This is not an exact answer to your question but has useful information:

I did a search on the hypertufa forum for "fire" and the first choice was "tufa Under Fire" which focuses on putting tufa objects in kilns to see what happens to it.

Here is a link that might be useful: go here

Hey Crafty --- Ordinary bricks, mortar & concrete are not suitable for high temperature use, so unless the cement itself is rated for "refractory" use, I would advise against it. Regular old Portland-based concrete routinely explodes when exposed to high temps. Look into "fire brick"..."refractory cement"...or "Ciment Fondu" before you go building any kind of fire containment out of cement based materials. Below is a link to a Glossary of Terms for cement, concrete & Hypertufa that might be useful. It covers all those terms & issues. Good luck & play safe --- Tango

Here is a link that might be useful: Cement, Concrete & 'Tufa Glossary