Mortar for firebrick arch

[aurican33]

So I'm working towards my build, doing a cast dome and I want to put a firebrick entrance and arch on it because i think it looks great. Can I use my castable refractory as mortar for the firebricks? (its proprietary from mt. savage, not home brew). If not, will the Rutlans refractory cement on amazon work? My supplier sells "Refractory Mortar" at $60 for a 10lb bag... i cant imagine i'll need that much dry mortar just to do an arch and to mortar the dome to my hearth right?

[david s]

Most castable refractory has resonantly fine aggregate but still around 5 mm making it unsuitable as a brick mortar. You could sieve out the most coarse aggregate or use the home res mortar 3:1:1:1 sand, hydrated lime, OPC, powdered clay.

[aurican33]

david s Thanks for much for the info. So given I can't really use the castable refractory, I decided to go and buy a 50lb bag of drymix heatstop II. My Father in Law wants an oven built at his cottage so I'll need the extra anyways.

Man I'm getting close to casting the dome and I'm getting nervous! This project has been insane.. I'm just a computer programmer, never done anything like this ever but I want a wood fired oven SO BAD. Took me 2 months to build the right kind of trolley.. I had to get 1000lb casters, learn how to use a miter saw, learn how to use a drill, a jigsaw... Well, time to learn how to use a mortar mixing drill bit! uhhhh .. I can use one of those to mix castable refractory in a bucket.. right?

[david s]

No, it’s much better to mix it in a barrow with a spade. You can then asses the consistency a lot better. The stuff goes off very quickly so don’t mix more than half a 25 kg bag at a time.

[aurican33]

So TIL I should check this forum before I just gung-ho go at it. I did use the bucket and I tried 25kg at first. I did not get to the bottom of the bucket before it was unworkable. So I used a smaller bucket and made many many many small batches (like 5lb/batch) and that went well. The consistency wasn't perfect between each mix but all in all the dome ended up great! I left it with plastic on it for 24 hrs and am now air drying for a few days. Looks plenty strong, thick and wonderful! I do have a couple hairline cracks but i'm not concerned at all.

Next up, cutting ceramic fiberboard and laying firebricks! Whee what a project!!!

[david s]

Too late now as you’ve already finished the cast, but for future use or anyone reading this thread, castable refractory goes off fast and mixing tools, bucket or barrow must be washed out thoroughly before making a new batch or any residue left behind will accelerate the new batch. Sorry, I should have mentioned this in my previous post. The same actually applies with standard cement, but the reaction is nowhere near as marked as it is with calcium aluminate cement.