continuous refractory cast: dome, vent & chimney liner

[Jeffrey-O]

I wanted to share my build concept with the forum in the sincere hope that users will challenge any inadvisable notions.

I intend to build a backyard 36” WFO with gas assist. The floor cooking surface will be purchased high-quality refractory brick. I want to cast the dome (@2.5” thickness), vent (also @ 2.5”) AND chimney stack (6” ID @ 1” thickness) from engineered refractory mortar, and I am thinking to use 2-3% 12mm chopped basalt fiber for crack resistance. I was sortof on the fence about this for the refractory dome because there is some sort of binder agent in these chopped fibers (otherwise, they would just be a pile of hair), and I’m not certain what the binder’s composition is. But I’m thinking that WHATEVER it is (let’s assume UNhealthy), it’ll surely be burned out in the initial curing fires, and so I needn’t worry about health effects. Is that reasoning reckless?

Anyway, the concept is to cast the dome, vent AND chimney liner in refractory around one cleverly-conceived mold, so that it’s all continuous. After carefully removing my mold, I would insulate the dome at 3” thick. Has anyone used basalt felt mat for this purpose instead of the common ceramic fabric? I was thinking to do 1” insulation around on the top/sides if the vent/entrance and also around the chimney. Final render would be a basalt fiber-reinforced Portland concrete, probably with Edencrete admix and a little colloidal silica, which I would limewash for aesthetics. Would it be prudent to do a final water-proofing on type of this render?

A spark arresting chimney cap would go on top. Anything ill-conceived about what I’m describing? I like the idea that there is continuous refractory on all interior surfaces, covered with continuous insulation, finally covered by continuous render.

[david s]

The problem with a monolithic casting is the uneven heating and therefore uneven thermal expansion it will experience. This usually results in the casting to decide that it needs an expansion joint which results in a crack. From my own experience and also from what I've seen on other one piece domes, is that it usually results in a vertical crack opposite the oven mouth.This does not effect the operation of the oven as a dome is a self supporting structure. My own oven at home is a one piece dome and has not cracked, although my first mobile oven with an identical dome cast from the same material, of identical thickness, out of the same mould did crack in that position. After 8 years of use I rebuilt the oven, but replaced it with a 3 piece, multi sectioned dome. Some oven manufacturers do offer one piece domes as an alternative to their multi sectioned ones, especially for mobile applications. Road travel and vibrations are a problem if the dome and gallery are multi sectioned, so some prefer a one piece dome even if it does increase the risk of a crack developing at the back. As the dome sees much higher temperatures than the gallery this difference in thermal expansion is an even greater problem and a good case for at least having a separate casting for the gallery.

Regarding your proposed use of "engineered refractory mortar", You need to be careful because it will not contain the required burnout fibres that reduce the risk of steam spalling. They recommend that a thickness of 1/8" not be exceeded, I think for this reason. You would be better to obtain a dense castable refractory which does contain the burn out fibres. Howevrr, a far cheaper alternatives is to use the homebrew as a castable, but you’d need to add the burnout fibres to it. It is far more user friendly as well as costing peanuts compared to castable refractory which is designed to withstand service temperatures around double that of an oven. You are paying for high temperature aggregates not needed. It also contains calcium aluminate cement CAC which is tricky to work with, especially if you've not had experience using it. before. There are a number of well documented homebrew casts on this site that you may find useful. Generally the recommended reinforcing for castable refractory is melt extract fibres (stainless steel needles). Needles because their thin size can dissipate excess heat to the refractory that surrounds them and stainless so they won't rust in a heat environment that accelerates corrosion.

