Help I have been procrastinating, and cannot find info on whether a 40lb bag of refractory cement (pricey, I know) can substitute as well for my 1/2 Halverson clay 1/2 vermiculite. Our drought this summer made all my clay sources too heavy, my grandkids (and I) cannot roll it out enough to knead the insulation in and make hard "mud pies" I pound in between the two metal skins of the pipes. So a friend came by and asked why we are working so hard. Just get some refrac' cement.
This is my third rocket stove. My first one didn't work as well as my 2nd because I used well casing for the inside burner liner, and packed the rest with vermiculite (mostly against the inside well casing, 3/8"+ steel) and sand and river rock gravel. The oustide burner liner is a 55 gal drum. Over that is a discarded 30" dia industrial water pressure tank. The vent is 8" sched40 gas pipe, steel, welded and bolted (coupling) to old pressure tank, with 30' on the floor and 15' up to the elbo that exits the house. It works fine after 30 minutes of burning carefully with small wood until the core gets hot enough to rocket.