Limestone screenings for thermal mass?

Hi, newbie here. I'm wanting to build a RMH for my new shop. I have about a third of a dump truck load of screenings left over from the base material under the slab and I was thinking that it might be suitable to use as my thermal mass. My thought is to make a cinder block enclosure, pack the cells in the block tightly with the screenings and then fill in the enclosure with a few inches of screenings, lay in the ductwork and continue filling in with the screenings. When moistened, these screenings can be packed very tightly. (The base for my slab was so tight that a fully loaded mixer truck didn't even leave ruts when he backed over it.) Anyway, I could then dry the screenings out with a few test fires and some fans.

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.

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.

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.

Great! I think I'm going to give it a try.

Thanks for the input!