I'm adding tiles to a shower stall in quite a lengthy (for me) repair job. For this job, since a shower will be subjected to a lot of running water, I'm supposed to use a waterproof grout.

I bought an "all purpose" grout (Made by Mapei). It is a portland cement based, "unsanded, polymer modified" grout. But nowhere on the box does it say "waterproof". Am I on the right track? Or is there a grout that is really waterproof that I should be using.

I would love to hear from someone who knows.

William