Philip Baldwin : I am not sure exactly what you mean when you wrote about ''Chimney mortar'' but most of these Dry Rockhard! Their expansion rates will be different
than the metal ! I expect that they will soon crack, and fall apart. Refractory Cement is kinda $pendy and I am very sure of failure of this material too! The best use of
a Refractory Cement in a
Rocket Mass Heater RMH Build is for glueing broken Bricks back together !
These materials Are not insulating !
Actually counter-intuitively, Rusting is not even close to the reason that Iron / Steel Metals fail ! !!! A quick google search for '' High Temperature Hydrogen Attack '',
or Hydrogen and Steam Embrittlement Will help understand the Fate of Iron / Steel metal in those locations.
A large part of the difference between the metal core of your Rocket and the barrel, is the barrel will only occasionally see 1,000ºf -not the 1,600ºF and above that you
can expect from its core !
The Barrels Heat Exchanger acts exactly like the Heat Exchanger within the " Blower cabinet '' of any of the Fossil Fuel Fired Forced-air Furnace. Both are made of very
similar Ductile Steel, All F.F.F.F-a.Furnaces Heat exchangers use a Thinner gage ductile steel than even the lightest Barrels/Drum ! The furnaces Heat exchanger usually
Gets a 20 year warrantee !
Hope this is timely, and you find it useful. For the Good of the Craft ! Big AL