Concrete to bond rock?

[justinl]

IMO it would be perfectly fine to use relatively small amounts of portland cement. amount of cement relative to volume of water will determine how rapidly the cement will change the Ph, so it has to be considered on a case-by-case basis. You would just have to keep a closer eye on Ph for about a month than most people do on their tanks; I don't think it would be too hard to buffer or perform water changes to combat it.