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Insulation and ISTRA40 Mortar Mix questions - South Australia

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  • Insulation and ISTRA40 Mortar Mix questions - South Australia

    Hi,

    Apologies, I posted this on "getting started" but then found there is a Regional Forum section. This is probably a better place to post my questions.

    I am based in Adelaide. I am about to start my dome build. I plan on using firebricks for the oven floor and clay pavers (230 by 114 by 70 from LittleHampton Bricks and Pavers) for the Dome.

    Just wondering if anyone can help with a couple of questions.

    1) I poured/layed 120mm of 5:1 vermicrete on top of the reinforced concrete top about a week ago. It is slowly curing. From what I have read this should be sufficient for insulation under the floow however after reading a few threads by some advocating CalSil board on top of the vermicrete I am starting to doubt myself. Do you think the vermicrete is enough or should I add 25mm of CalSil board on top as well?

    2) Regarding the mortar mix - I was thinking of using ISTRA 40. ISTRA40 is a high temperature calcium aluminate cement. I have used this in the past to construct a brick charcoal grill and it worked well and set extremely hard (although it goes off quite quickly so small batches at a time). I used a ratio (weight) of 4:1:0.4 of dry washed sand:ISTRA40: water in the past as recommended by Bianco and Adelaide Brighton Cement. I called ABC the other day and confirmed this mix for pizza oven construction and they agreed it was appropriate. Has anybody in Australia had any experience with this product? Does my proposal sound reasonable? Forno bravo recommend a different mix with the Calcium Aluminate cement and also recommend adding fireclay.

    Cheers

  • #2
    1. The addition of the cal sil board over the vermicrete will give you super underfloor insulation. As 100mm of 1:5 vermicrete is considered adequate and you have 120mm, then you probably have plenty. The addition of more is subject to the rule of diminishing returns. But it is prudent to give it plenty of time to dry before building over it as it is difficult to eliminate moisture from under the floor. If you haven't already done so, drill a few holes through the supporting slab from underneath so moisture will have an escape path as it runs away from heat. (See my attached experiment on vermicrete drying.)

    2. Most oven builders have found the home-brew mortar mix perfectly adequate, more worker friendly and cheaper than proprietary calcium aluminate products. I don't think the extra price is worth it, although for the top third of the dome it may be a better choice. The oven floor bricks are best laid loose without mortar. The addition of clay whilst adding refractory properties and stickiness, also increases shrinkage.

    If you are planning on laying the clay pavers on edge then the 50mm thickness is an insufficient mortar joint to survive the thermal cycling and resultant expansion contraction of the oven. Perhaps backing it up with a refractory layer or home-brew over the top may work. In addition just because the pavers are made of clay does not ensure that they will be adequate to withstand the environment of an oven with internal fire, after all they have been designed for a different purpose. Some pavers are ok some not. Hopefully your supplier can advise you. Generally the creamy coloured pavers are more likely to be a better choice.

    Vermicrete insulating slab copy.doc.zip
    Attached Files
    Last edited by david s; 05-22-2020, 05:17 PM.
    Kindled with zeal and fired with passion.

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    • #3
      Originally posted by david s View Post
      1.

      If you are planning on laying the clay pavers on edge then the 50mm thickness is an insufficient mortar joint to survive the thermal cycling and resultant expansion contraction of the oven. Perhaps backing it up with a refractory layer or home-brew over the top may work. In addition just because the pavers are made of clay does not ensure that they will be adequate to withstand the environment of an oven with internal fire, after all they have been designed for a different purpose. Some pavers are ok some not. Hopefully your supplier can advise you. Generally the creamy coloured pavers are more likely to be a better choice.

      Vermicrete insulating slab copy.doc.zip
      The pavers are 230 by 114 by 70 mm. What do you mean by laying the clay pavers on edge? I was going to use half bricks for the dome.

      Comment


      • #4
        Originally posted by Papasa View Post

        The pavers are 230 by 114 by 70 mm. What do you mean by laying the clay pavers on edge? I was going to use half bricks for the dome.
        Ok, but you did post "I plan on using firebricks for the oven floor and clay pavers (230 by 114 by 70 from LittleHampton Bricks and Pavers) for the Dome."
        Kindled with zeal and fired with passion.

        Comment

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