Even more info
Non porus tiles laid on a non porus surface you would use a different glue, because mastic pretty ,much works by evaporating/ drying, I have heard of problems when used in that configuration. If you want to do that use something like Ardex X77 this will tile over glass metal , just about anything, ok in a wet environment so much so it can be used to tile swimming pools. Although in this case the gyp rock would dissolve if it was wet.
