Years ago when I repiled my house in gisborne, I made the piles myself, using pumice, sand, and cement. This was the procedure in those days to make the piles lite for dragging around under a house, made perfectly fine concrete.
However as to impurities of reef tank significance, that will probably vary from one batch of cement to another. Unless buying a reef tank designed product it is probably not possible to know what may be lurking in the cement.
Garf make concrete plugs for their frags, they first soak them in vinegar, then cure them in salt water for a couple of months. I think this is not so much to establish a biological filter, but more because new concrete can release major calcium / alkalinity and effect these levels in the tank.
IMO the safest plan would be to make a few trial rocks, go chuck them in a rock pool at the beach for a few months to leach whatever they may leach, then introduce to the tank one at a time, monitoring levels and observing livestock carefully.