Homebrew Castable - larger aggregate?

[Ronstarch]

Hi All,
Without opening the Pandora's box of whether the Homebrew mortar mix that is discussed here can be used as a castable "refractory" for casting an oven, I plan to give it a shot with an upcoming build. This is the Portland, lime, fireclay, sand, sand, sand diy homebrew, with SS melt extract fibers/burnout fibers.

My question: One difference between the Homebrew and some commercial castable refractory products is that the Homebrew only has sand as an aggregate (because it was originally designed as mortar, not castable). In trying to use this stuff to cast a whole oven, would it make sense to add some larger aggregate, like some type of pebble-sized material? I figure this would add strength, much like concrete. Risk is obviously spalling/explosion if the wrong material is used. So is there a low cost larger aggregate that would stand up to oven temperatures that could be added to the Homebrew to make it more like concrete from a structural/strength standpoint? Anyone done something like this?

I know, "at that point you should just use a commercial castable refractory." Granted, but humor me here please, if you have a suggestion.

Thanks!

[UtahBeehiver]

PM David S, he is our casting expert. But I believe that the SS melt fibers give you the strength you need for the casting material.

[david s]

Hi All,
Without opening the Pandora's box of whether the Homebrew mortar mix that is discussed here can be used as a castable "refractory" for casting an oven, I plan to give it a shot with an upcoming build. This is the Portland, lime, fireclay, sand, sand, sand diy homebrew, with SS melt extract fibers/burnout fibers.

My question: One difference between the Homebrew and some commercial castable refractory products is that the Homebrew only has sand as an aggregate (because it was originally designed as mortar, not castable). In trying to use this stuff to cast a whole oven, would it make sense to add some larger aggregate, like some type of pebble-sized material? I figure this would add strength, much like concrete. Risk is obviously spalling/explosion if the wrong material is used. So is there a low cost larger aggregate that would stand up to oven temperatures that could be added to the Homebrew to make it more like concrete from a structural/strength standpoint? Anyone done something like this?

I know, "at that point you should just use a commercial castable refractory." Granted, but humor me here please, if you have a suggestion.

Thanks!

This is a very good question. Some materials have unsuitable thermal expansion rates at different temperatures. Proprietary castables contain high temperature aggregates that can withstand temperatures around three times greater than we would use. Unfortunately the 500-650 C range is where many materials undergo rapid and quite variable thermal expansion. If you think about it a brick oven is a castable with extremely large aggregate. The best aggregate you can use therefore is broken bricks which are already sintered and quite refractory. . Everything I’ve read about castables though always use nothing larger than 6 mm.

[Ronstarch]

David, thanks for the reply, makes a lot of sense. I called around a few places near me, and no one has firebrick grog or sells damaged firebrick at a discount. What I do have access to, in abundance, is regular old clay brick from houses built in the 1890s-1920s. I could smash them up and mix the small pieces into the homebrew. Think that could work, and be worthwhile, or would it actually be worse than no large aggregate at all?

[david s]

You should sieve out the fine dust and not add it to the mix. Crushing the bricks with a hammer is pretty hard work. I’ve done a bit of crushing dense firebricks. After you’ve done 5 you’ll know you’ve done some work.