Making dome with perlite and concrete

[Biggin0612]

Hello everyone, I am getting ready to build my dome and with all of the research I have found it says to build my dome using an excercise ball forming the cement and perlite mixture around it. My problem is finding cement, it seems like noone has any Portland white and all my local store has is Portland Limestone type II cement and rapid hardening cement. Will either one of these work to set my dome?

[Sixto]

You don't need White Portland cement, ($$$) unless you want that look. Gray Portland cement works just fine ad is less costly. Look for a local Masonry supply house, typically the staff is more knowledgeable than a big box store.

Also perlite is usually added as a separate layer outside the blanket insulation, as an additional insulating layer and also to get a more solid and rounded substrate for the final plaster or render layer. Your don't want to add perlite to your dome refractory casting because the purpose of the castable refractory cement is to store heat in its mass, and perlite both weakens and reduces mass of the dome.

Look at this thread for a PDF recipe for homemade castable refractory....https://community.fornobravo.com/for...-homebrew-dome

Also look at this thread for a process similar to what I think you're doing.... https://community.fornobravo.com/for...able-wfo-build

[Biggin0612]

Sixto,
thank you for the info. one question though... i am not doing the dome with fire brick, just the floor, will the refractory work just fine if i cast it by itself ( like what i was going to do mixing the perlite with cement to make the inner wall of the dome ).

[Sixto]

If "by itself" you mean no perlite, that's my recommendation. If you mean Cement only - I assume then you are buying a proprietary refractory castable powder (which contains all the necessary ingredients) instead of using the homebrew recipe discussed in the links i added on post #2, which have Sand, Lime, and powdered FireClay in addition to Portland Cement, and often add fibers (Stainless steel needles and/or Alkaline-Resistant Glass Fibers) to strengthen the dome and resist cracking.

In either case, insulating above and below is important, and that's where I would say the perlite goes. Note that if using perlite below the bricks, you'd use a 5:1 (Perlite to Portland cement by volume) ratio for strength, to hold up the dome...but when using perlite around the outside of the dome, the ratio changes to 10:1 for additional insulation value since that is typically covered by plaster and does not have to be as strong.

[Biggin0612]

The information you have given is very beneficial and I thank you very much

[david s]

The perlite/cement mix cast over a gym ball seems like a simple idea, but is fraught with failure. Its popularity derives from people doing research via YouTube. Not only does it require moving into position once cast, it also requires some sort of formwork because you only require the top half of the ball, As the material for the inner oven a mixture of Portland cement and perlite is unsuitable from a number of angles. Firstl, perlite’s inclusion in a mix reduces conductivity but increases insulation. This means the material is both slow to heat and produces an inability to store heat because of its low thermal mass. Secondly the perlite in the mix drastically reduces the strength of the mix once it has set. Such a dome made from this material will be very vulnerable to bumps and abrasions resulting in bits falling off.
If the only cementious material binding the mix together is Portland cement there is a further problem in that temperatures higher than 300C begin to degrade Portland cement. The inside surface of such a mix will easily see these higher temperatures because of flame impingement on the dome.
You may get a number of successful cookies with such a set up but it certainly won’t last. We’ve invited a number of times, folk with perlite domes, to report on their longevity. So far none have, I suspect they’ve all failed. Check out the homebrew cat ovens, cast in situ over a sand mould, in the other ovens section on this forum.

[Biggin0612]

david s,
Thank you for your input, I too have been one of the youtube researchers, but unlike many of them I have been looking further into it as I do not want this to fail and wanted to hear from folks that have tried, tested and succeeded. I have also been looking into the refractory cement and building out of fire bricks, just haven't figured yet how i can make the dome shape with the bricks. If i were to lean towards building with bricks and then insulating would it have to be in the shape of a dome in order to be efficient or would a square oven do just as well?

[david s]

The hemisphere with a front cross draft flue has proved oven centuries to be the best all round design. Because a hemisphere is a self supporting structure it requires no additional bracing or buttressing. Departure from it as in the flatter Neopolitan lizza ovens require bracing at the base to contain the outward thrust on that area. The hemisphere also has the advantage of being a very efficient combustion chamber with even circulation and excellent fuel economy. (Think hemi cylinder heads for cars) cubic chambers may be easier to build, but result in poorer circulation, cool spots and inherent structural weakness. Many first time builders opt for half barrel designs because of the perceived simplicity of construction. However, because of their need to include bracing or buttressing to counter side thrust at the base, this design becomes more difficult than the dome.

[Biggin0612]

david s,
thank you again, I have been doing a bunch of reading throughout these forums, and your name pops up a lot . As I am now seeing this will be a bit more of detailed build than I originally thought it would be, but I am SOOOO up for the challenge. A pizza oven will complete my outdoor ( I like to call it ) restaurant. I have a flat top, hog rotisserie, large double burner gas top, and smoker grill. My wife and I love to cook and entertain and this oven will be the perfect final puzzle piece. I am sure I will have tons more questions and will post my progress, all I have right now is my 4'x4.5'x4" slab done and letting that cure a few more weeks before I build the sides and top slab.