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Working with Vermiculite - Forno Bravo Forum: The Wood-Fired Oven Community

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Working with Vermiculite

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  • Working with Vermiculite

    Hi everyone, I have a question about the ratio of vermiculite to portland cement that will be poured on top of the hearth. The forno bravo site recommends 5 parts Vermiculite to 1 part Portland cement. Does this ratio go by weight or simply by volume? For instance for every 1lbs of cement do I have to weigh 5 lbs of vermiculte, or does it mean for ever bag of cement, I simply refill the empty cement bag 5 times with vermiculite and mix that? I assume it's by weight but that would mean a hell of a lot of vermiculite!

    I'm planning to build my vermiculite layer 68 inches long by 65 inches wide by 3.5 inches high. How much vermiculite and cement would that represent?

    Can someone help me with this?

  • #2
    Re: Working with Vermiculite

    Originally posted by Nick in Laval View Post
    Hi everyone, I have a question about the ratio of vermiculite to portland cement that will be poured on top of the hearth. The forno bravo site recommends 5 parts Vermiculite to 1 part Portland cement. Does this ratio go by weight or simply by volume? For instance for every 1lbs of cement do I have to weigh 5 lbs of vermiculte, or does it mean for ever bag of cement, I simply refill the empty cement bag 5 times with vermiculite and mix that? I assume it's by weight but that would mean a hell of a lot of vermiculite!

    I'm planning to build my vermiculite layer 68 inches long by 65 inches wide by 3.5 inches high. How much vermiculite and cement would that represent?

    Can someone help me with this?
    The ratio is by Volume. I used a Plastic pitcher to make my measurements. Do not use the bag for measure as it will not give a repeatable size. It is easy to use a pitcher or bucket to make the measurements.

    I did 7/1 vermiculite and portland. Some have indicated a 50/50 mix of perlite and vermiculite give a more workable material but I have not used this mix.

    I dampened the vermiculite with a garden mister on the end of the garden hose till damp but not wet - mixing with a shovel to dampen overall, and made a slurry from the portland and water in a 5 gal pail, to pour over the vermiculite and mixed in a wheel burrow with a shovel.

    You do not want to use a concrete mixer it must be mixed by hand.

    Chip
    Chip

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    • #3
      Re: Working with Vermiculite

      Originally posted by Nick in Laval View Post
      ...question about the ratio of vermiculite to portland cement that will be poured on top of the hearth. The forno bravo site recommends 5 parts Vermiculite to 1 part Portland cement. Does this ratio go by weight or simply by volume?...

      I use pearlite as it is cheaper out here in Northern California and readily available in 2 cu. ft. bags at HD.

      As mentioned, I measure by Volume and I have switched to a 3:1 ratio (3 parts Pearlite to 1 part Cement) to get a little more strength out of the mix.

      I dry mix with a hoe in a wheelbarrow and then slowly add water, it is a very different (very dry looking) mix compared to grout, but it does harden.

      Eric.
      Thanks, Eric

      My 42" dome build pics in Napa

      My build thread part I

      My build thread part II

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      • #4
        Re: Working with Vermiculite

        3/1 mix will not provide much insulation value, at 3.5 inches I have 2.5 inches of 7/1 vermicrete and 2 inches of FB ceramic board on top of that, and I know I still lose heat into the slab. Wish I had another 2 inches of Vermicrete or FB Board.

        The vermiculite will compress but it will be based on the type, it is available from fine to coarse and I used medium grade. Mine compressed about 30% but buying more than you need should not be a problem because you will want more loose fiil for over the dome and you can easily take back an unopened bag of portland but 1-90 Pound bag should be more than enough for the insulating slab.

        Chip
        Chip

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        • #5
          Re: Working with Vermiculite

          Chips right on, you should at least 5-1, 7-1 would be better, more insulation is on of the top ranked if I could do it over statement
          http://www.fornobravo.com/forum/f8/p...s-i-18098.html

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