Great Idea Al. I have messed around with concrete a bit, but just to make bases for branches for my dragons. One product that may help you is a waterproofing agent. I use Bondall Silasec but this is an Aussie product so you probably won't be able to get it (
www.bondall.com .) This not only waterproofs but makes the concrete set in about 1/4 the time.
Another idea is to pour your concrete into a mould. Coat the inside of the mould with oil (I use olive oil,) pour the mix in, wait for 24 hours and pop it back out.
