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Northern Ontario oven build

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  • #16
    Made more progress this week. A bit of head scratching as I learn but coming along ok. I know mortar joints are supposed to be as thin as possible or around 1/8" but when I looked at many photos on line, the outer gap seems much larger. Does the 1/8" refer to the inside width? I did cut the second course bricks a bit to reduce the exterior gap a bit. I think the 3rd course will require both more cuts and side tapers. Going to work on an adapter or table sled to do that. Cutting the angles was not easy with my saw, despite being a 10" saw. Fun project so far.

    I am a little worried about getting the opening form out. I put wedges under it to give me some vertical clearance but did not allow for side clearance. And I stapled it together. Should have screwed it. I will leave it till I have a few courses about it or I have to move it. I have an oscillating saw and I think that might be the ticket to getting it out.

    Any input thus far would be appreciated.

    Cheers,
    Shawn
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    • #17
      I had the same though about the joint gaps. What type of mortar do you use?

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      • #18
        I believe the reason the manufacturers of refractory mortar recommend using very thin mortar joints is because of the risk of steam spalling. In a thick mortar joint moisture has much further to travel which increases the risk of steam spalling. The addition of burnout fibres in the product is not included as it is with castable refractory. If you can access some fine polypropylene fibres and add these to your mortar mix they will do the job of allowing moisture to escape , providing a fair measure against steam spalling. I believe the same addition should also apply if using homebrew mortar. Only a very small amount is required, but mixing about double the normal time is required to get proper dispersal, as they tend to clump.
        Last edited by david s; 09-13-2021, 12:16 PM.
        Kindled with zeal and fired with passion.

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        • #19
          Mmmm... I went and finished filling in lower row today. Figures... Eyeroll...
          I did the third row (course) today. Gaps were much better.
          Where would I find these fibers? Are they the same that are added to resin to bulk it up?

          Would cutting up fiberglass matting work?

          Thank you for the input

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          • #20
            No, not fibreglass, you need something that will melt at a very low temperature. Very fine polypropylene fibres do the trick. They melt at 160C. They are used primarily to reduce or eliminate early shrinkage cracking in standard concrete, but work well as burnout fibres for refractory materials. I use Sikafiber 48/19, which is a two pack
            system.The fine fibres are the ones to use, don’t add the thicker ones to the mortar, but you can use them to increase flexural strength in a standard concrete mix. There are other brands who sell much the same product.
            Kindled with zeal and fired with passion.

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            • #21
              Originally posted by Petter View Post
              I had the same though about the joint gaps. What type of mortar do you use?
              Thermobond 915 is the Mortar

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              • #22
                Ok. 3/8" seems to be the maximum recommended thickness. I also used Calcium Aluminate mortar like you. My opinion is that as long as it does not crack while curing, you are fine. I used a wet brush about 10-15 minutes after each batch was mortared into place, 10-20 bricks. This smeard all excess mortar into position, tightened the joints and filled them nicely. Then, I let it set for ~30 minutes before cleaning the bricks on the inside. No cracks after curing. I had some really wide joints... There are several ways to do it, but I did it this way and it worked well.

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                • #23
                  I believe the thin joint recommendation is a result of the inevitable phases transformation that occurs when the chemically bonded water is released (not steam from wet mortar). This happens over a broad temperature range well above the boiling point of water. The shrinkage caused by the water release may crack or spall the mortar. Thinner joints reduce this effect.
                  ​​​​​​
                  However, you are building a dome, so a crack or two will not affect its stability.

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                  • #24
                    Thank you Petter . I agree, although I don't have anything to base it on. As much as thinner is better, I don't think the dome will collapse on me for that reason, but I will try to get smaller joints on each new level as the vertical forces become a larger factor and my brick cutting skills get a little better.

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                    • #25
                      Originally posted by Petter View Post
                      I believe the thin joint recommendation is a result of the inevitable phases transformation that occurs when the chemically bonded water is released (not steam from wet mortar). This happens over a broad temperature range well above the boiling point of water. The shrinkage caused by the water release may crack or spall the mortar. Thinner joints reduce this effect.
                      ​​​​​​
                      However, you are building a dome, so a crack or two will not affect its stability.
                      It is the removal of the mechanical water rather than the chemical water that is far more potentially damaging to any refractory material for the temperatures and method to which we fire. Whether mechanical or chemical in nature the removal of both is enhanced if the burnout fibres are included.
                      Here’s a pretty good document explaining water and refractory material.
                      https://thinkhwi.com/curing-dryout-refractories/
                      Kindled with zeal and fired with passion.

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                      • #26
                        Petter I found Fiberstrand 150 fibres locally, 1 lb bags. It says on the spec sheet about 0.67-1 lb/cu yd of concrete so, as you say, I won't use much. Does that proportion sound about right for my application? The specs on Thermobond 915 do not say how much one bag yields directly, but rather 80 bricks with 1/8" joints. Would I be wrong to use one bag of fibres per one bag of refractory?
                        Last edited by Shawnr; 09-15-2021, 11:56 AM. Reason: Corrrect typos

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                        • #27
                          Yes they’re the right fibres and dosage about right, although if using as burnout fibres you don’t need as many as when using them in a standard concrete mix. I don’t know the weight or size of your bags, but if making a batch of mortar use about 2g for every 10 kg of dry mortar. Pull the fibres apart, spread them through the dry materials and mix for double the time you normally would as they tend to clump and resist dispersing throughout the mix.
                          Kindled with zeal and fired with passion.

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