If it is limestone sand/gravel than it is calcium carbonate. No different than the shells the chickens lay. It is a biological product with a ph of 8.4 and it will try to buffer liquids to that ph so it may "melt" after some time from rain and need to be replaced. That would be the only negative of it. It is perfectly safe. Now if it is hydrated lime than that is calcium hydroxide and would be horrible for chickens or any animal. It is the stuff that causes skin burning, internal bleeding, and the like. 2 very different substances make sure yours is calcium carbonate and you are OK.
Here is more info if you need it:
http://en.wikipedia.org/wiki/Calcium_hydroxide
http://en.wikipedia.org/wiki/Calcium_carbonate
Here is more info if you need it:
http://en.wikipedia.org/wiki/Calcium_hydroxide
http://en.wikipedia.org/wiki/Calcium_carbonate