the amazon reviews here have a number of people saying that it hardens into lumps in the crabs' shells and even a few crabs died. I would not recommend based on that.

What I use for sand is actually some children's play sand that I got at Lowe's. It holds its shape very well when moist and it's fine enough grit that the crabs can dig in it easily. Additionally, it's super cheap compared to substrates marketed towards hermit crabs or pet owners.