Concrete to bond rock?

[Delphinus]

Has anyone here used concrete to bond rocks, as opposed to say building a PVC skeletal structure, or using acrylic or fibreglass rods after drilling holes?

Noticed Ching Chai's doing it on his new tank build so got to thinking about it. He says he's using regular concrete. I thought this would make the pH of the water unpalatable for quite some time?

Or is it OK in small enough doses?

I remember a few years ago about a polymer based concrete which I think is used as a patching compound, but I couldn't find the exact brand name (I think it was discontinued). And I kind of forget the name now anyhow.

I did find another polymer based patching compound though at Home Depot, but I'm not sure how strong it would be. Maybe it's OK?

Or is it really OK just to use regular old Portland cement and just use as little as possible? (Call me a skeptic, I just don't see me getting away with it, is all..)