Hope someone else who has worked on styrofoam w/see this, DAISY. I covered a foam wig head, and tried several adhesives and had a lot of trouble. It's been so long I have forgotten what I used, but IF I try styrofoam again, I'll slather a cover of thinset b/f I tile it. I've read that some cover w/duct tape or something similar. I've also thought of Gesso - that might work. Why don't you post your question on the other side since it's on topic and asking for information.

I just used GEII in it, pretty thick in case any absorbed in. Someone said one glue ate the ball away, but I forgot which glue. Some people have mentioned a coat of tape over it, or a thin coat of thinset or something like that

I realize this post has been here for awhile, but I'll respond anyway. I've done a number of styrofoam ballswhatever, and yes glues that will hold the mosaic will also eat the styrofoam! I've had no trouble at all by covering the ball or whatever with duct tape first. It works like a champ.