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  • Perlcrete question

    Hi All,

    I have finished pouring (shoveling really) a 4 inch insulation layer of perlcrete (5-1 perlite-Portland cement mix). It has set in forms for 7 days. I kept it covered under a tarp for the period, but noticed that as it cured, the surface had the texture of a bowl of rice crispies. The surface was very loose, and could be eroded fairly easily with a light swipe of my hand.

    I removed a piece of the form today. The Perlcrete seemed to hold up well... a clean vertical side, with only a few bits of perlcrete residue.

    Is it normal to have such a loose top? How should I stabilize it. I am thinking that a skim coat across the top to level and lock in the loose perlcrete is the thing to do. I think I will also buttress the sides of the perlcrete with red brick.

    I feel like I cannot trowel the fireclay/sand mix in its current state.

    Please advise how you would proceed in this circumstance. Thanks

  • #2
    Re: Perlcrete question

    A thin skim coat of regular mortar won't hurt. Better to leave the forms on while building the oven, though so you don't beat up the edge.

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    • #3
      Re: Perlcrete question

      Thanks for the quick reply.

      I'm glad to hear that. I think I will try to vacuum (not getting too close to the perlcrete) the surface before I skim coat to get the smoothest surface possible.

      Leaving the forms in place was also my wife's suggestion. Needless to say, it seems like a better idea now...

      Thanks again

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      • #4
        Re: Perlcrete question

        One other thing, does the consistency of the perlcrete seem about right?, and is it normal for the surface to be so loose and pebbly?

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        • #5
          Re: Perlcrete question

          It's too late now, but you can get a pretty good surface if you trowel it firmly once placed. You'd be better off allowing the slab to dry more for a couple of weeks, then use a dry 50/50 clay, sand mix of about an inch thick to place your floor bricks on. If you build over it now you will be locking in moisture that will be hard to remove later.
          Kindled with zeal and fired with passion.

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          • #6
            Re: Perlcrete question

            It will always be loose and pebbly. The required strength for this application is only compressive strength, of which it has plenty.

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            • #7
              Re: Perlcrete question

              Ok, that's what I thought.

              Man, I hate the thought of waiting two ore weeks.

              Thanks again!

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              • #8
                Re: Perlcrete question

                You can wait, or you can proceed. I laid the floor the next day.

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                • #9
                  Re: Perlcrete question

                  If you have a little perlite left over, you can make your skim coat out of it. First, wet some perlite to control the dust. Then, with a gloved hand, grind it over an old window screen. Just about any rough handling will break the larger chunks into smaller particles. If you don't have any left over, try to get a few small bags of fine perlite. (Just make sure they have no additives)

                  The fine perlite, mixed at the same 5 to 1 ratio, will finish out smooth on top of your coarse layer.
                  Last edited by Gulf; 09-06-2014, 04:55 PM.
                  Joe Watson " A year from now, you will wish that you had started today" My Build Album / My Build

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                  • #10
                    Re: Perlcrete question

                    Here's why it is advisable to dry your perlcrete layer before building over it.The perlcrete layer takes around double the quantity of standard concrete so even after hydration there is a huge amount of free water to eliminate. Allowing the sun and wind to do much of the work for you eliminates a lot of angst and time trying to do it with fires.
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                    Last edited by david s; 09-06-2014, 04:45 PM.
                    Kindled with zeal and fired with passion.

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                    • #11
                      Re: Perlcrete question

                      If you start building fires as soon as your floor is down, it is not a concern.

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                      • #12
                        Re: Perlcrete question

                        Originally posted by Tscarborough View Post
                        If you start building fires as soon as your floor is down, it is not a concern.
                        Probably better to wait until the dome has been built at least.
                        Kindled with zeal and fired with passion.

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                        • #13
                          Re: Perlcrete question

                          Why? Water doesn't want to stay water under heat, so begin getting rid of it as soon as possible. I built a fire in my vault as soon as I took the falsework out, and every time I worked on it after that. When it was done, I was ready to cook pizza.

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                          • #14
                            Re: Perlcrete question

                            Originally posted by david s View Post
                            Probably better to wait until the dome has been built at least.
                            Good advice.
                            My Build:
                            http://www.fornobravo.com/forum/f8/s...ina-20363.html

                            "Believe that you can and you're halfway there".

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                            • #15
                              Re: Perlcrete question

                              Originally posted by Tscarborough View Post
                              Why? Water doesn't want to stay water under heat, so begin getting rid of it as soon as possible. I built a fire in my vault as soon as I took the falsework out, and every time I worked on it after that. When it was done, I was ready to cook pizza.
                              I'm not saying your approach won't work Tom, but a 4 ft square vermicrete slab 4” thick is around 120 cm x 120 cm x 10 cm = 144 litres (32 gallons). Around a third of that will be water, about half of which will be used up in the hydration process, but there is still a huge amount of free water left in it.
                              How long would you expect to take to boil dry a 15 gallon drum of water. A long time and plenty of fuel to do it. By putting the floor bricks on top before drying is like placing a lid on your boiling drum of water. Allowing nature in the form of sun and wind to do much of the drying for you, if the weather will allow you, is a prudent approach. Some builders report that their ovens take months to dry properly and this is why.The water under the floor is the hardest to get rid of.
                              Last edited by david s; 09-06-2014, 09:38 PM.
                              Kindled with zeal and fired with passion.

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