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Help! Vermiculite problems

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  • Help! Vermiculite problems

    OK. I am working on the hearth of my Pompeii oven. I called every garden center in Southeast Michigan. They either did not carry vermiculite or they were sold out. Finally, after finding a web page that tipped me off that vermiculite is used in the construction of swimming pools, I found a pool supply store that sold me six bags of a special mix of vermiculite and portland cement (called A-TOPS Pool Mix, comes in 2 cubic feet bags for $18.95 each).

    I mixed it with as much water as it could hold and shoveled in to the form on top of the concrete stand. It is now the second day and I am concerned that it is not setting properly.

    I can still push my finger into the top surface. The vermiculite mix is firm but not super rigid. Is this the way it is supposed to be? Will the edges crumble when I take the wood form away? Am I really supposed to build a brick structure on top of this soft base?

    Any guidance would be appreciated!


  • #2
    Re: Help! Vermiculite problems


    I have not used the mix you refer to. If the ratio of portland to vermiculite is the same as a post I found on a pool forum (7:1) then I would think you would be OK. The mix is very crumbly indeed, I have more than a bit missing from the edges of mine. But it does have some compresive strength and should hold OK. If memory serves, my mix was "soft" for more than 3-4 days. Give it a few more days and see how well it hold weight.
    Wade Lively


    • #3
      Re: Help! Vermiculite problems

      I was a little nervous also but, it took about a week for the vermiculite layer to harden up. That said, it is still on the crumbly side and pieces around the edge chipped off in spots.


      • #4
        Re: Help! Vermiculite problems

        I think that your mix is a little on the light side with cement, but it might come up in rigidity over a week or so. I mixed my hearth vermiculite insulation 5 parts : 1 portland cemrnt, see:

        thread #2 for the finer details and pictures. It took 3-4 days to stiffen up and after a couple of weeks, it felt like a fairly 'firm' cork type consistency.
        After the dome was built (the following week-end, I covered it with a thermal blanket and mixed 3 1" layers of the same vermiculiter cement and topped it with a 3/4" layer of water proof render.
        See how it goes over the next 5 to 7 days. If it is still very crumbly still, I might be tempted to remove an outer layer (say 6 inches or so and remix a little more cement 5:1 and relay it to take the load of the bricks of the dome, which will be quite considerable. It will bond onto the existing layer but would be much stronger and harder.
        Prevention is better than cure, - do it right the first time!

        The more I learn, the more I realise how little I know

        Neillís Pompeiii #1
        Neillís kitchen underway


        • #5
          Re: Help! Vermiculite problems

          Originally posted by Stiennon View Post
          I mixed it with as much water as it could hold

          Hey Richard and welcome to the forum,
          I have 2 concerns:
          1. 7:1 is pretty lean on the cement side (as mentioned by Nissaneill) This is also what the FB guide recommends.
          2. "as much water as it could hold" is the BIGGEST concern and was my first thought.
          Mixing this vermiculite as most on here will agree I'm sure is tricky at best. the key is to have it mixed very well BEFORE the water and then add just enough water for it to hold together. Since you used a bag mix we can assume it was mixed well prior to water.
          So this is where we are at with your mix, did you add just enough water to make it hold together like.....loose garden soil when you squeeze it it stays together other wise its kinda like the crumb topping on a muffin.
          Or was it wet and soggy? too much water and the cement is greatly diluted thus the strength. also too much water will make the vermiculite softer that it should be and susceptible to compression if not placed correctly. Id say wait at least a week and see if it comes up to strength. The mixing of cement products even bagged mixes such as spec mix or your basic concrete mix form H.D. can be DRASTICALLY altered by the mixing method, water content and mixing time. Mix until moist, mix until sticks to trowel, mix until free flowing, all are "relative" in definition unless specified by weight. the good news is if it doesn't get hard you can re use the mix... MAYBE? For now patients...
          Last edited by Unofornaio; 06-30-2007, 08:12 AM.


          • #6
            Re: Help! Vermiculite problems

            Well, the recurring theme here is that everyone who mixes vemiculite cement has misgivings - it doesn't feel right the first time, but generally after a week all is well. Hang in there. Most of the comments here about this can be used if it completely fails, but usually it just works!


