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Insulating slab question - Forno Bravo Forum: The Wood-Fired Oven Community

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Insulating slab question

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  • Insulating slab question

    I finished my structural layer and poured my insulating layer using about 4 parts vermiculite and perlite to 1 part portland cement... no other aggregates. It is only a day since the pour, but it seems like I could easily a knife through it. In other words it more resembles hard sand, than cement. Is that normal? What should this layer feel like?

    Thanks in advance.

  • #2
    Re: Insulating slab question

    Originally posted by cholme View Post
    I finished my structural layer and poured my insulating layer using about 4 parts vermiculite and perlite to 1 part portland cement... no other aggregates. It is only a day since the pour, but it seems like I could easily a knife through it. In other words it more resembles hard sand, than cement. Is that normal? What should this layer feel like?

    Thanks in advance.
    Your description and concern are common threads here.

    The vermicrete will support a lot of weight, despite its crumbliness. Give it a couple more days and you should be ready to start
    Lee B.
    DFW area, Texas, USA

    If you are thinking about building a brick oven, my advice is Here.
    Our One Meter Pompeii Oven album is here.
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    I try to learn from my mistakes, and from yours when you give me a heads up.

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    • #3
      Re: Insulating slab question

      You can increase the strength of the insulating slab by adding sand to the mix, but you also reduce the insulating value. The 5:1 mix has been proved to have adequate strength, so it's best to stick to the plan.
      Kindled with zeal and fired with passion.

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      • #4
        Re: Insulating slab question

        LeeB pretty much covers it. It will have very good compression strength, but always (even when fully cured) remain crumbly. Give it a full week to cure if you still have an uneasy feeling, it will be hard then (but still crumbly).
        Don't let the rain get to it, you wil have a runny oatmeal mess if you do...its far from weatherproof and absorbs water better than a sponge.

        RT

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        • #5
          Re: Insulating slab question

          As for any portland based material, after the initial set (10-12 hrs), keep it wet for at least 7 days. If you inadvertently allowed it to dry out, you may have to rip it out and re-pour.
          Last edited by Neil2; 06-19-2011, 03:10 PM.

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          • #6
            Re: Insulating slab question

            Mine was "crumbly" for days. But I kept it covered, and voila! a week later, it had the consistency of styrofoam. Yes, I could dig at it if I wanted, but it was pretty solid.

            Just layered some on top of my dome. Perfect curing conditions - the oven is under a canopy, but it's going to be raining for the next few days (100% humidity).

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