Castable mix for oven

[wood11]

I had a recommendation from a member two months ago with respect to using crushed firebrick as an aggregate to make a castable mix.
One option was: 3 parts crushed firebrick, 0.5 parts fireclay, 1 part calcium aluminate cement (by volume).

My question: Can the shape be cast in one step? The concern is cracking as the mix sets and cracking during use. The thickness is 50mm.

[fox]

How big is the oven, how do you intend to build it - over sand, in a mould or some other method? Can you buy ready made refactory cement? I don’t think commercial castable refractory cement has clay added but some have stainless steel needles.

[UtahBeehiver]

Davis S is our casting expert on the FB forum. See if you can PM him and talk with him directly.

[david s]

I had a recommendation from a member two months ago with respect to using crushed firebrick as an aggregate to make a castable mix.
One option was: 3 parts crushed firebrick, 0.5 parts fireclay, 1 part calcium aluminate cement (by volume).

My question: Can the shape be cast in one step? The concern is cracking as the mix sets and cracking during use. The thickness is 50mm.

Yes you can cast in one step. Fine cracks will occur during use but never seem to get worse over time.Building in smaller units (bricks) is a way of allowing this uneven heat to be accommodated. Any large cast refractory is subject to cracks because of the uneven heating and resulting thermal expansion. Because the structure is in the form of a dome it can’t fall down.You can increase the strength of the casting by adding stainless steel needle random reinforcing, added to the mix at least 2% by weight. It makes placement more difficult, they are not called needles for nothing. Actually the correct name is melt extract fibres.
You can use crushed firebrick as an aggregate, but if making a casting for the inner dome it needs to be dense castable so the firebrick you are crushing also needs to be dense, not insulating firebricks which are extremely light. Dense firebrick is not easy to crush, i’ve Done some with a hammer and it takes a lot of effort. The brick needs to be crushed down to nothing larger than around 5 mm and not too much brick dust added. Sieve about half of it out. Really far better to purchase a proprietary dense castable refractory which contains the high temperature aggregates, burn out fibres and is formulated to make up a good workable mix. Making your own castable mix could result in a recipe for failure.