asked advice recently on best addy to stick tiles onto waterproof matting.told cement based flexi.made sense to me.went to pick bag up on way to job,advised that manufacturer say ordinary ready mixed can be used.only specify cement based for sticking matting.got second opinion from shop where customer bought matting.they said same.tried sticking one on with tubbed addy biscem showerproof.nothing adhered to matting.rang homelux technical out of interest.they said exactly what shops said.for any traders who come across this ignore them and use cement based.

In theory ready mixed will stick to matting but, as you have waterproofed the background, you have effectively created a sealed environment and ready mixed will not set,maybe never set, and if and when it gets wet it will certainly soften and fail, unlike cement based adhesive

Cement base all the time, not great advise from a supplier

ready mixed adhesive develop their bond by losing moisture which results in the polymer cross linking. If the background is waterproofed and a low water absorption tile is being fixed the moisture can only evaporate from around the periphery of the tile through the joint. The drying time will be very slow. It is also possible that the pH of the ready mixed adhesive could resoften an acrylic paint on waterproof membrane in the early stages of curing soon after drying due to the length of time the "wet" adhesive is in contact with it.

Cement based adhesives set hydraulically by chemical conversion of the cement and therefore do not need to lose moisture to develop a bond.

Given the above it would make sense to use a cement based adhesive over such compounds instead of ready mixed products

dock