I've tried rockdust but didn't really like it. Switched to non-treated wood sawdust I get for free from a local mill. Really soaks up moisture and is easy to rake the poo up off of, plus it's free.
My whelping pen has trex decking with 1/2" gap spacing, and for a smaller pen or raised pen I would never go back to anything else. Easy to clean, maintenance free, and will last for years to come.
Goodluck

Go concrete with a full top with a 5 ft over hang and you will be well sastified call me I can show you some nice pens !!! just got done building another set for my male dogs only .Pp

I would go w/ concrete fully covered as well, the extra area for a walk/feed/load area is a must in foul weather