drew, i wish i was closer so i could donate something "concrete", but since im not, here's a recipe for aragocrete! this is basically concrete base rock, but a recipe that was made up by GARF... id trust it. Seems easy and i think ill try it one day. maybe a project to do with the kidies? could be messy though.
http://www.garf.org/MPegs/AragocreteArch.html