for shells (clam, mussel) to compost away. I have shells that have been "bouncing" around in my pile for years now. This is not a "hot" pile, rather a "digester" pile for kitchen and yard wastes. Occasionally I'll toss in some dry molasses to add some "boost", but it is rarely turned or watered, etc. location directly under the shade of a Chinese
Tallow ( yeah I know - but it was here long before I arrived).

All shells from seafood are made of calcium (i.e. calcium cabonate - lime), a major mineral for soil building. It could takes years, if you don't grind them or break them up first.

I usually throw them away, or usde them as "path gravel", or decorations in garden beds.

I've seen shells in the limestone in my neighborhood. It takes eons of repeated acid rain for them to dissolve and turn into limestone.