What primer and adhesive - underfloor calcium sulphate

[Old Mod]

by Alan & Mark. Thanks guys.

 

[hmtiling]

I'll offer my advice and experience (for what it's worth) and without the previous sarcasm (apologies to all!)

Flatness - These floors should be lovely and flat, and for the most they are.
But don't assume they are - I have seen some absolute howlers in Lancashire/Manchester areas.

Laitence - massive confusion and conflicting arguments abound, but laitence is ALWAYS there no matter the claims of zero or low laitence. In my experience the laitence is always left on far too long (should be removed after 7 days or so) or else it's forgotten about until we come along and it's been literally baked on so it's not dusty, loose or flaky.
Some people insist on removing any and all laitence no matter what it's like. I know loose, friable laitence 100% needs to come up, but in my experience, if you need to get diamond cups on to remove it, then it can stay where it is - it's not going to affect the bond.
But hey, if you feel happier grinding it back then go for it. Personally, I wouldn't waste the time or money.
I will hire a floor sander and 60grit Silicon carbide pads which I find remove any and all LOOSE laitence easily, while not removing the baked on stuff.
(Be sure to vacuum properly afterwards of course)

Priming - I only ever tile heated screeds using Ditra/Rapidmat/Tilemaster matting, so I fix those using TM Anhyfix mixed quite loose and using a mosaic trowel. I will prime once with acrylic or SBR just to knock back the absorbtion and increase the bond of the anhyfix although it's not really necessary.

Cement adhesive - I like Anhyfix, but not for tiling - I prefer to use what I am used to, cement based adhesives and on top of a membrane you can use what you like.

Moisture - I use hygrometer boxes, as well as getting knowledge of when the screed went in, weather conditions and heating commissioning to determine when things are ready. Always be aware that a fairly new screed can give "dry" readings, only to "bounce back" with loads of moisture following wet weather, heating cycles, and plastering/painting - so in my experience, time and heating cycles are a far better measure than anything less than a carbide bomb test.

Testing - most adhesive company reps will come out and test for you FOC, so use them! Get them in early and use the service they offer.

Last words - With gypsum based adhesives, and membranes being a good idea on heated floors anyway, there's no need to be overly concerned about epoxy primers and etringite reactions nowadays. I wouldn't risk using cement based adhesive onto calcium suplhate screeds because quite simply; there's no need to.

Ultimately, different manufacturers will specify different methods.
Different screed installers will say different things.
If you're unsure - ask questions, use adhesive reps for testing and use common sense, and use what works for you.

(and if I am wrong about anything here, I will humbly listen and take it on board!)

Good post