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Perlite concrete mix incorrect? - Forno Bravo Forum: The Wood-Fired Oven Community

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  • Perlite concrete mix incorrect?

    So another issue I've encountered with my oven build. I ordered portland cement and mixed it on a 5:1 ratio with perlite, however after I got about half way through I realised they had sent me a concrete mix (containing sand and aggregates) instead of pure general purpose cement. I was stupid and didn't look at the bags and just assumed they were what I ordered.

    What would be the repercussion of still using this mixture as my insulating floor? Will it not be as strong and potentially crack under the weight or heat?

    I intend to have fire bricks on top of the floor too.

  • #2
    Not only will the addition of 5 parts perlite with the concrete mix result in an inadequate strength the sand and heavy aggregate will reduce its insulative quality. If it were mine id Be digging it out and doing it again.
    Kindled with zeal and fired with passion.

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    • #3
      Originally posted by david s View Post
      Not only will the addition of 5 parts perlite with the concrete mix result in an inadequate strength the sand and heavy aggregate will reduce its insulative quality. If it were mine id Be digging it out and doing it again.
      I still used 5 parts perlite with one part of the mix, so I'm not sure how it would reduce the insulate quality of the base? As for strength I would've though the aggregates would've helped, but then again it means there's less cement for bonding so I can see why the lower strength may be the case.

      I'm hoping to find the easiest solution as simply redoing it is a last resort due to the time and money spent on it already. If there are no other solutions then I suppose I will have to throw away my work and start again.

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      • #4
        I would heed David's advice, typical proportions of bagged concrete mix is 1 part cement, 3 parts sand, 3 parts aggregate or roughly 14% cement, not even close to the strength you need for the oven base of a 5 to 1 pcrete ratio need to support the oven.
        Russell
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        • #5
          The reason it wont insulate very well is because your mix contains aggregate and sand both having a density something like 18 times that of perlite. You could try placing some calcium silicate board 2 thick over your existing slab, but that would still not address its inadequate strength, yours being around 9:1 aggregate to cement. The cal sil board will also be more expensive than the materials for a new cast perlcrete insulating slab. Because it will be so weak the slab you have laid will be easy to remove.
          Check the attached table to see how much the addition of extra cement and sand makes to the insulation values of vermicrete (perlcrete the same).

          Click image for larger version  Name:	image_83170.jpg Views:	1 Size:	332.7 KB ID:	409932
          Last edited by david s; 12-03-2018, 11:06 PM.
          Kindled with zeal and fired with passion.

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