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Erica Wisner
Post     Subject: Fireproof Concrete mix

Yup. 'Refractory' is the word you want; a thin-set mortar is fine; ideally you want it to be rated for over 2400 F, but something that can handle about 2000 will do if that's all you can find locally.

We often use fire clay (refractory clay) instead of chemical-set mortars; it's easy to work with, and doesn't damage the brick if something needs to get taken apart later (like to replace a cracked brick).

You can use ordinary concrete and cement mortars in the casing (outer 3-4") of the heater, just not in the firebox area.
If you do use Portland cement products, be sure there is a good expansion joint (a layer of flexible insulation or something like that) between firebox and surface masonry, so the firebox doesn't crack the concrete when it expands with heat. Earthen masonry is a lot easier to patch than cracked concrete; and it's also possible for the force to be relieved inward in ways that screw up your firebox.

John Elliott
Post     Subject: Fireproof Concrete mix

Welcome to Permies, Mark!

Concrete is not generally thought of as "fireproof" because the intense heat of a fire can cause chemical reactions to take place in the concrete that weaken it. That said, there are refractory mortars that are useful for bonding chimney bricks together and they can be considered "fireproof". Generally they have a higher clay content and are made with more water than regular mortars so that they are runnier and make for a thinner bond.

Here is a link with recipes for refractory mortar.
Mark Leathers
Post     Subject: Fireproof Concrete mix

I am looking for a good fireproof concrete mix. I am pulling out my Schrader Stove and replacing it with a rocket stove. My stove now sets on a brick hearth and surround and I plan to enclose the whole brick area. The area is about 4x6x8. So, I want to build my fire box with brick and fireproof concrete. Any suggestions?

Mark S. Leathers
Rolla, MO.