Lovell, Ajax is right about surface prep. Take a rotary floor machine over it to sand any laitance off, hoover it and prime it but only once you (or your client) has taken moisture readings over the whole area for a couple of days and the RH is less than 75%. Moisture meters aren't that expensive, but if you don't have one, maybe suggest your client buys one (they sound keen to do this right, and without one, you cannot tell how dry the screed is).
Re gypsum based
tile adhesive, I have never used it, and have a personal loathing for gypsum based materials, especially for use on floors. Is that stuff water stable when it gets wet??? Do you need to use epoxy resin grout with it???
It does all sound a bit odd though, that the contractor who laid the floor says it is Code 6 M10 but the customer thinks that was out of stock and they might have used M13. Ask the contractor if there is a difference in price between M10 and M13. If M13 costs more then it's DEFINITELY M10, because even if they were out of stock, they wouldn't have poured the dearer material and only charged for M10, LOL...
p.s. You don't have a moisture meter, here is a Heath Robinson test you can do:
Place a plastic sheet on the screed surface and fix it down with impermeable adhesive tape (like Gaffer tape). The next day you will find condensation on the underside if there is still moisture in the screed.
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