What primer and adhesive - underfloor calcium sulphate

[Dumbo]

I live my life my the mantra of I think his name was Chris Carter who wrote the x files . "Trust no one"

To be fair J I will say what I think I have heard in the past but if im wrong I will put my hand up .

 

[Old Mod]

No I asked that before and all I get is . You need to carbide bomb test it .

That’s the only wAy we can get a moisture reading by volume. And it’s instant, unlike a humidity test, which takes 48hrs.

 

[thickbed]

If using flexbone, as far as I'm aware, the screed must be 87% rh or lower

 

[Ajax123]

I'm enjoying this thread...

 

[One Day]

I'm enjoying this thread...

 

[Ajax123]

Its a bit like time travelling back 5 or 6 years...

 

[Ajax123]

If using flexbone, as far as I'm aware, the screed must be 87% rh or lower

Why?

 

[Dumbo]

Why?

Bal/flexbone claim max moisture content of 1.5%cm whatever that is .

 

[J Sid]

Its a bit like time travelling back 5 or 6 years...

for many of us, these screeds are no more common then they were 5 or 6 years ago ( thank god) so going over old ground is a good thing imho.

 

[thickbed]

Why?

Im pretty sure that's what they said when I went on the BAL course

 

[Ajax123]

1.5% moisture in anhydrite screeds will be 92%rh with a tolerance of 5% built into the test. With flexbone this becomes irrelevant because the mat is not bonded to the screed and because the mat allows the moisture to migrate. there is a continuing myth that trapping the moisture in the screed leads to the screed breaking down. It does not. Ettrignite can only form in the presence of moisture. your primers and adhesives are largely water mixed which is why Cement based adhesives should not if at all possible be use. If they are then a good chemical barrier is needed and acrylic just does not do the job well enough. The other major issue that I come across im afraid is preparation workmanship e.g primer not applied evenly, adhesive too wet, primer not applied in enough coats etc etc. The use of calcium sulphate adhesive eliminates completely the risk of ettringite and so is a better option. Calcium Sulphate screeds can often be used at higher RH than cement e.g 80% or some allow 85%. If the screed is very wet e.g. up to 99%RH then Norcross offer a guaranteed system. BAL do not offer a gypsum based adhesive.

 

[Spare Tool]

Following this as have first ever anhydrite to tile in Feb (it's only the second one I've ever actually seen round these parts) so all tips however old or new are informative..

 

[Ajax123]

Technical - http://gypsol.co.uk/technical

 

[One Day]

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!)

 

[J Sid]

thanks @impish