Heat Mass Battery Ideas Anyone?

Besides cob (and water) what is the best material / method for a heat mass battery?

I've a rsmh to heat my shop with.

Thinking of a concrete block "form" to hold pea gravel mass in place around the first 10' or so of 8" horizontal exhaust pipe exit flue.
I'll insulate behind and under the mass battery bed to keep the heat from dissipating into the wall and floor.

Pea gravel is not the most efficient heat sink I realize, but easy to remove when the time comes.

Any comments?

Thanks.

We've been kicking over some ideas in this thread. Maybe you want to specify if you are talking about heat mass solids, or if you are open to using phase transition materials like in the other thread.

I have read through that thread.

Most of those ideas are a bit more than I can tackle at this time given the time constraints.

As mentioned, I'm thinking of pea gravel surrounded by concrete blocks.

Anyone used this method? Or some similar material?

I'm familiar with phase change material used in solar hot water heating applications.

Anyone used this material for a heat sink battery for a rsmh?

Thanks.