Announcement

Collapse
No announcement yet.

Most suitable grog granularity

Collapse
X
 
  • Filter
  • Time
  • Show
Clear All
new posts

  • #16
    Re: Most suitable grog granularity

    If you are set on designing your own castable you should also consider the addition of burn out fibres to assist drying the refractory when you begin heating. Proprietary castables usually have these added already.you need fibres that burn out at very low temperatures like polypropylene which melt at 160 C and then burn at not much more leaving a network of tiny pipes. You need less than you would think because they are extremely fine (finer than human hair) but require extensive mixing for adequate dispersal (about double the mixing time that you need to make the mix workable) A proprietary castable mix already has these fibres dispersed through the dry mix in the correct proportions so this is something you don't need to worry about when you buy the correct product (another reason I advised using a proprietary castable)
    The castable I use has pretty fine aggregate, about 2mm max.
    Kindled with zeal and fired with passion.

    Comment


    • #17
      Re: Most suitable grog granularity

      Thanks David,

      I had come across the burn-out fibre advice... I was going to add natural rope.

      In what form would you add polypropylene?

      So are you suggesting that I could get away without a coarser aggregate - if 2mm works for a larger oven, could I not get away with 1.5mm for a smaller one (66cm?)
      My oven on a pallet build thread

      Comment


      • #18
        Re: Most suitable grog granularity

        Originally posted by di11on View Post
        Thanks David,

        I had come across the burn-out fibre advice... I was going to add natural rope.

        In what form would you add polypropylene?

        So are you suggesting that I could get away without a coarser aggregate - if 2mm works for a larger oven, could I not get away with 1.5mm for a smaller one (66cm?)
        Natural rope may work once the fibres have burnt away and your oven gets wet again, but they won't help much for the first time you fire because they won't melt or burn out at a really low temperature. Once you've reached around 200-250 C, a danger point for stream explosions. Your natural rope is unlikely to have burnt away, so won't be that much use.
        The polypropylene fibres I use are actually designed for concrete compression strength enhancement, but do a fine job as burn out fibres for refractory. Mine are around 15mm long, you should be able to get them from specialist concrete suppliers. You may also want to consider the addition of stainless steel needle reinforcement for your castable, particularly if you plan on casting in one piece, because the uneven heating creates more stress on large castings. These stainless steel needles are known in the trade as melt extract fibres and are available from refractory suppliers. They should be added at least 2% by weight of dry material used and need good mixing for even dispersal. They make the mix more difficult to handle though, they're not called needles for nothing.
        Regarding aggregate size, the size of your oven has little to do with the mix and the size of the aggregate used. I should think your 1.5 mm aggregate should be fine.
        Last edited by david s; 05-15-2014, 03:43 PM.
        Kindled with zeal and fired with passion.

        Comment


        • #19
          Re: Most suitable grog granularity

          I have read (probably the same) literature on poly fibers providing a means for moisture to escape refractory castings. However, I am not convinced. 99% of the fibers in a given mass will have no avenue to allow the melted fiber to escape and thus provide a means for the moisture to also escape. Melted polypropylene has the same mass as un-melted, and it can't just disappear so I do not buy the theory.

          They certainly do not hurt anything, and help prevent cracking before they melt, so I would still suggest adding them, but silica glass fibers, especially coated ones, would be preferable for both compressive and tensile strength as well as helping with shrinkage cracking.

          Comment


          • #20
            Re: Most suitable grog granularity

            Thanks a million for this Dave - really appreciate all the detailed information. I intend casting mine in sections... so hopefully this will mitigate the explosion risk?

            So, the only advantage of using a coarser aggregate is in reducing the volume I need, right? If this is the case, I might just get away without pounding bricks (since I have more than enough grog already)?

            I did take note of your recommendation to use a commercial castable product. I looked into it and this was all I could find in France. At €166 per 25kg, it would have cost me €1200 for the amount I need... and that's before shipping!

            If I were to pound bricks, do they need to be refractory?

            Edit... would using polypropylene rope do the trick? How thick do the fibres need to be or can they be?
            Last edited by di11on; 05-15-2014, 07:46 PM.
            My oven on a pallet build thread

            Comment


            • #21
              Re: Most suitable grog granularity

              The aggregate is not used to reduce the volume of cementious material for that reason. Concrete (refractory or otherwise) is a matrix of cementious material surrounding a graded aggregate such that the amount of cementious material is both consistent through the mix and minimized, but not because it reduces the amount of cementious material, that is a side effect.

              Obviously, the aggregate has to have the same thermal characteristics as the "glue" so the best would be the same cured castable material reduced to proper gradation, and second best would be a refractory material with similar properties. Firebrick is second best, common fired brick is third, and some natural materials like basalt are a long third.

