Concrete to bond rock?

[xtreme]

Here is an arch that we managed to create over the weekend. The two joints don't look very natural but hopefully with time the coraline and what not will blend them in so they aren't as noticeable. Next attempt will be a pillar of some sort.

[Black Phantom]

I built a bunch of rock for my tank a few years ago. Instead of stacking like you did I filled a box with marine grade sand, hollowed out the shapes I wanted and then filled them with my mix. That way you don't have to support the structure and you can make some wild shapes, caves, etc. And the finished product looked very natural.