anhyrdrite screeds

[JOHNNYCLARKE]

About to start a job monday and just found out the screed is pumped anhydrite.
Screed been down 2 weeks over water underfloor heating so I have been on to mapei/weber etcc and screed company for advice.
They all say the same that you should allow 1mm thickness of screed per day drying before tiling.
This is similar to fibrous screeds for underfloor heating unless where ditra mat or similar used, in which case can tile onto screed within days.
But you must scrape skin off screed and prime floor first in either instance with anhydrite.
This screed apparently expands rather than contracts and can blow tiles off subfloor.

So I know what route to take, using ditra which I quoted for over fibrous screed that would have been down for only two weeks as well.

MY QUESTION IS , HAS ANYONE HAD A PROBLEM WITH TILES NOT ADHERING TO THIS ANHYDRITE SCREED BEFORE.

APPRECIATE ANY INPUT

[gaz4266]

just make sure you prime well

[Cliff Anger]

cant emphasise this enough - yes we get phonecalls almost daily about failures on anhydrite screed - this stuff should be banned !!!
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Anhydrite is made from Gypsum and is not chemically compatable with cement adhesives.

The anhydrite MUST be completely dried out....prepared and primed so it is completely sealed . if the primer fails the adhesive fails !

Much has been written about the formation of ettringite crystals - which form when cement, gypsum and moisture get together - they expand pushing off the adhesive and the subsequent tiling

just what you wanted to hear hey johnny?

[Cliff Anger]

Get the contractor to put in writing that the screed is suitable for tiling on, and they take full resposibility for any de-bonding or walk away

[Sir Ramic]

Can someone explain how Anhydrite screed can be recognised correctly. I believe it is a lighter colour, is this correct?

[Grace'sDad]

Just a quick thought before I go to bed (finally!) but isn't BalBond SBR meant to prevent anhydrite / cement cross reactions?

Surely this is a cheaper and quicker alternative as long as the screed has dried of course?

[TilingLogistics]

Provided the screed is totally dry. I was told you could run a rotary machine across with carborundum blocks, remove dust, prime and then tile. I also thought you could Ditra with a contact adhesive?

Kev

[JOHNNYCLARKE]

cliff

Thanks for your advice.
I was thinking of using a black very coarse scrubbing pad on my 16inch scrubbing machine before applying mapei primer G.
Do you think this will do the job?

Have recently started using mapei after meeting one of your reps on apex grange stand at stone show and my guys are really happy with the keraquick for laying ditra /insulation boards over underfloor heating and keraflex maxi on normal heated floor screeds.
Am going to try kerquick on limestone next time round as it seems to give you more time than other brands ive used that go off in ten minutes when mixed with a whisk in warmer weather which is no good for a stone fixer who needs time to adjust stone when laying long courses.
Will start using your fibreplan over electrical heating if it is as good as mira xplan has been for us so far.
ANY STONE FIXERS OUT THERE REALLY SHOULD LOOK AT MAPEI PRODUCTS .
REAL QUALITY ,GREAT TO USE AND APEX GRANGE GIVE REALLY GOOD PRICES WHEN BUYING BY THE PALLETT.

johnny

[JOHNNYCLARKE]

dave

just read your advice to a previous post last year about this and very good advice too.
Much Appreciated.

Johnny

[Branty]

Tiling to Anhydrite, is a lot like tiling to finish plaster, with more worries. I.E it wetter, thicker, and has a shinier finish.
Removing the laitance and the shine is essential. Probably more so is checking the moisture content. As Cliff says, You can do all the prep you like, but if there's to much moisture in the screed, the adhesive won't bond.

Your risidual content has to be below 0.5% before you can fix tiles direct to it. I believe you can use ditra if the risidual moisture content is 2%. In both these instances you will need a meter reading if you want to be sure.
It's all well and good your builder saying it's been down for four weeks, if the first two, it was exposed to the elements. Get him to do a moisture reading.

[JOHNNYCLARKE]

getting a bit confused now.
Mapei and Weber both said I could put ditra down with a rapid flex quickset even if screed only 2 weeks old.
Now presumably screed is still not completely dry.
So Mapei said take skin off top and apply primer G which i think is accrylic.
So any moisture left in screed should go down and sideways if it cannot get up through primer.
Is this correct?