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Crumbly Vermiculite/Portland Cement - Forno Bravo Forum: The Wood-Fired Oven Community

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Crumbly Vermiculite/Portland Cement

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  • Crumbly Vermiculite/Portland Cement

    Hi. I poured an insulated hearth, following the instructions provided by Forno Bravo. I have noticed that around the edges, there is some crumbling of the vermiculite. How solid should this top layer of the slab be? I do not want to build my hearth on a slab that will not hold up. unfortunately, i had not noticed the fb board that is sold by forno bravo until after i had poured the insulating cement. I am wondering if I should tear it out and go with the board....
    Any thoughts based on experience out there?

    Thanks,
    Brian Covey

  • #2
    Re: Crumbly Vermiculite/Portland Cement

    brian i put a course of bricks on the concrete hearth to hold the vermiculite in it will take the weight of the oven

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    • #3
      Re: Crumbly Vermiculite/Portland Cement

      thanks, for your help.

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      • #4
        Re: Crumbly Vermiculite/Portland Cement

        no props mate enjoy your building

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        • #5
          Re: Crumbly Vermiculite/Portland Cement

          I had the same experience that you did. My solution was to retain the existing insulation and pour a 1.5 in. refractory cement surface with steel mesh reinforcement ( I used a product called bull fencing 3/16 in. diameter ) this gives you a nice smoth surface for your bricks and also with the 4 in. refactory surface you can cook bread.

          By retain what I did is pour a 2 in.x5.5 in. cement wall to retain the perlite insulation, the harth slab was large enough for the wall.
          Last edited by Lill Dave; 10-06-2011, 05:14 PM. Reason: clarify my retain comment

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          • #6
            Re: Crumbly Vermiculite/Portland Cement

            Wait as long as you can to let the vermicrete layer dry as much as possible before covering it over. It is really hard to eliminate all that water in the vermicrete layer later.
            Kindled with zeal and fired with passion.

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            • #7
              Re: Crumbly Vermiculite/Portland Cement

              My local refractory expert told me you can use rock salt to level off over the vermicrete if you need to, it is a really good and cheap insulator, he also said you can use it instead of vermicrete. good luck.

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              • #8
                Re: Crumbly Vermiculite/Portland Cement

                I would think that salt will dissolve if it gets wet and also will contribute to corrosion problems. I think you are better off using 50/50 sand, fireclay if you just want to get a good level surface to put the floor on. Add some lime to it if you want it stronger and waterproof.
                Last edited by david s; 10-06-2011, 11:08 AM.
                Kindled with zeal and fired with passion.

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                • #9
                  Re: Crumbly Vermiculite/Portland Cement

                  I left the form on my perlcrete slab until I was finished with the oven to prevent damage to the edges.

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                  • #10
                    Re: Crumbly Vermiculite/Portland Cement

                    I found that due to the porosity of standard vermiculite and perlite that when it is mixed with Portland Cement to form an 'insulating concrete' that an excessive amount of water is needed in the mix. Given that in standard concrete, the more water you add the weaker the resulting concrete becomes, it seems logical that these vermiculite mixes are going to be inherently weak due to the amount of water that is needed to 'wet out' the mix (as the water is soaked into the aggregate, unlike normal concrete).

                    To overcome this I found a supplier who makes 'Sealed Perlite', Perlite treated such that it has no pores and does not absorb water. I made my dome insulation layer using the 5:1 ratio with this Perlite and used chicken wire for the reinforcement, and even at only 50mm thick it feels as solid as any concrete I've ever seen. My neighbor used normal vermiculite with the same ratio and ended up with a crumbly mess that he broke off and threw away!

                    Has anyone else ever used the sealed perlite before?
                    The Melbourne Fire Brick Company

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                    • #11
                      Re: Crumbly Vermiculite/Portland Cement

                      I have been using Vermiculite/Portland for many years as a swimming pool base.
                      Is this the same product you guys are using?
                      It comes in bags already mixed. Not very expensive either.

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                      • #12
                        Re: Crumbly Vermiculite/Portland Cement

                        Could you describe the grain size and characteristics of the mix you use buckeye? Does it absorb a lot of water, more than a standard bagged premix concrete? How would you describe it once it's set?
                        The Melbourne Fire Brick Company

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                        • #13
                          Re: Crumbly Vermiculite/Portland Cement

                          Vermiculite/Portland cement concrete 5:1 mix will support up to one hundred pounds per square inch.

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                          • #14
                            Re: Crumbly Vermiculite/Portland Cement

                            Originally posted by benguilford View Post
                            Could you describe the grain size and characteristics of the mix you use buckeye? Does it absorb a lot of water, more than a standard bagged premix concrete? How would you describe it once it's set?

                            Could you describe the grain size - I used medium grade it comes in fine medium and coarse- coarse is about the size of a bb for bb gun or a bit larger. medium is 1/2 that size and smaller, not sure what fine looks like.



                            Does it absorb a lot of water
                            - Yes, I made 6:1 batches of 3 cu/ft of vermiculite, mixed in 2 gallons of water into the vermiculite and then added the 1/2 cu ft of portland slurry (portland and a half gallon of water) dirzzled over the wetted vermiculite while mixing in with a shovel.

                            Sets similar to a wine bottle cork.
                            Chip

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                            • #15
                              Re: Crumbly Vermiculite/Portland Cement

                              I prefer using vermiculite to perlite because I don't like the dust the perlite contains. I've tried coarse, medium and fine vermiculite and generally prefer the medium grade. They all seem to be about the same density but the fine vermiculite needs more water in the mix. I tried a 50/50 vermiculite, perlite mix with 10% by volume cement and for some reason this brew proved to be the best yet, I will continue using it.I should think the vermiculite and perlite vary considerably depending where it's from and how it's processed.
                              Last edited by david s; 10-07-2011, 09:22 PM.
                              Kindled with zeal and fired with passion.

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