              Comment


              • #22
                Re: Most suitable grog granularity

                Thanks again TS... think I'm getting my head around it.

                So if I understand correctly, without a coarser aggregate, my mix would be something like a mortar concrete and would require a higher percentage of cement.

                Here's what I'm thinking:

                31% Refractory Cement
                34% Sand
                17% Chamotte 0-0.5mm
                17% Chamotte 0-1.5mm

                Have I got the ratio of cement to fine aggregates ok? I.e. roughly 2/1 fine aggregates / cement?
                My oven on a pallet build thread

                Comment


                • #23
                  Re: Most suitable grog granularity

                  Originally posted by di11on View Post
                  Thanks a million for this Dave - really appreciate all the detailed information. I intend casting mine in sections... so hopefully this will mitigate the explosion risk?

                  So, the only advantage of using a coarser aggregate is in reducing the volume I need, right? If this is the case, I might just get away without pounding bricks (since I have more than enough grog already)?

                  I did take note of your recommendation to use a commercial castable product. I looked into it and this was all I could find in France. At €166 per 25kg, it would have cost me €1200 for the amount I need... and that's before shipping!

                  If I were to pound bricks, do they need to be refractory?

                  Edit... would using polypropylene rope do the trick? How thick do the fibres need to be or can they be?
                  Go for the Prosicast 140 (dense castable) It is still way in excess of what you require (good for 1400 C) and has the correct aggregate, good thermal shock characteristics etc. etc. and only E33.34 for 25 Kg That's cheaper than i can buy it in Australia. Three bags may do you for a small oven with 2" walls.
                  What diameter oven are you planning on?
                  Last edited by david s; 05-16-2014, 01:52 AM.
                  Kindled with zeal and fired with passion.

                  Comment


                  • #24
                    Re: Most suitable grog granularity

                    Originally posted by david s View Post
                    Go for the Prosicast 140 (dense castable) It is still way in excess of what you require (good for 1400 C) and has the correct aggregate, good thermal shock characteristics etc. etc. and only E33.34 for 25 Kg That's cheaper than i can buy it in Australia. Three bags may do you for a small oven with 2" walls.
                    What diameter oven are you planning on?
                    Ha, don't know how I didn't see that - might actually have been a viable option! With shipping and the work I'd have to do to bring back what I already have, it would be a good bit more expensive.

                    There are some pictures in this thread of what I'm planning. Was going for a 66cm oven with 8cm walls... by my calcs needing a volume of around 70L. I'd need 7 bags of that stuff to be sure... so that would be €233 plus shipping. For reference, my grog(chamotte), sand and refractory cement cost me around 130+15+60 delivered... but I'd need one bag of cement anyway for other stuff... assuming €120 for shipping (if I'm lucky) I'm saving around €180 doing it the risky way :-)
                    My oven on a pallet build thread

                    Comment


                    • #25
                      Re: Most suitable grog granularity

                      8 cm walls are pretty thick. I do mine at 6 cm and it's plenty. Recalculate and you'll find you'll save heaps.
                      Kindled with zeal and fired with passion.

                      Comment


                      • #26
                        Re: Most suitable grog granularity

                        Originally posted by di11on View Post
                        Thanks again TS... think I'm getting my head around it.

                        So if I understand correctly, without a coarser aggregate, my mix would be something like a mortar concrete and would require a higher percentage of cement.

                        Here's what I'm thinking:

                        31% Refractory Cement
                        34% Sand
                        17% Chamotte 0-0.5mm
                        17% Chamotte 0-1.5mm

                        Have I got the ratio of cement to fine aggregates ok? I.e. roughly 2/1 fine aggregates / cement?
                        I would go with David's suggestion, in a concrete mix the cementius material must coat the outside of of each particle of aggregate, the design of the mix, selection of aggregates, is intended to allow them to pack together, as the surface of a sphere is a cube of the radius you can see that a single large stone takes less cementius material then 1000 s of small stones,
                        Of more importance then compressive strength is controlling shrinkage you are attempting to compete with a mix designed in a laboratory by experts with years of testing the % chance of equaling their result is ? Again if cost is the only driver and you are not concerned how long it lasts go for it

                        Comment


                        • #27
                          Re: Most suitable grog granularity

                          Thanks for the input guys.

                          If I was starting again from scratch, I think I would go for that castable product - but I am where I am.

                          Also, as I mention in this thread, I am living in a rented property here in France and it has to be a temporary installation. It will essentially be an oven on a pallet so realising that anything could happen when trying to move it some day, I was initially reluctant to put too much money into it. So I'm trying to balance numerous objectives here.

                          That said, I'm planning on casting in sections and I'm hoping the mix will be of better quality than the homebrew mix.
                          My oven on a pallet build thread

                          Comment

                          Working...
                          X