John Cunkelman

No. As in the square or round concrete liner for chimneys. It is kinda brown or tan in color.

TYVM  for all the information. I think I will just use the pipe as a form and use cob or concrete.  could i use chimney liner and then surround it with cob. I am a guy who likes to build it as "bullet proof" and durable as i can.

Thank  you for the fed back.  I can see cob and concrete holding up in the cooler heat runs for the thermal mass and holding shape after the duct work has rusted out or such.  If I were to use gravel or rocks around the duct work, what is the life span of it, if I build a "good" rocket stove?

I am new to the site and new to rocket stoves. I have been reading and watching videos. It all makes sense other than the pipes that run in the ground or thermal heat bank. I see that most seem to have duct work pipe. How does this hold up or is it only form for cob fill. Can you use concrete instead of cob?