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Cast dome with 2nd hand arch. Queensland.

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  • #16
    That’s fine. Either side up, it doesn’t matter. It is the gaps between the tiles that you want to create.
    Kindled with zeal and fired with passion.

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    • #17
      Very hot here today (for the gold coast), not going outside until some shade is over my work area.

      My question is about the sand part of the homebrew.

      I got some brickies loam for the cast before xmas thinking I would get some casting done and because the shops shut over the holiday period. Now I am not sure if it is correct after reading some FB posts about brickies loam/sand for homebrew. One post said that it already had clay in it. Is this correct? Do I have to adjust the homebrew recipe or look for different sand?

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      • #18
        Basically any sand will work ok and actually a mix of grain size is better than sand that has the coarser grains sifted out. Some brickies loam does naturally contain a little clay, but probably not enough to cause you any problems. Too much clay can impart more shrinkage than you want, leading to shrinkage cracks. You could test the clay content of your sand by simply filling a glass jar 1/2 full and topping it up to 3/4 full with water. Give it a decent shake and let it stand for 24-48 hrs. The sand will separate from the water with a small layer of clay in between, if there's any there at all.
        The homebrew already has a high clay proportion and extra could impart the shrinkage cracking. However, as it really helps to bind all the other ingredients together to make it easy to apply, apart from imparting refractory qualities, it is an important ingredient.
        Last edited by david s; 01-04-2023, 01:07 AM.
        Kindled with zeal and fired with passion.

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        • #19
          Because of the hot weather, I suggest you time your application of the dome cast for the late afternoon, say around 4 pm. That way the casting will be out of the sun and will be slower to go off. Also the use of chilled water will help. In summer I have to always do this to avoid wet casting material going off too fast. We’re in Townsville so the heat is an even greater problem. Two people applying the wet mix helps get the job done quickly, but make sure you wear rubber gloves or you’ll regret it. Cover the casting the next morning, but don’t use black plastic which will also increase temperature if the sun hits it.
          The addition of the fine polypropylene fibres, apart from their use as burn out fibres to assist water elimination, also assist greatly in reducing early shrinkage cracking which is their primary function for standard concretes.
          Last edited by david s; 01-04-2023, 01:12 PM.
          Kindled with zeal and fired with passion.

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          • #20
            Now I'm back from camping, time to get into the oven again.
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            Here is a pic of the sand in water after letting it sit over night. I can see a very narrow band of clay on top (about 1-2mm) Do I need to adjust the homebew mix to account for this?.

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            • #21
              Laid the fire bricks down and cut the the pieces with an angle grinder (125mm). Soaked the bricks for a couple of hours while I exchanged the correct cutting disc, the inner diameter was incorrect. One disc cut about eight bricks before slowing down. Very happy the bricks are very flat with no edges catching on a trowel to test it.
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              • #22
                There is negligible clay in your sand so no need to reduce clay in the recipe.
                When cutting firebricks the inhalation of brick dust is a dangerous activity, you really need to use a wet saw. Even after soaking the bricks for ages it still makes a large amount of dust as you no doubt found. However it seems like it's too late now. Also a cheap dust mask is inadequate as you've probably also already found out when you blew your nose.
                Kindled with zeal and fired with passion.

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