Billy, if you're willing to risk it using the concrete, it's not a problem. Just be aware of where the concrete has been. Make sure to be extremely mindful of the pH level for the first 5 years. Yep, 5 years. If the concrete get soaked for that long, pH will stabilize out. If the concrete don't get submerged, try 20 years for more.
The best type of concrete to use is damaged and broken concrete materials from the local plants. Do not use any colored concreted. Just plain straight up gray, or the base color of the sand and aggregates that the concrete bonded to.
Make sure to get porous concrete, and not the sealed. The porous have a greater chance to leech out the pollutant on first flush (using garden hose), before introducing into your pond. Sealed concreted may have smaller pores, but the pollutants are trapped at much greater level due to large bonding surfaces within the smaller pores.
