Troweling refractory domes on sand forms

[taharvey]

Has any one had experience of making a dome on a wet sand form, troweling refractory cement to thickness to make a casa style oven?

1. Have you had luck troweling onto the vertical surfaces without an external form? (use low slump mix?)

2. Any thoughts about using ceramic fiber in the refractory mix to reduce cracking?

3. Also, I'm concerned about cracks/leaks in a indoor installation. any thoughts on how to make the lining as durable as possible?

[wild_willy]

Re: Troweling refractory domes on sand forms

Hi Taharvey
Check out the thread on "Castable Refractory Concrete".
I used fatty sand (Brick sand) as a mould then covered it with builders plastic sheet before covering with castable refractory concrete. This stops the sand from absorbing water from the mix and reducing its strength. If you mix it right there is only a little slumping going to happen for the first 6" up the dome or so. I ended up with a cast dome of at least 4" with around 6" at the base line. Don't add to much water to the mix as this weakens the mix and you'll have the mix slumping. I sort of held in in place with a wide plastering type trowel for a little while so that it cured slightly and stayed in place. Mix in small batches (one 20Kg bag at a time. It took under 2 hours to complete. No need for an external form, it gets easier as you work up the dome. Trowel the mix on in stages around the base and keep pushing it up into the right place until it starts to hold its form. Not sure about added ceramic fibre.
After gentle curing over about 2 weeks it works fine - no cracks at all and cooks a mean pizza.
Happy to post pics or supply more info if you need.
Cheers Willy

[ERASMO]

Re: Troweling refractory domes on sand forms

Pics would be great!!

[david s]

Re: Troweling refractory domes on sand forms

Castable manufacturers sometimes add natural fibres which help eliminate slump cracks but they're really there to burn out at low temps which leaves minute pipes which steam can escape from. Reinforcing is usually added in the form of stainless steel "needles" which are not actually needles but flat, curly, non pointy things.A one piece dome is subject to extreme stress due to the uneven heating. The top of the dome gets really hot first while the base is quite cool. This creates uneven expansion. The dome is best made in sections to accomodate the heat expansion.

[Balthazar]

Re: Troweling refractory domes on sand forms

Has any one had experience of making a dome on a wet sand form, troweling refractory cement to thickness to make a casa style oven?

1. Have you had luck troweling onto the vertical surfaces without an external form? (use low slump mix?)

2. Any thoughts about using ceramic fiber in the refractory mix to reduce cracking?

3. Also, I'm concerned about cracks/leaks in a indoor installation. any thoughts on how to make the lining as durable as possible?

I have done exactly what you are asking without a problem. I formed my dome using "brickie's" sand and styrene cut to form the curve. I then covered it in a thin plastic sheet(see pic), and then trowelled on my refractory cement with stainless steel needles. When dry, I dug everything out through the doorway and peeled off the plastic sheet from the inside, which left a nice smooth surface.
The pics show the refractory layer ONLY (before I insulated it).