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How should vermiculite concrete look once it is cured? - Forno Bravo Forum: The Wood-Fired Oven Community


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How should vermiculite concrete look once it is cured?

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  • How should vermiculite concrete look once it is cured?

    Hello ,
    we just poured the vermiculite/cement mix and are not really suire if it came out right. Can anybody describe how its consistency, feel, look etc should be?

    Should it be hard as concrete or still relatively soft?


  • #2
    Re: How should vermiculite concrete look once it is cured?


    What was your ratio? At 5:1 or 6:1 you get something that looks like oatmeal. At first you think it will never hold anything, but it will. It takes longer to set up than regular mortar and concrete, but if you used a good ratio, you should be fine.

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


    • #3
      Re: How should vermiculite concrete look once it is cured?

      Hi Jim,
      thanks for the reply. I used a 6 to 1 ratio.
      Although I used a lot of water it still was somewhat dry looking and I had to compact it real well to make it hold together.
      Now after about 20 hours it seems to set, but it is still somewhat brittle.
      I think if I took a screw driver I could probably easily dig a hole.
      The "left overs"that spilled over can be easily crushed between the fingers.

      One thing I wasn't sure about also is that we used a Vermiculite product that is used for filling masonary wall cavities and has some kind of water repellent on it.
      Do you think that might be a problem?

      That was the only Vermiculite we could find here. (i posted a sep thread on that question.)
      Thanks again


      • #4
        Re: How should vermiculite concrete look once it is cured?

        I've read here that the water repellent is a concern - however, you've already poured it, so let it harden then perhaps you may want to compression test it. The water repellent might affect how it cures. As far as being able to drive a screwdriver into it - mine felt like that (and I used horticultural grade, no repellent). Artists use vermiculite concrete for sculpture because a few days into the cure it is very easy to cut/shape, then it firms up further.


        • #5
          Re: How should vermiculite concrete look once it is cured?

          To the vermiculite under the hearth I was to 6:1 ratio due structural issues.
          To the vermiculite over the dome I was 8 or 9:1.
          Pictures following.

          Attached Files


          • #6
            Re: How should vermiculite concrete look once it is cured?


            I'm guessing you had to take a lot of care to make sure your dome vermiculite went on so smoothly, and that it adhered.

            Any tips on that?
            Pizza Ovens
            Outdoor Fireplaces


            • #7
              Re: How should vermiculite concrete look once it is cured?


              From a mason's point of view, I'd have to say that the water repellant would affect the absorption rate in your mix. However, as Maver points out, it's done now. As I said, these mixes take longer to set and cure by their nature and the relatively small amount of cement used as a binder. You'll have to wait and see, but I still think you'll be fine.

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


              • #8
                Re: How should vermiculite concrete look once it is cured?


                Well, let´s say...was a piece of cake <g>
                I wrote the following sentences a long time ago, in any thread of this site

                'Some tricks to mix vermiculite and cement that not all the people knows.
                First at all, the isolation will be better if the vermiculite is loose, meaning that if you could maintain the vermiculite grains all together without the use of any “glue” (cement) it will be a better temperature isolator. Use the bigger relationship between vermiculite/cement.
                I used a 7:1 vermiculite/cement mixture (9:1 could had done the work too) in a single layer of 3 inches below the oven hearth as same as on dome. In a Scott type oven, the loose vermiculite over the barrel is just maintained by the surrounded walls, without cement.
                Second, and very important thing, the mixture of vermiculite/cement need to be done by mixing well the dry components at first, without water´s addition. This procedure let the vermiculite particles be totally covered by the cement and prevents it to absorb a greater quantity of water than necessary.
                The water will be added sprinkling gently and slowly, with continuous mixing the mixture until slightly wet (you could take in your hand and see a loose mixture that separate if you do not hold it).
                The goal is that each particle or grain of vermiculite covered by cement be wet but no overflowed.
                This almost dry (slightly wet) mixture will be kindly distributed around the dome only tapping it against the chicken wire (do not force it, be gently). Below the hearth will be equally distributed in the prepared box, as loose as possible.
                When wet you could feel like a lot of grains firmly joined together. In the next day (or 6/8 hours later) it will have cork consistency. Three/four days later you could beat it and feel it as a drum and it will have a strong and firmly consistency.

                I hope this help.'




                • #9
                  Re: How should vermiculite concrete look once it is cured?

                  Thanks for all your help!
                  One more question please so that we can continue building this oven without loosing sleep over having used the Vermiculite with the water repellent coating...

                  Even if the insulating layer holds together OK (it looks/feels pretty good by now).., is there a potential problem with the oven heat and the Silicon coating? Can the heat impact the insulating layer?

                  I also read in the Pompeji instructions - too late - in the hints section that "horticultural Perlite/Vermiculite and not Industrial or Masonary Insulation (silico Coated) should be used.
                  However there was no explanation, why?!
                  Can anybody explain?

                  Thanks again..


                  • #10
                    Re: How should vermiculite concrete look once it is cured?

                    the recommendation, and James will correct me if I am off base, is that the Silicone is a lubricant and is used a a loose fill in for insulating the core of the hollow block. Having it coated helps it slip down the hole. In terms of mixing with cement the coating prevents a strong bond from happening.

                    Think of it as cooking eggs. In my copper bottom 50 year old Revere Ware (that was made in Riverside, CA) pan, when I crcak an egg and plop it onto the hot surface it sticks to the pan like no tomorrow. To keep it from sticking I first treat the pan with some butter. If I take an egg an crack it over a hot teflon coated pan it doesn't stick.

                    This is kind of an extreme example as the Perlite/Vermic is not coated as well as a Teflon style pan. However, the purpose is the same. The mere mixing and tumbling of the media will break some of it up and give a bonding plane it just is not as strong as it could be. Even with using the non coated stuff you will find that the final product is somewhat friable (crumbly). As it is being used in compression it will probably be ok.


                    • #11
                      Re: How should vermiculite concrete look once it is cured?

                      VV, for what its worth - coming from a fellow serf I agree with Jim and Maver, you should be fine. I followed the directions to the letter and you description of the end result sounds like what I had/have. No, it doesn't set up hard and rigid like concrete - yes, I can dig it out with a screwdriver, and yes - it will compress under too much pressure. I actually (foolishly) put a "dent" in the area surrounding my dome by kneeling on the hearth slab while cleaning the inside of the dome during construction. I am not at all concerned, other than my kneeling and rubbing a bit of the edges off while constructing, I've had no issues.
                      Because of the silicone issue, you might want to give it an extra couple of days of cure time, just to be safe. It does seem to get harder (and compress less) over time. Mine is 5 weeks old, was standing and kneeling on it again during the insulation process this past weekend......no "dents" this time around - but a screwdriver could still do serious damage.
                      Good luck and keep us posted