I've tried many different methods of keeping rock piles stable and together. For larger projects, the very best way to connect large piece of rockwork together is by first drilling a 5/16th hole all the way through. Then using 1/4th inch CPVC that will thread through the opening(s) in the rock. Then I would use AquaStick by Two Little Fishes or Hold fast, by Instant Ocean to fill the top hole. These are aquarium-safe putty-like epoxy. It can also be applied underwater.
I had originally tried silicone a long time ago and it didn't last. It took too long to cure, ws difficult to keep rock piles stabilized while curing and the visible areas around the connections was not aesthetically settling to look at.http://ecx.images-amazon.com/images/I/41y6LgscfEL._SX425_.jpg