floor tiling soon after screeding.........

[DHTiling]

In perfect conditions with perfect preparation it is perfectly reasonable to use an acrylic primer and cement based adhesive over an anhydrite screed. However I am not convinced that you are dealing with the perfect site here. I will let Mapei stand or fall by their on advice in respect of ther own materials. You have undoubtedly read other posts on adhesive types and the reasons for choosing different types with different screeds. For me it is all about building in robustness in order to protect you and your client in the long term from problems. I can say that I am unaware of any issues where a gypsum adhsive has been used. If you go the cement way you must make sure that the screed is below 75% relative humidity measured with a hair hygrometer. You would need to ensure a high end acrylic primer is used or better still use a water dispersible epoxy primer.

You are right that the material can be force dried. - as it is 40mm I assume he has decided not to go for wet underfloor heating so force drying would involve space heating and dehumidifiers. I say this because generally 40mm is not deep enouh over a standard domestic wet system - needs to be 30mm cover to pipes so 15mm pipe plus 30mm cover is 45mm total minimum depth - I would usually specify 50mm nominal in this situation.

Perfect answer from a perfect member..