- GoldenHas donated $ to the upkeep of GPL
The water in the slurry doesn’t have to go anywhere. It is needed in the finished concrete (or mortar, thinset, etc). The water is part of the chemical reaction and is retained in the hardened product.
There is a little bit of water (especially in self leveling concrete that evaporates). As we all know it is responsible for the low number of home runs at Target field. That said I wouldn’t worry about it behind your schluter mat.
As for using Schluter waterproofing, my dad always says he will sell them to you if you insist, but he wouldn’t put it in his bathroom. These were his words when I told him I was planning to use a Schluter mat when I renovated my bathroom 4 years ago.