Regarding the basalt fibres, should be ok for the service temperature of an oven, the ones I use don't appear to have any binder, but have a range of fibre thickness, some around 0.3mm down to some as fine as human hair.
My observations with fibre additions in concretes, both OPC and CAC basesd, has demonstrated that the thinner the fibre the greater the difficulty of adequate dispersion (clumping issues). The very fine pp fibres requiring extended mixing time. As the basalt fibres (at least the ones I obtained) also contain some very fine fibres, they too require the same extended mixing treatment. By contrast the AR fibreglass fibres I also use, as used by the concrete countertop manufacturers, have a uniform thickness of 0.5mm and disperse much more easily. Regarding the performance of both the basalt fibres are marginally stronger than the AR fibres up to 400C, but marginally weaker over 400 C and have a slightly higher temperature resistance, but both ok in the range to which we fire. The cost of the two is a further consideration, but both are expensive. I paid around the same for both, but the AR fibres I prefer to work with. I currently use 50/50 of each.
As you have also mentioned Edencrete, which we also use, they have not done any testing on its use with CAC only OPC. We use it in the concrete supporting slab, not the refractory. I imagine that it requires even more mixing based on the observation described. It is predispersed in a liquid pre diluted additive, but as the fibres are not visible it’s hard to tell. My own tests showed a 50% increase in flexural strength over control samples without it.

Reports of builders attempting brick builds of mobile ovens have resulted in reports of the falling to bits pretty quickly. The cause presumably being the weakening of the bonds at the mortar joints because of thermal cycling, expansion and contraction. A crack in the inner dome is highly unlikely to affect the outer render when there’s a thick insulating layer between the two.

A further disadvantage of a monolithic casting is the problem of relocating it, however, if you use the sandcastle method it can be cast in situ to make relocation unnecessary.

Regarding uneven expansion, experience shows that loosely laid small area floor bricks last better than either mortared floor bricks or large section floor bricks. This is because the loose laying allows each brick to expand individually. The same principle applies for brick units or refractory sections in the dome.

[UtahBeehiver]

FYI, David S is our resident casting expert and has by far the most experience and valid info on casting, so follow his advice.

[Jeffrey-O]

Thanks, David S. OK, understood that burn-out fibers are a must and this provides permeability for steam to avoid spalling. So maybe I'll cast a one-piece dome with a dense castable refractory with burn-out fibers (with some basalt fibers for good measure--and fingers crossed that it won't crack) and cast the vent/gallery separately and then join with mortar (giving some overlap on the exterior). Maybe chimney refractory liner as a separate casting too? Is any type of refractory mortar suitable, or is there something in particular I should seek out?

I'm intrigued that such a crack as you describe doesn't affect the operation of the oven. What's happening thermodynamically? Would you expect that the outer rendered layer where the crack is ends up being ever-so-warmer than the rest of the render?

I can understand how transporting a monolithic cast would be prone to cracking. Especially if it's not at equilibrium temperature throughout the refractory and it experiences vibration. A variable expansion gradient throughout that is looking for any way to equalize, and the vibration would do the trick! But what happens with brick and mortar domes, or even the a 3-piece dome--through their heat cycling? There will always be uneven heating, and some unequal expansion. Is it that the more joints means smaller movements per joint? And so the effects are less pronounced/perceptible than all the movement happening at a single crack?

[david s]

Thanks, David S. OK, understood that burn-out fibers are a must and this provides permeability for steam to avoid spalling. So maybe I'll cast a one-piece dome with a dense castable refractory with burn-out fibers (with some basalt fibers for good measure--and fingers crossed that it won't crack) and cast the vent/gallery separately and then join with mortar (giving some overlap on the exterior). Maybe chimney refractory liner as a separate casting too? Is any type of refractory mortar suitable, or is there something in particular I should seek out?

I'm intrigued that such a crack as you describe doesn't affect the operation of the oven. What's happening thermodynamically? Would you expect that the outer rendered layer where the crack is ends up being ever-so-warmer than the rest of the render?

I can understand how transporting a monolithic cast would be prone to cracking. Especially if it's not at equilibrium temperature throughout the refractory and it experiences vibration. A variable expansion gradient throughout that is looking for any way to equalize, and the vibration would do the trick! But what happens with brick and mortar domes, or even the a 3-piece dome--through their heat cycling? There will always be uneven heating, and some unequal expansion. Is it that the more joints means smaller movements per joint? And so the effects are less pronounced/perceptible than all the movement happening at a single crack?

I've added more to post #2 for you to re-read.

[Jeffrey-O]

Thanks for this input, David S. I'll have a look at some of these "homebrew" refractory mixes. Looking to get started in earnest early in the spring and hopefully be cooking this summer.