Please accept this from a newbie. I am adding a few processed egg shells to my food mix figuring that this additive will help with pH some. I read that the tiny bits may be used as grit, too.
Again I am no authority.
Carol

I am buying a old blender today at the thrift store...
We buy free ranging eggs & the shells are great...
Will grind them up to a near powder & include a bit with each feeding...
May put the ground up egg shells into a mason sprinkle jar..
Bob

I've been using a mortar and pestle. The occasional shell I put in more or less intact.