Strengthening Inside of the Dome

[Vivi88]

Hey guys!

So I finished my inner refractory dome using the gym ball method. It worked relatively well, however one thing I've noticed is because I was mixing multiple refractory bags in a wheelbarrow several times, the mixture wasn't always 100% consistent. This caused certain layers to drop down slightly, overlapping the lower layers that had hardened. This means the inner done is uneven and at certain areas looks cracked. I'm thinking of adding a layer on the inside of the dome to even out the overlapping and fill in the cracks.

How would you recommend I go about doing this? Should I simply grind out the dome a bit to be even and then add a layer, or just add a layer on top of the overlapping?

[david s]

If it’s a one piece casting you are going to get some fine hairline cracks anyway. Might as well just leave it. As a tip if you happen to do another, castable refractory should be washed out of your barrow and tools after each mix as any that has gone off will just accelerate the subsequent batch when mixing. Chilled water also helps in delaying it going off giving you more working time.

[Vivi88]

If it’s a one piece casting you are going to get some fine hairline cracks anyway. Might as well just leave it. As a tip if you happen to do another, castable refractory should be washed out of your barrow and tools after each mix as any that has gone off will just accelerate the subsequent batch when mixing. Chilled water also helps in delaying it going off giving you more working time.

Well it was meant to be a one piece casting but I ran out of materials half way up the dome (I underestimated how much of the refractory would sag down resulting in me having to use more than intended) so I finished the top half of the dome the following day.
I'm worried as well that if I now add an inner layer into the dome that this could cause issues when I start curing the dome as the water vapour would be trying to escape a much harder outer dome layer that has had 2+ weeks to cure already.

I wouldn't say the cracks are hairline, they are big but I don't know if thats a result of the oven actually cracking or the way I layered the refractory.

[david s]

Calcium aluminate cements do not require extended damp curing like Portland cement. They achieve full strength pretty much in 24 hrs, but any filling or subsequent layers should be done when the material is still moist, somewhere between wet and dry. It’s your call whether to add more, but you may now need to dampen the casting first. Another trick to help the bonding is to paint the cracks with a thin slurry of a richer and finer mix prior to filling them. To achieve this simply sieve out the aggregate with a fine sieve before adding water to it. Leave it for 5 mins before mixing up some unsieved castable to peanut butter consistency to fill the cracks. I think you’re asking for trouble if you try to add a thin layer over the whole of the inside surface.

[Vivi88]

Calcium aluminate cements do not require extended damp curing like Portland cement. They achieve full strength pretty much in 24 hrs, but any filling or subsequent layers should be done when the material is still moist, somewhere between wet and dry. It’s your call whether to add more, but you may now need to dampen the casting first. Another trick to help the bonding is to paint the cracks with a thin slurry of a richer and finer mix prior to filling them. To achieve this simply sieve out the aggregate with a fine sieve before adding water to it. Leave it for 5 mins before mixing up some unsieved castable to peanut butter consistency to fill the cracks. I think you’re asking for trouble if you try to add a thin layer over the whole of the inside surface.

Thanks so much, I'll give your method a go and avoid doing the entire dome. I don't know if it'll just be worth trying to grind out the cracks if they're not too deep.

Also how long is the shelf life on most refractories? I have a opened bag from two weeks ago that I can use to fill in the cracks, but I've heard that too much exposure to oxygen actually makes the mix go off and no longer work as well under heat.