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What Concrete for a cast flue? - Forno Bravo Forum: The Wood-Fired Oven Community

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What Concrete for a cast flue?

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  • What Concrete for a cast flue?

    I am getting to the point soon (hopefully) of starting my flue for my 42" Pompeii and have decided that I would like to cast my own using wood forms.
    1. What type of concrete should I/ can I use?
    2. Can fibers be used for reinforcement?
    3. Are there min and max lengths of the chimney?
    Thanks, Eric

    My 42" dome build pics in Napa

    My build thread part I

    My build thread part II

  • #2
    Re: What Concrete for a cast flue?

    I found the following mix on the web,

    Ingredients:

    River gravel or crushed fire bricks
    Sand
    Calcium Aluminate cement. (if available in different grades choose the better one)
    Lime (lime is usually cheaper, about 80% of the cement price)
    Water
    You can add into this mix a little standard Fireclay if you like.

    Mixture: (parts ratio is 3 x 2 x 2 x 0.5, plus water)
    3 shovels of the gravel or crushed firebricks
    2 shovels of sand
    2 shovels of the cement
    half shovel of lime

    -----------------------------------------------------------------
    1. Has anyone used this mix?
    2. Where do I get "Calcium Aluminate Cement" (around Napa, CA)?
    Thanks, Eric

    My 42" dome build pics in Napa

    My build thread part I

    My build thread part II

    Comment


    • #3
      Re: What Concrete for a cast flue?

      I would not recommend any gravel, although crushed firebrick is fine. Depending upon where you are located, the gravel could have a very different rate of thermal expansion than the rest of the ingredients and cause major spalling at high temps.

      Comment


      • #4
        Re: What Concrete for a cast flue?

        Have you used this mix design?
        Or used another?
        Thanks, Eric

        My 42" dome build pics in Napa

        My build thread part I

        My build thread part II

        Comment


        • #5
          Re: What Concrete for a cast flue?

          There are pre-mixed castable refractories out there. I would not feel comfortable casting my own custom flue with a site-mix, but I did use one to fill in the gaps around my terracotta flue and firebrick.

          Comment


          • #6
            Re: What Concrete for a cast flue?

            Yes you would be better to buy a premixed castable refractory ie just add water. The flue doesn't get that hot (as evidenced by the inside being black ie under 300 C) so shouldn't be too much of a problem, although if it is relatively thin and the outside is exposed to the ambient temp then expect the flue to crack, due to temp difference. You really need to insulate it a little and have an outer skin to prevent this.
            Kindled with zeal and fired with passion.

            Comment


            • #7
              Re: What Concrete for a cast flue?

              Calcium Aluminate cement. (if available in different grades choose the better one)
              Lime (lime is usually cheaper, about 80% of the cement price)
              We are warned off mixing calcium aluminate cement and lime: They are apparently incompatible.

              With flue tile so cheap, why would you mess with this difficult material?
              My geodesic oven project: part 1, part 2

              Comment


              • #8
                Re: What Concrete for a cast flue?

                Originally posted by dmun View Post
                With flue tile so cheap, why would you mess with this difficult material?
                I want to cast my own because I want an offset and dont want to have to pie cut a bunch of 13x13 liners.

                Making the forms is the easy part.


                Eric.
                Thanks, Eric

                My 42" dome build pics in Napa

                My build thread part I

                My build thread part II

                Comment


                • #9
                  Re: What Concrete for a cast flue?

                  Flue as in the whole vent area?
                  I cast my vent using premix, just added water. I added some crushed firebricks to add a bit more volume to the mix. 25 lbs of this Rutland refractory cement premix was just enough to fill the form.
                  George

                  My 34" WFO build

                  Weber 22-OTG / Ugly Drum Smoker / 34" WFO

                  Comment


                  • #10
                    Re: What Concrete for a cast flue?

                    Is this the material?

                    If so, does anyone know where to get it (or a suitable replacement) in Northern California, I live in Napa, CA and work in Pleasant Hill, CA. (a bedroom community about 30 minutes from San Francisco)

                    I did check their website and called a few places (mostly Ace Hardwares stores) and was told that they did not carry that material.
                    Thanks, Eric

                    My 42" dome build pics in Napa

                    My build thread part I

                    My build thread part II

                    Comment


                    • #11
                      Re: What Concrete for a cast flue?

                      I ordered mine, along with other related materials, from these guys. Shipping was free too.

                      Castable Refractory Cement - 25 lb.
                      George

                      My 34" WFO build

                      Weber 22-OTG / Ugly Drum Smoker / 34" WFO

                      Comment


                      • #12
                        Re: What Concrete for a cast flue?

                        Originally posted by fxpose View Post
                        I ordered mine, along with other related materials, from these guys. Shipping was free too.

                        Castable Refractory Cement - 25 lb.
                        Fxpose, thanks

                        Did you use the premixed style?
                        Is there an advantage/disadvantage to using either premixed or a dry mix?
                        Thanks, Eric

                        My 42" dome build pics in Napa

                        My build thread part I

                        My build thread part II

                        Comment


                        • #13
                          Re: What Concrete for a cast flue?

                          Originally posted by EricU View Post

                          Did you use the premixed style?
                          Is there an advantage/disadvantage to using either premixed or a dry mix?
                          I used the dry mix where you add water. That 25 lb pail comes in dry powder form. You just add water.
                          George

                          My 34" WFO build

                          Weber 22-OTG / Ugly Drum Smoker / 34" WFO

                          Comment


                          • #14
                            Re: What Concrete for a cast flue?

                            OK, since actually obtaining refractory cement is harder and more expensive (very), I have been thinking of two other ways to do my chimney liner;
                            1. Cut firebricks in half the long way and just build up my own while I am doing the exterior decorative chimney and fill the void with the pearlite concrete mix.
                            2. Use fire brick to make my transition then do the exterior decorative chimney, then use a sonotube liner or some kind of interior form and fill the void with the pearlite concrete mix

                            Any thoughts?
                            Thanks, Eric

                            My 42" dome build pics in Napa

                            My build thread part I

                            My build thread part II

                            Comment


                            • #15
                              Re: What Concrete for a cast flue?

                              Perlite cement mix against the hot side is not very strong or refractory solution 1would be better IMO.
                              Kindled with zeal and fired with passion.

                              Comment

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