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tdkamila 04-11-2010 02:39 AM

Silicone in pearlite
Why is it important to use pearlite without silicone for pearlite cement insulation? If you look in the Pompeii Oven instructions under Hints and Tips They caution against using pearlite with silicone added and recommend using "horticultural grade pearlite". Can someone explain why? I can source the masonry grade with silicone easily. Getting the other requires me to order and ship.

Thanks Tom

RTflorida 04-11-2010 12:45 PM

Re: Silicone in pearlite
The thought is the portland cement will not bond with the perlite (or vermiculite) due to the silicone. Since the mixture inherently has a somewhat crumbly texture when hardened, you wouldn't want something in it to compound the crumbly nature.

Not being a chemist or engineer and having not tried the silicone perlite, I have no idea if the silicone actually effects the bonding or not; but it certainly makes sense that it could.


david s 04-11-2010 02:05 PM

Re: Silicone in pearlite
Perhaps the silicon addition for masonary grade perlite is an attempt to prevent extra absorption of water into the perlite grains.

dmun 04-11-2010 05:43 PM

Re: Silicone in pearlite
I believe that someone used the stuff successfully: Why don't you ask your vendor for a sample (explaining the problem) and do a test. After a week it should be hard, the consistency of cork, and only a little crumbly around the edges.

kebwi 04-11-2010 07:55 PM

Re: Silicone in pearlite
Can't speak for the silicon perlite, I only used horiticultural, but I also used attic-insulation (silicon) vermiculite and didn't have too much trouble. It is true that while mixing it up with portland and water sometimes the portland would slide off exposing the golden vermiculite, but I just mixed it up more thoroughly and stopped messing with it.

Seemed to work fine.

My biggest problem with perlite was that even the most modest mixing rapidly destroyed the perlite, rendering it as fine dust. The vermiculite stood up to the abuse of mixing much better...and I mixed very gently: giant wheelbarrow, long gloves, slowly churning the mixture up to my elbows without any tools. That's as gentle as it gets.

I would go vermiculite for that reason alone. Silicon or not doesn't seem to be an important deciding factor to me.


Tscarborough 04-11-2010 07:57 PM

Re: Silicone in pearlite
It doesn't matter if it turns to dust, the essential property is the same. Siliconized perlite is useless for gardening, but it doesn't really matter for this application.

david s 04-11-2010 10:52 PM

Re: Silicone in pearlite
Surely if the grains break down to dust you will end up with an increase in density, because each grain is filled with a lot of air. Certainly when mixing it and this happens you end up with a reduced volume which is in turn an increase in density. This is the main reason I prefer to use vermiculite rather than perlite because it doesn't break down as easily and does not produce that irritating dust.

tdkamila 04-12-2010 02:32 AM

Re: Silicone in pearlite
Thanks for the replies. This will be the second oven I built. I work at a Vocational school in Central Mass. and I am designing this one for the students to construct at school. I would like to get the materials from one local source to make it easier for the future owner. On my oven I contained the concrete/pearlite mix within the hearth by pouring the hearth with a circular form two inches off the bottom the diameter of the dome plus insulation. It worked fine but I ordered the horticultural pearlite on line. It is much easier to have the future owner set up an account at a local masonry supply than have to depend on them to order something on line and then waiting for a delivery. It makes scheduling the project easier on our part.
Unless someone has firsthand knowledge I guess this may be a non starter. By the way what happens to the silicone when it reaches a temperature of one thousand degrees? Any chemists out there?

Thanks Tom

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