dome mortar

[Key]

So, I have some pre-mixed refractory mortar. But the joints cant exceed 1/8th inch(3mm). my plan was to set the dome brick with this stuff then fill the back of the dome, where the gap is more like 1/2 inch, with type S and Fireclay mix in a grout bag.
Does anyone see a problem with that? I saw that Stonylake might have done this but it doesnt look like they are active on the site.

[UtahBeehiver]

You can back fill with what we call "Home Brew" 3 sand, 1 lime, 1 portland, 1 fire clay. This can also be used to do the entire dome if you wish. Confirm the refractory mortar is air set not heat set before you proceed..

[Key]

it is heat set! is the concern that they float or move after the arch support is removed? will the back fill mortar hold enough until it is heat cured?

[UtahBeehiver]

Above my pay grade

[Key]

Thanks for the information! I remember stumbling upon your build awhile back. Man, that is sharp! I love the little details like the engraved sun in the top of the dome. love the copper.

[Gulf]

+1 on Russell's answer. I would just go with the homebrew and skip the heat set.

[Key]

Joe, did you use silica sand or play sand? I want the joints pretty tight. I just looked at your build and its really nice! i guess you didn't have any cracking?

[Gulf]

i guess you didn't have any cracking

Ha ha! You wont me to lie to you? Every masonry oven has cracks. All materials shrink and expand with heat. Something has got to give. If the mortar is the correct strength the cracks will be more like separation lines zig- zagging along the bed and head joints upward toward the apex. If the mortar is stronger than the brick, the crack or cracks will shoot almost straight up regardless of the joints splittling some bricks. Mine are the latter. I used a refractory mortar called Firerock. It was an air-set mortar described as an adhesive for 1/8th" joints. It got expensive very quick so I elected to set the brick with it and come back later to point the joints with homebrew. If I had it to do over again, it would be all homebrew.

Edit:

did you use silica sand or play sand?

I would have to go back and check, but I think that it was all purpose sand. I do remember sieving it through an old window screen to remove some almost pea-gravel size aggregate.

[Key]

this is all super helpful! I have already set the walls with the heat set stuff but I will do the arch with home brew. thanks for the heads up on the cracking, I wont freak out so bad when I see that happen.