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Perlite Concrete on free form igloo - Forno Bravo Forum: The Wood-Fired Oven Community

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Perlite Concrete on free form igloo

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  • Perlite Concrete on free form igloo

    Has anyone used lime in the perlite concrete to make it stickier. I am have a time getting it thick enough on the near vertical surfaces?

  • #2
    Re: Perlite Concrete on free form igloo

    What kind of perlite/cement ratio are you using? Is it too wet (or too dry) maybe? What kind of structure are you putting it around - some chicken wire or other reinforcement?

    I've only setup my base insulation layer with perlite/cement, but I know that my first attempt would have been too dry and crumbly to stand up vertically. This weekend I did an additional layer on top and felt like I got the water content right.

    The only other suggestion I've heard is to start at the bottom and work your way up.
    Pizza Oven Picture Gallery
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    • #3
      Re: Perlite Concrete on free form igloo

      My mix is 5 perlite to 1 portland cement. It is placed on expanded metal lathe which I have covering 1 to 2 inches of mineral wool. I have been working from the bottom up and eventually I think I can be successful. I was just wondering if a little lime in the mix would make it stick together better?
      Attached Files

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      • #4
        Re: Perlite Concrete on free form igloo

        Some lime might help, but make sure you use rubber gloves. I think a 5:1 is way too rich. You are adding much more weight in cement than perlite which will make you mixture denser and a poorer insulator. I use 10:1 and add 3 litres of water for every 10 litres of vermiculite (perlite probably the same) The mixture will seem too dry and crumbly when you apply it but it hardens up ok and is left with lots of air spaces between the grains, which is what you want. This layer does not need to be strong, merely enough to hold it together so you can apply your outer shell over it. Let it dry well, cure then do the outer shell.
        Kindled with zeal and fired with passion.

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        • #5
          Re: Perlite Concrete on free form igloo

          I wouldn't use the lime. Use a 10:1 or 12:1 mix. You have to play around with the water ratio. You may find it easier to do in several courses, going up 6 inches or so and letting it set for 3 or 4 hours before placing the remained.

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          • #6
            Re: Perlite Concrete on free form igloo

            I started at the bottom of the soldier wall and went all the way around and then by the time I got to the end I went back to my starting point which was slightly firmer now and began with another six inch or so "row" and went all the way around...I continued to do this until I reached the top of the dome. It stuck fine to the original dome. Didn't use any lath or chicken wire.

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            • #7
              Re: Perlite Concrete on free form igloo

              Thanks to good advice from other posters, I leaned up my mix and put it on as dry as I could. By starting at the bottom as you suggested I was able to the perlite mix to stay long enough to set. I applied in several sessions.

              The chimney has been my last project. It runs over the oven and up the back. I decided to do it all of clay tiles rather than have a metal middle piece. I tied into an existing chimney. I had some tricky miters to cut and a reduction from 13 1/2 to 8 1/2. I got it all together tonight at lit a paper fire. It drew perfectly.
              Marty

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              • #8
                Re: Perlite Concrete on free form igloo

                the chimney sounds interesting. like to see some pictures of it.

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                • #9
                  Re: Perlite Concrete on free form igloo

                  Here are a few pictures of the chimney. It seems to draw flawlessly which was a big concern of mine since the oven is in our kitchen. I have a little fitting and mortaring to do before its completed, but there is no sign of leakage even with the loose fitting tiles and a very smoky fire. Is anyone aware of a product I could use similar to fiberglass mesh tape to wrap the joints with in addition to the fire-clay. I am guessing that after time the fire-clay will crack and drop out of the horizontal joints. With some mesh the clay would at least be held in place.

                  I plan on covering the whole chimney and oven with adobe after I have test fired for a while and after my stock of adobe thaws out (April). I will be building an armature out of expanded lathe to allow an air space for expansion around the flue tiles.
                  Attached Files

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                  • #10
                    Re: Perlite Concrete on free form igloo

                    Man! I am liking that oven! that is a saweet chimney!

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                    • #11
                      Re: Perlite Concrete on free form igloo

                      Marty,
                      A mixture of vermiculite and cement, 10:1 will work well as insulation around the flue tiles, then you have something to stucco against. It is slightly flexible and works well as a thermal expansion layer.
                      Kindled with zeal and fired with passion.

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                      • #12
                        Re: Perlite Concrete on free form igloo

                        Hey Marty, cool looking flue work, I was wondering how big the oven is and if is was build partially into the brickwall behind it ? As far as the fluejoints go you could wrap the pipe in regular metal hardware screen, like the type for windows, and skim coat the flues with a portland, fireclay and sand mix. a little extra protection and even insulate after that. And then your adobe brick, did you make those?

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                        • #13
                          Re: Perlite Concrete on free form igloo

                          I'm thinking you may want to mound up some vermiculite concrete under that chimney to give it better support, instead of the occasional brick. I don't see any reason you couldn't use the fiberglass mesh that's used to bind sheets of cement board on your joints, with refractory mortar, of course. That's not normally done, but it's normally laid close to vertical.
                          My geodesic oven project: part 1, part 2

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                          • #14
                            Re: Perlite Concrete on free form igloo

                            That looks to be a little over the maximum recommended 30 degree offset. If nothing else, you should add a good vertical stack at the other end to help with draw. As well, you should keep it very clean to prevent a flue fire.

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                            • #15
                              Re: Perlite Concrete on free form igloo

                              1. The flue angle is under the recommended angle of 30 deg--closer to 45. I was constrained by the location of the existing chimney and a concrete bond beam that goes around it. As we've made a bigger fire today there was some smoke escaping out the front. Not much, but more than is acceptable. The vertical stack currently is about 7'. I am going to add to that and see if it takes care of it. If not I may try opening the throat a little wider. Its about 2 inches narrower side to side that the flue liner on top of it. I am also thinking the unsealed joints in the chimney may be impacting the draft.

                              2. I plan do plan on supporting the chimney as suggested with perlite cement after I get the kinks worked out. I am a little concerned about resting all the weigh on the oven because of anticipated movement of both the oven and the flue.

                              3. I put a match to some fiberglass tape used for cement board and it burned readily. I don't know that the chimney would ever get as hot as an open flame, but I would rather find something that could take the heat better. Hardware cloth may be the ticket as you suggested, John.

                              3. The oven is 36" dia and it does recess into the adobe wall. I insulated between the adobe and the fire brick with about 3 inches of mineral wool filling the gaps with loose perlite. The adobe bricks were site made by my wife and I and anyone else we could persuade to help. 10,000 in all over 2 summers.

                              We are hoping to get it hot enough to cook in by the weekend. Thanks to all of you for your interest and suggestions.

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