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Hearth question - Forno Bravo Forum: The Wood-Fired Oven Community


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Hearth question

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  • Hearth question

    I have a question about pouring the hearth. I understand the first layer of the hearth is structural concrete, but for the insulating layer it says in the instructions to mix approx 5:1 portland cement and vermiculite. Looking at portland cement at my local home store, it states on the bag to mix the portland with a ratio of sand and aggregate. When doing the insulating layer, do you include this sand and aggregate or just the dry portland cement and the vermiculite.
    Thanks in advance, and to all of those that make this a great site to learn from.

  • #2
    Re: Hearth question

    Just the dry portland and vermiculite (or perlite). The directions on the bag are usually for concrete (mix with sand and gravel) or mortar. You are mixing a specialty mix for insulating. This was not made up here - you can google for perlcrete or vermiculcrete (may have to check the proper word blend on the latter) and find that there are other uses. When you mix it, you will think you are doing something wrong - it's not a conventional masonry mix texture - it does not adhere well to itself and feels unlikely to harden. Just carry on and pour it in your form - as long as you follow the directions and are in the ballpark it will work fine. The final product after hardening will take quite a load but does not handle abrasive forces so do not be alarmed if the edges rub off easily.


    • #3
      Re: Hearth question

      thank you very much for the info Maver. I will let you know how it goes.


      • #4
        Re: Hearth question


        Just to reinforce what you said, that's exactly right. It looks very strange the first time you do it, but the ratio works just fine. Don't add sand or aggregate, just Portland and vermic.

        "Made are tools, and born are hands"--William Blake, 1757-1827


        • #5
          Re: Hearth question

          Looks like were at about the same stage in the building process. I just built the walls today and will try to build the hearth next week. I'm glad you asked this question because I was thinking in the same direction you were. Thanks everyone for your fast feedback on these type question because more people benefit then just the person asking the question.


          • #6
            Re: Hearth question

            As roundmanone said, and just for the people benefit, it is important to repeat the whole process.
            The vermiculite or perlite and the portland cement need to be mixed dry and after that, the water will be slowly showered while continously mixing.
            This procedure let each particle of vermiculite be 'covered/surrounded' by portland before get wet, protecting it from to much water and making an homogeneous final product.