sorry about that I always try to explain things and then when I go back and read over it I realize if some one was saying this to me it probably wouldn't be that helpful.
so I took some pictures for you
A) pur fill or waterfall sealer foam. this is the good stuff in my opinion for grout in terms of expansion foam. you have to cut the skin off of it before you put the grout on, but since it expands your probably gonna want to cut it into shape anyway. top = with skin Bottom =with out
B and C) both are great stuff (thats the brand I'm not saying it's great) I just got them at two different places. I don't like using this for grout as much because even when its cured its soft and squishy, which adds a risk to it having some give a little, then grout hardens and could cause it to crack if your going for a thin coat. I have used it befor and it does work but it's not as good. again you want to cut the skin off. Top = with skin bottom = with out
D is the kind of Styrofoam I'm talking about. hope the picture helps heres another one close up
here's a pic of two different kinds of the black expansion foam, ones beckett and ones pur fill which also comes in a can with the twist on tube. Tetra also makes one. I'm not trying to push you towards expansion foam the strofoam works as good or maybe a little better I just lean towards using foam cause I hate the fuzzy dust the the Styrofoam makes, it gets ever were.
and if you do use foam hopefully you wont be dumb like me and not use gloves cause it's a struggle to get off, LOL
hopefully this is a little more helpful, any other questions just ask
-Matt