In the background

[Mike Cecil]

It was a common source of grit additive for making mortar (the wet sloppy stuff, not the ordnance!) but I think that's largely a thing of the past now. Shell-grit mortar was common in early colonial construction in Australia, for example.

'Wicked-pedia' also state -

"Shell grit is coarsely ground or broken seashells. It is used, among other things, by birds as a source of calcium for egg shell production, and to aid digestion.

Other uses include protecting plants from slugs or snails[1] and in aquariums."

Mike

[Hanno Spoelstra]

Seashells indeed

Mostly used for seashell lime which is used as a soil fertilizer, as I understand it.