Yes indeed. Bentonite clay is very interesting stuff. But there
is one caveat for using it as a grounding electrode backfill. It
does swell when it is moistened. But it also contracts when it
dries out. For those of us who live in a dry climate, this means
that the fill can shrink away from both the hole and the
conductor. This is not a good thing.
There is a mix of chemicals to be used for grounding electrode
backfill that avoids this problem for the most part. I believe
it is even available as premix in bags. But unfortunately, I
don't have the proper name (yet) to search for. So, this week
when I did the earth terminals for my new QTH, I had the drilling
company buy the separate components and we mixed them on site.
The formula is:
75% Gypsum
20% Bentonite Clay
5% Sodium Sulfate
I believe this is available from galvanic protection distributors
and is known as "standard galvanic anode backfill". I'll know
for sure in a day or two. I'm tired of trying to mix these
powders on a windy site and we'll be needing a lot more for the
conductor trench. If I locate a trade name, I'll post it here.
Anyhow, it works this way... The gypsum absorbs and retains
water. So the moisture level seen by the bentonite in the mix
does not vary enough to cause the shrinkage problems. The Gypsum
has very little expansion or shrinkage due to external moisture
level variations and it improves the conductivity of the mix.
There is enough Bentonite so that the expansion of the Bentonite
still operates to pressurize the electrode and hole walls for
good contact. For this to work, the mix must be put in dry and
then have moisture added. The sodium sulfite is probably not
strictly necessary in a system with no continuous source of DC
current for the earth terminal. But it helps prevent polarizing
the earth terminal if there is some DC flow.
73, Eric N7CL
>From: "Hsu, Aaron" <aaron.hsu@unistudios.com>
To: <towertalk@contesting.com>
>Date: Wed, 7 Oct 1998 15:47:31 -0700
>
>> Guy Atkins wrote...
>>
>> From: Guy Atkins[SMTP:4nradio@accessone.com]
>> Sent: Wednesday, October 07, 1998 1:19 PM
>> Subject: [Antennas] Sources for Bentonite?
>>
>> Can anyone tell me the kind of store or supply house where I
>> might find the mineral bentonite, used to enhance ground
>> conductivity (i.e. around ground rods)? Some years ago I
>> visited a refining plant in Wyoming where bentonite is mined,
>> and once brought back 30-50 lbs. of it (I was given a torn bag
>> of it for free). Now I'm in need of more bentonite, but I
>> don't plan a trip to Wyoming any time soon!
>>
>> This is excellent stuff: very conductive, non-corrosive to
>> ground rods, and swells to make an extremely tight fit between
>> the earth and the ground rod(s) in an antenna system. Now, if
>> I can only find some more of it! Any suggestions? Thanks.
>>
>>
>> OK, this message intrigued me and I wondered, "what's
>> Bentonite?" A search on HOTBOT (http://www.hotbot.com) came
>> up with over 5000 hits. Going through the first few yielded
>> info and suppliers. Canadian Clay Products, Inc. had the
>> specific answer I was looking for...what is it? The
>> description is at the bottom.
>>
>> Most of the world's Bentonite comes from Wyoming and So
>> Dakota. I found several uses for it - land fill, "Galvanic
>> Anode Cathodic Protection" for guy wires, intestinal tract
>> cleansing, gound sealing and grout, waste water treatment,
>> livestock feed additive, beer and wine clairifier (during
>> brewing/aging), drilling mud, geosynthetics, and many more.
>> Is there anything this stuff doesn't do?
>>
>> ---------------
>> from Canadian Clay Products, Inc. website...
>>
>> What is Bentonite?
>> Bentonite is a clay mineral which is largely composed of
>> montmorillonite, which is mainly a hydrous aluminum
>> silicate. It is a highly colloidal and plastic clay with the
>> unique characteristic of swelling to several times its
>> original volume when placed in water. Bentonite was formed
>> from volcanic ash deposited in an ancient sea, and modified by
>> geological process into the present Sodium Bentonite. The
>> Bentonites were calculated to have accumulated between 74.5
>> and 70 million years ago. At that time, the Wilcox area was
>> the center of a huge shallow inland sea which stretched from
>> the Arctic Ocean to present day Mexico and was at least 1600
>> kilometers wide.
>>
>> Bentonite has been called the clay of a thousand uses. Sodium
>> Bentonite is noted for its affinity for water which gives it
>> tremendous selling properties. Soduim Bentonite contains
>> exchangeable sodium cations. When dispersed in water it breaks
>> down into small plate-like particles negatively charged on the
>> surface, positively charged on the edges. This unique ion
>> exchange is responsible for the binding action which takes
>> place. Bentonite's small plate-like particles provide a
>> tremendous potential for surface area. It forms thixotropic
>> gels with water even when the amount of Bentonite in such gels
>> is relatively small. These characteristics give Bentonite an
>> enormous range of potential uses.
>> -------------------------
>>
>> ...and it does seem to have an ENORMOUS range of uses!
>> Interesting stuff.
>>
>> - Aaron Hsu
>> athsu@unistudios.com
>> dae@pacbell.net
>> No-QRO Int'l #1,000,006
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