            • #7
              Re: Help! Vermiculite problems


              Is the slab firm without any loose material? If there's not enough cement then the vermiculite stays loose & can almost be brushed out, If this is the case then I would scrape out the loose material & pour another slab on top of what's left. If you can't scrape it out then it's probably OK. When the pad's cured its still kind of springy, Nissaneill's analogy of cork is a good one

              This is how mine looked the first time. I had added too much water & mixed in a mixer, not good, the slab ended up with some loose vermiculite where the cement had kind of washed off. I mixed another batch by hand 5:1 dry first & then less water & re poured another 3" on top of the old slab. I also took this opportunity to install a split brick sub floor

              In both cases I seem to remember it setting in a day or 2, maybe I'm wrong but I think I would have freaked out if it was still wet after a week.

              Pouring this slab is miniscule compared to the work you'll put in on the dome & rest of the project, not sure of the consequences of building on a soft pad but personally I pour some more.




              • #8
                Re: Help! Vermiculite problems

                I just poured my vermiculite slab this past week and it is now 4 days old and it is starting to get stiff. Was a little worried the day after I poured it that the mix was wrong because it seemed so soft. But now no worries.

                "Opportunity is missed by most people because it is dressed in overalls and looks like work." Thomas A. Edison


                • #9
                  Re: Help! Vermiculite problems

                  Thanks for all the input everyone. By the time I left for California the vermiculite was starting to get that firm, cork-like, feel that several of you described. I think I am going to be OK. If I did it again I would leave space around the periphery for some regular concrete just so the edges don't crumble away.

                  I am looking forward to sharing more with this excellent forum!



                  • #10
                    Re: Help! Vermiculite problems

                    Im in western michigan and found vermiculite availible at menards. I know it doesnt help with the hearth but it might still help for the loose fill on top of the dome.


                    • #11
                      Re: Help! Vermiculite problems

                      Hey, it worked! I am just back from my three week trip to California, China, Hong Kong, Singapore, Tokyo, Seoul. The vermiculite set up very nicely. All of my forms dropped right out.

                      I did have to cut two of the supports that held up the plywood for the platform with a Skill saw.

                      Thanks to everyone for their assurances.


                      • #12
                        Re: Help! Vermiculite problems

                        Hello Steinon...

                        I've got the same exact problem as you and in my third day right now. I'm terrified that I either used the WRONG Vermiculite Product or have the mixture all wrong. How is your Slab holding up with the load of the dome and the other materials???

                        Did your Slab look anything like mine below? Please comment on Sandy Texture and Color which is a sandy brown. 3 Days in and mine is Wet and Moist as well....


                        • #13
                          Re: Help! Vermiculite problems

                          its very strange mix - I was worried about mine too

                          Dont worry ...i found that mine is like cork you can push a sharp knife in and even leave a depression if you push you finger tip on it hard

                          But after a week or so it becomes quite dense remember the weight of the finished oven is over a large area

                          the mix was like oatmeal / porridge see a few pics they should set your mind at rest..the mix was 6parts vermiculite - 1 part cement


                          • #14
                            Re: Help! Vermiculite problems

                            delayfox. Relax! The vermiculite mixture will indeed harden. Within a week it will be as hard as, well, cement! My oven has had no problems at all and is now officially a year old. It is the center of our social life. Last night we made the best calzones ever. Even the french bread came out great. Best crust I have ever achieved.

                            I like your design. Keep posting pictures as you get to the next steps!

                            My pictures are here at Flickr.



                            • #15
                              Re: Help! Vermiculite problems

                              Thanks Steinnon...

                              Was freaking out... all that time and money, mostly the time. I saw your photo's looks great!!!

                              As I am about to embark on the DOME, and hoping to finish before the cold weather settles in NY, any advise on DOME Construction.

                              1. High Heat Mortar... did you opt to buy pre mixed or did you make from scratch?

                              2. DOME: Couldn't tell from your photo's but is the first row of firebrick MORTARED on to the oven floor?

                              3. DOME ANGLE: Did you use the a form for the dome, wedges, or free hand? How long did it take you to complete the dome (ie. just the dome, not the insulation, etc.)

                              4. Did you use the blanket sold at Forno Bravo for insulation?

                              5. FIRE BRICK CUTS... How did you cut yours in half and do you think a TILE SAW would work?

                              6. Stupid Question: How did you create the diameter of your circle on your oven floor? Can't figure out how to create a 36" Round template?

                              Thanks for your help... keep an eye out under my pictures... will post them on my string marked "Greetings from NY".