High pressure washing with straight water is good to get rid of loose chunks, dust, etc. Let dry in the sun for a few days. It's amazing how long it takes to dry out. You can use a heavy hammer and concrete chisel to break your rocks into shapes and sizes that work for your design. Gluing them together is actually kinda fun. I mark out with masking tape the shape of my tank on the concrete floor in my garage. I use "apoxie sculpt" (two part putty which comes in colors) to "cushion" the connection between rocks. But I put some superglue gel on both rock surfaces - place the putty between them - then push them together. If I need "instant" hardening, I use "insta-set super glue accelerator" by Bob Smith. Just spray the putty & glue connection. Instant hardening. Works great for ledges, towers, etc. But like others have said, consider that you may have to pull apart your reef structure at some point. So don't make your pieces too big. Hope this helps. Cheers!
