Ours always have access to grit, so I've not experienced this but I expect that they are just fillling up the crop and will just top up from now on. I'm always surprised at how much our use, considering they are out and about.
I give our chicks (8 weeks and 12 weeks and confined to runs) seed trays with soil and they go nuts for it. Maybe sand is a bit dull? No beasties?