Terra Cota Stack

Can Terra cota chimney liner be used as a stack liner, either round or square instead of building with fire bricks

In what capacity are you using the liner? for the riser? They can be used, but they can crack from the extreme temperatures--I've heard of people here on permies cutting them to create an expansion joint--remember that they still need to be insulated to offset the temperatures inside and outside of the riser.

The terra cota can not handle the rapid temperature changes and it will crack. Cutting them to create an expansion joint has been done but not very often and I have yet to hear of one lasting for a very long time. And you would need to wrap it with an insulation blanket.

Currently the best riser to have if you can afford it, is ceramic fiber blanket inside of standard stove pipe. Known as the five minute riser (how long it takes to make one) it can handle any temps up to and over 2300F

If cost is an issue, than a Walker style fireclay / perlite  riser is under $30.00 plus labor.

A low tech riser can be made using clay, straw and puffed rice ( or popcorn ). Since they're stacked in units they're less likely to crack as they have expansion joints.

Mj Phillips,    The folks at Dragonheaters  Flue build part 1 use the clay flue pipes as an effective bell skin but advise to line them (in particular the first bell) with firebrick splits to prevent them from cracking from the thermal shock as Thomas pointed out.

The burn chamber (along with the riser) are meant to be very insulative and light in weight to keep the heat in for clean combustion whereas the chimney liners are dense and heavy which is what you want for your mass.  

With that said, the video which Graham posted shows a very small ratio of insulative material and lots of density from the clay and sand. A nice little DIY cooking stove to make on the cheap but not really something I would call insulative that a rocket mass heater would need to burn cleanly at the temps that can now be reached more easily with ceramic fibre board/blankets. I have made several stoves like in the video before which taught me a lot about the dynamics of rocket stoves (and were fun to build) but found that they cracked terribly and started to crumble apart from the heat.

Well, i have done several risers with it. Would it be tona, schiedel, or else. They work. But crack badly. What helps, is to mortar these, with stuff  like fondulit or mapegrout. A high temp mortar. This defeats the advantage of using those. Which is a light ish weight riser, which would heat up fast. And be standalone.