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Durability of burn chamber with perlite or vermiculite?  RSS feed

 
Posts: 6
Location: detroit
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Tinkering with ideas for my first RMH.  Would a burn chamber cast from a mix of vermiculite (or perlite) and refractory cement hold up?  Thanks in advance for any input.
 
gardener
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Perlite, vermiculite doesn't hold.

But casting mortars are usually ready made. You can't add  anything. And far more long lasting than our diy mixes.

Jmo.
 
pollinator
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Must say that I used to make my own 'insulating' firebricks from a mix of loose vermiculite and high alumina cement and they worked very well in various woodstoves.

As an experiment I lined the burn chamber of my workshop 'J' tube rocket and they have also stood up to the crazy high temperatures very well. Maybe slightly more fragile than the commercially produced vermiculite boards but also a lot cheaper...
 
Gregg Brazel
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Location: detroit
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Thanks for the input.  I think I'll give the homemade chamber a go as I like working with concrete.  I plan to make the stove body out of precast slabs that can be disassembled, so if anything fails internally I won't be sol.
 
pollinator
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It works, look up Matt Walkers work on YouTube or over at Donkeys rocket stove forums.
Matt tried to use a minimum of refractory cement,for reasons of cost.
I have had some luck using RapidSet, an alumina based cement available at home depot.
I replace the usual portion of aggregate with perlite,and add Roxul stone wool insulation fibers for crack resistance.
My results are quite durable, but perhaps too dense.
 
Gregg Brazel
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Thanks for the info, William.
 
Posts: 39
Location: North Alabama
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I don't have any experience with pre-mix refractory mortar, but vermiculite with Calcium Aluminate works pretty well. I can dig up the recipe if you need it. I built a J tube from it and although it developed a couple of cracks from shrinkage in the beginning, they have not progressed or caused any problems. I used the stainless needles for crack control and other than the two cracks mentioned, the stove has held up very well to about 50 or so very hot firings, hot enough to glow pink hot in the feed tube. It really holds the heat in the fire path and insulates well enough that at 2" thick it is only about 200 degrees on the outside around the combustion critical portion of the stove.

A number of people don't believe you can use vermiculite in this type of fabrication. Some folks however don't know the difference between agricultural vermiculite and concrete aggregate vermiculite, night and day difference when it comes to mixing it with any kind of cement, and while it's true the agricultural stuff won't work, the aggregate works just fine. Vermiculite is made from a type of feldspar which is actually very good for high temp construction, it is chemically similar to the clays used for ceramics.
 
Gregg Brazel
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Location: detroit
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Thanks for the info, Duane.  I didn't know the dif between ag and construction grade.  I'll have to look for the latter.  I found a study about vermiculite having good properties for high heat applications, but the full text was a pdf for purchase which I didn't buy. 
 
John Harrison
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This is the stuff I used in the UK Vermiculite Insulation

I mixed it dry with high alumina cement and then added enough water to make a trowelable consistency. I made the 'bricks' in shallow moulds, pressing the mixture down slightly with a plasterers float. Before it all dried I added a thin (2mm or so) almost slurry like layer of high alumina cement/water mix to the top surface which helps to give the bricks a bit more resilience to damage without adding too much mass.

Some of the bricks have been in regular use for over 7 years in some of our woodstoves and are only now showing signs of significant damage.
 
Gregg Brazel
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Location: detroit
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Thanks for the input, John. Will def keep that in mind.

And hat tip to William for leading me to Walker Stoves.  I might use one as the core depending on price. Great ideas in any case.
 
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Hi all,
So this is a little off the wall, but I was thinking about these high temperature burn chamber materials currently being used and I don't think they are quite there yet because the operating temperatures are on the upper threshold for these commercially available materials.

The burn chamber operating temperature is closer to the combustion chamber of a rocket engine than it is a furnace. So I got to thinking about materials that have a really high operating temperature - like certain ceramics.

I did some digging and noticed that modern high temperature ceramic technology has a very close relationship to a slightly different material - synthetic gemstones.

For example, synthetic ruby is made by chemically bonding Aluminum Oxide to a small amount of Chromium Dioxide at a temperature of around 2,000 Celsius. So what if the burn chamber was made from ruby? How would that be achieved?

The first task would be sourcing an affordable bulk quantity of Aluminum Oxide. It happens to be used as an industrial blasting medium and can be purchased in 50lb bags for about $100. So far so good.

The next challenge is finding a binding agent to mix with the dry materials so that it can be poured into a mold, and burns off cleanly during the firing process. After a bit of reading, propylene carbonate is a good candidate, and is used in all sorts of things like cosmetics.
The final mixture needs to be about 2-3% by weight for effective binding, and it is soluble in acetone. A slightly cheaper option is ethylene carbonate.

So we have a ingredients for synthetic ruby in bulk quantity. The challenge now is curing it at such high temperatures. This is normally done in a special kiln, but seeing that these chambers and risers are so big, maybe it's better to make the mold as the kiln. We know that portland cement turns to dust at these high temperatures, and I'm wondering if it would be a great material for making the casting mold? If the mold is constructed as a j-tube for example, then you could incorporate it into a rocket forge type of set up, and deliberately burn the chamber out by using forced air to increase the chamber temperature. As the cement chamber disintegrates, the internal ceramic chamber cures.

Does this sound crazy?
 
Satamax Antone
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Yes, because you have forgotten that melting point of ruby is 2050C°

https://www.azom.com/article.aspx?ArticleID=1948

And that some more practical, tho costly refractory pourable mixes can reach a good 2600C°


But they're most certainly less costly than artificial ruby.
 
Nick Kitchener
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Satamax Antone wrote:Yes, because you have forgotten that melting point of ruby is 2050C°

https://www.azom.com/article.aspx?ArticleID=1948

And that some more practical, tho costly refractory pourable mixes can reach a good 2600C°


But they're most certainly less costly than artificial ruby.



I have not seen any commercial refactory rated to 2,600 degree C. There are some rated to 2,600 F, which is less than 1,500 C. Using it in a burn chamber is pushing the operational spec...

As far as ruby is concerned, I said it has a melting temp of approx 2,000 C, and the exact melting point as you pointed out is indeed approximately 2,000 degrees. The challenge would be reaching and maintaining such a high operational temperature necessary for curing this refactory material. This is why I think a sacrificial mold might be a possibility.

If anyone knows of a commercial refactory with an operating temperature of 2,000 C (3,600 F) then please post a link
 
Satamax Antone
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Duane Hylton
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Well it is not a commercial product per se, but here is the patent number for some geopolymer recipes that will certainly fit the bill. I use example 3 with enough vermiculite to make it just firm enough to pack well. It has withstood being fired in my j tube with no signs of cracking or degradation. In fact until I fired it hot there was some effloresence that went away after getting red hot.

patent no. 4,472,199
 
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