I've experimented with a small TLUD biochar stove made out of food cans. It worked really well, produced charcoal and cleanly flared off the wood gas for cooking; it really was impressive.
The problem, though, was that it only produced a little bit of biochar at a time, burned out in about half an hour (TLUDs are batch stoves) and over time the cans will burn out. I'd like to scale up to a bigger size, 20 gallons or so. But I don't have he ability to work metal, and any metal will wear out over time.
Could I cast 'ferro-refractory' cement with refractory cement and metal lath or chicken wire and rebar? Or would this just implode?
Here is a link for those who do not know what TLUDs are.
I suspect that the heat might weaken the metals you describe.
To keep cement/concrete together (defeat cracking) add fibers. I favor Roxul rock wool insulation this application,it can take the heat. I add it to water and blend it in,then I add the enhanced water to the dry ingredients.
Another idea is to use stainless steel. Two large mixing bowls, cheep stockpots (Harbor freight has a cheep set of 4),kitchen sinks....