One of my concerns is that the lime might react with the galvanized metal ductwork. Does anyone have any experience or insights on this? Any feedback would be much appreciated.
Glenn Herbert wrote:I can't speak about reactivity, but if the limestone packs as tightly as you describe, it will probably work fine as the mass. Sand makes a poor thermal mass because there are tiny air spaces between all the grains. Your limestone with a full range of particle sizes should be able to fill all the air spaces and have good particle-to-particle contact.
Add a bit of quicklime, sand and gravel, and you have a nice roman concrete. Gravel and stones would increase the mass, fill the space faster, and save you some work. I don't think limestone would react much with zinc.
Thanks for the input!