Grounding Solutions would be happy to help you with this process. All it takes is for you to email us your soil resistivity data and a site plan, and we can take care of the rest. If this is something you would like us to do for you, please email Michael and firstname.lastname@example.org
But if not us, please get someone to help you with this important design process.
We hope you found this information useful, if you should have any questions, please do not hesitate to contact us again in the future.
The Engineering Team at E&S Grounding Solutions
Photo credit: E&S Grounding Solutions