42" Pompeii Oven in Jamaica

[wthwaites]

I've laid out 4" of ceramic fiber blanket and have started my stucco render as a finish. I'll post pics tomorrow.
For the stucco recipe, I have been using david s recommended recipe ratio of 4:1:1 (Sand: Hydrated Lime: Cement) but without hydrated lime as I have not been able to am not able to source it. So, technically the ratio is really 4:1 ( Granite Sand: Cement).
Will the lack of hydrated lime be a problem? I hope not since I'm about 1" thick so far with the render and approaching the finishing coat!

[MarkJerling]

Aha, thanks for explaining!

[wthwaites]

The cast was done over some formwork (thin plywood cut to size and wedged on top of bricks). It was several layers over a two day period until it got to the same thickness of the side walls.

[MarkJerling]

I got the correct fibers! So i managed to pour the slab for the squirrel tail chimney.
Now I can move on to the final stages for the build - insulation and then the final render!

Nice! How did you do it? Did you do pre-cast slabs, or did you cast it in place over some form of formwork?

[wthwaites]

I got the correct fibers! So i managed to pour the slab for the squirrel tail chimney.
Now I can move on to the final stages for the build - insulation and then the final render!

[wthwaites]

I was told it was glass fibres for concrete; I will double check tomorrow. If they aren't, they can't be used?

Apparently the glass fibers I got are purposed for building boats. I guess they are not the correct fibers needed.

[david s]

I was told it was glass fibres for concrete; I will double check tomorrow. If they aren't, they can't be used?

If they're not they will react with the cement and get destroyed. The AR fibres are coated in zirconium for protection against chemical reaction with the cement. Some have far more coating than others. How you tell, I don't know, but if they're pretty cheap, off eBay or similar, they may not have much zirconium. I pay around $200 for 20 kg. If they say suitable for concrete, then presumably you have the correct product. Use a reputable supplier and you should be ok.

[wthwaites]

Make sure they are the AR alkaline resistant ones.

I was told it was glass fibres for concrete; I will double check tomorrow. If they aren't, they can't be used?

[david s]

Make sure they are the AR alkaline resistant ones.

[Gulf]

I'm rooting for you. I have a mental problem with needles of any kind lol. I'm looking forward to how it all turns out.

[wthwaites]

I got Fibreglass fibres, so I'll be moving ahead with casting the slabs with both that and the polypropylene fibres. That's the best I can do with what I can source. Lets hope for the best!

[wthwaites]

Ok Thanks David. I'll search around for fibreglass fibres.

[david s]

The reason melt extract fibres (stainless needles) are recommended is that being a very small diameter they present a much larger surface area than thicker stuff. This enables them to dissipate their heat more easily to the refractory that surrounds them, thus reducing expansion issues of the two different materials. In normal ambient conditions eg red in a concrete beam the rate of temperature change is slow enough for the two materials to be quite compatible. But when extreme heat is applied > 200C/hr, then problems are likely to occur. You could try using some AR fibreglass fibres in the castable, glass has a melting point of around 900C so is well above our temp range.I use them as well as the stainless needles.

[wthwaites]

So the fibers aren't needed?
I was able to find needles, but they weren't stainless steel and was only available in 55 lb buckets at $250 - way out of my price range! For the amount I would be casting (essentially a 2' x 4' x 3" triangular slab divided into three pieces), I also think that the purchase would be a waste.
I'm all ears for alternative solutions. I may be able to find stainless steel threaded rods. Will that suffice?

[david s]

The polypropylene fibres will melt at 160C so are not suitable for reinforcing in this application. Once they have melted they leave behind a network of minute pipes that steam can pass through. This reduces the incidence of steam spalling provided heat up is taken slowly.
The recommended reinforcing for refractory is stainless steel needles, but their use is not mandatory. Their drawback is their expense and difficulty in handling when placing the castable.