use a 2 part epoxy. I read a thread on RC that use 2 part epoxies to glue sand all over a foam structure. Check this RC link
Reef Central Online Community - DIY Rock - Joe's Version
You may also want to check with
Geothermal Aquaculture Research Foundation Home Page they have a way to make realistic rock out of sand and cement. If you made your tower out of this stuff it would become
live rock in time and help with bio filtration. Or you could also coat the
pvc with it
