Casting a flue gallery over outer arch for chimney connection

[Steellearning]

Can I use a regular concrete mix to cast a flue gallery over my outer arch for my chimney pipe connection or do I need a castable refractory mix?
Is there some sort of home-brew I could use if I need a refractory mix? I've attached a photo of a flue gallery. Mine will not be so elegant

[david s]

Can I use a regular concrete mix to cast a flue gallery over my outer arch for my chimney pipe connection or do I need a castable refractory mix?
Is there some sort of home-brew I could use if I need a refractory mix? I've attached a photo of a flue gallery. Mine will not be so elegant

No, normal concrete will fail there, it's too hot. You need to use either castable refractory (best) or homebrew 3:1:1:1 sand, lime Portland cement, clay (second best)

[Steellearning]

No, normal concrete will fail there, it's too hot. You need to use either castable refractory (best) or homebrew 3:1:1:1 sand, lime Portland cement, clay (second best)

Thanks again David
I can get a hold of castable refractory, do I need to reinforce it with rebar or can I use it as it is since it is just a small piece? I wouldn't know where to get steel fibres for reinforcement.

[david s]

I don't think rebar is a good idea. For the sudden heat up the heat will rush to the denser steel raising its temp relative to the surrounding refractory and hence different expansion rates. At least this is what I think happens. The industry standard is stainless needles which won't rust like ordinary steel. Remember that heat accelerates any corrosion problems too. Being small and thin they can dissipate their heat to the surrounding refractory more easily than a thick lump of steel. If you can't access the ss needles just cast without reinforcing.

[gypsygirl]

Hello! I was about to ask the same thing! I'm considering casting with refractory cement to allow for a flue and chimney. We are retrofitting a chimney onto a 12-year old traditional cobb pizza oven. My next question is can I then bond the new flue gallery to the mud oven using the sand/clay mixture OR will the refractory cement stick to the oven, which is all made of natural materials (clay, sand, straw, etc)?

[david s]

I doubt whether you'd get a decent bond onto the dry cob. Try roughening and wetting the cob first.

[gypsygirl]

Thank you David. I'll make sure to do that. Wherever I use more cob mix, I also intend to add sodium silicate to the mud for extra 'stickiness'. I have decided to abandon the casting idea and think I'm back to replacing the arch firebricks, which seems to be the best method. Not being a skilled mason, trying to figure out how to create the opening at the top of the arch for the 6 inch double wall stove pipe's anchor plate is the next challenge.