Reef Safe Cement With Good Local Availability

[molotov]

Hope you don't mind me busting into this thread. I had a piece of my aquascape fall down. When I originally built the scape I used the 2-part epoxy and it was super strong. Four months later it breaks off and fell off the supporting structure.

What would you recommend to fix this? Would I have to pull the pieces out and use cement to get them back togther? If I did take the pieces out, how long would they have to cure for before I put them back in the tank?