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

 

[Deleted member 15]

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 !!!
----
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

 

[tiler burden]

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

 

[Gazzer]

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?

 

[Gazzer]

Is this of any use ?
Broken Link Removed
broken link removed

 

[Branty]

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?

The wat I see it, is its all about the moisture. If there's too much in the screed then that has to affect the bond to the ditra no matter what adhesive you use.
If you can get some reading that says its less than 2%, then you can use ditra.

 

[JOHNNYCLARKE]

just been to look at job now with a diy dampmeter and it feels and looks damp still after 24 days down.
Screed is apparently 40 mm thick which should mean 40 days to dry.
Dont know what to do as committed myself after talking to the mapei tech guy on friday who said would be ok.
If i put this accrylic primer G down will this stop any moisture from screed coming up or just some?
I thought if it really seals in screed then any moisture would move sideways and up wall which would be fine would it not!

CLIFF IF YOU LOOK AT THIS I WOULD REALLY APPRECIATE MORE ADVICE OR CAN CONTACT ME DIRECT.
I AM DUE TO START MONDAY BASED ON MAPEI INFO BUT NOW IM CONFUSED.

THANKS JOHNNY

 

[Branty]

The only problem I see Johnny is the moisture could affect the primer coating.

 

[JOHNNYCLARKE]

Branty
You mean you think primer might not stick to screed.

Have you had experience with this yourself.
My problem is that I put in my quote that I could put ditra down if screed had been down for five days or over but not fully dry. This is fine for normal screeds for underfloor heating and client gave me go ahead and paid for ditra for 180 sq mts.
But I dont know if this is ok on this anhydrite screed.
Ive only had four of these out of maybe 300 in last four years!!The builder was told he should let screed dry 1 mm a day but ignored it because he has laid on this screed before early and had no problems and did not tell me .

It was only when I found out what screed was down last week did I contact screed firm and told should wait.
The checked with Mapei and Webber who both said ok to put ditra down but neither mentioined the 2 % min moisture and I have only heard this now.

I could say they should wait but this is 7k of work over next two weeks and I am fully booked for months afterwards.

 

[Branty]

Johnny I think you need to get some advice from Schluter.
I have fixed to anhydrite in the past. The first one came up, since then I always got a meter reading. One of below 0.5% means its ok to tile onto.
I don't think there's a difinitive figure when it comes to using a decoupling membrane. I think TTA put a figure out of 2%, but not sure.
Give Schluter a call, see what they say.
The problem with this screed over a normal sand and cement screed, is the huge volume of water.
Try giving BAL tech a call tommorrow and see what they say.

 

[JOHNNYCLARKE]

Branty
Thanks for advice .
I will check with schluter tomorrow of course.
Could not get them over weekend.
Have put start off for tomorrow till see what everyone says.

Johnny

 

[Sully]

Branty,

I don't want to hijack Johnnys post but my query is along similiar lines.

Went to see a new build extension job yesterday and the flooring was at an early stage. I could see concrete between concrete beems on the floor and the house owner advised that building regs had specified a 'floating floor' due to the soil / trees near the house. Over this will be piped underfloor heating working of the megaoflow system and this will be covered in 'screed'. I asked the householder to get me more details from the builder. Is it likely to be anhydrite screed? Or put another way, when / why do they use an anhydrite over a sand & cement. Total area is 40sq/m.

Secondly on a sand and cement screed - is it minimum 4 weeks drying or 1 day per mm. I also need to spec a 75mm screed on another job so just want to be sure that it's not 75 days drying time.

Andrew

 

[JOHNNYCLARKE]

sully

sand/cement screed has different drying time to anhydrite. The BS has changed in last few years and is something like 25mm for 1st week and then 1mm a day thereafter.
I always work on a month but that is me not the ruling.
You will find very few contractors stick to the new regs as it is simply too long for them to wait.
And the other main difference is you definitely can put ditra down on screed as soon as you can walk on it so say schluter. I always say min 5 days.
You can check any of this info by calling the technical dept of your adhesive manufacturer provided they are a big outfit set up for it.
My problem was it being weekend and getting one piece of advice from a tech dept and then hearing more detailed info on this forum that I cannot check till monday.
Johnny

 

[GazTech]

A List of Screed Products used in Floor Screeding

The list below helps explain the difference in the types of screed products used in floor screeding.

Sand and Cement Screed
Sand and cement screed is otherwise known as traditional screed. Sand and cement are the components of screed and are combined together with water using a screed pump or mixer. Sand and cement screed is ideal for every situations such as ground floor slabs, suspended floors and as a lightweight topping. It can also be used over insulation and with underfloor heating. Fibre Screed
Fibre screed, also called PP fibre screed or reinforced screed, is ideal in any situation especially sites with high foot traffic and can be used with underfloor heating systems. Polypropyene fibres (refered to as PP fibres) are combined with sand and cement screed. The use of fibres inhibits shrinkage cracking, reduces settlement cracking, improves cohesions, increases strength, permeability and the screed is more shatter and impact resistant. It can also be used over insulation and with underfloor heating. Polymer Screed
Polymer Screed is ideal for concrete ground floor slabs, precast concrete, suspended floors and when a thinner depth is required. It combines sand and cement screed with a polymer resin, such as a SBR Bonding Agent. It is a very high quality bonding agent in which a carboxylated styrene butadiene copolymer latex (SBR) replaces conventional PVA. This increases durability, reduces shrinkage and cracking, increases flexibility and toughness, permeability and bonding strength. SBR latex, unlike PVA, cannot be re-emulsified once cured. It can be used over insulation.

Anhydrite Screed
Anhydrite screed, refered to as flo, flow, poured and liquid screed, is ideal for commercial buildings. It is a self leveling screed which is poured directly onto a floor for a fast application of 2000 square meters. Anhydrite screed reduces cracking, is light weight, protein free, environmentally friendly and high strength. It can also be used over insulation and with underfloor heating. Fast Drying Screed
Fast Drying screed is a combination of sand, cement and an admixture such as Isocrete K-Screed, Ronacrete Uniscreed or alternatives. It is suitable for interiors and exteriors. It is a specialist hydraulic binder which allows for rapid drying and will harden within 24 hours and become completely dry within approx. 7 days. It can also be used over insulation and with underfloor heating. Latex Screed
Latex or Waterproofer Planicrete is added to mortars, screeds and renders to increase mechanical strength and adhesion to the substrate. It is a synthetic rubber latex for improving adhesion of cementitious mortars and is a latex based on synthetic polymers, which cannot be re-emulsified in water after hardening and are also resistant to saponification. Click for a full explanation of saponification.

 

[GazTech]

Power Floated Concrete
power-floated concrete (or any concrete with a shiny finish) needs to be suitably abraded/scarified/scabbled in order to expose aggregate prior to tiling, otherwise there may not be an adequate bond strength with the adhesive.


The recommended bed thickness of the adhesive should not be exceeded....Gaz

 

[grumpygrouter]

Really interesting thread this guys. Thanks to all that have contributed.

 

[White Room]

Schluter also do a piped under floor heating when carried out to their spec they guarantee against any cracking

 

[Cliff Anger]

just been to look at job now with a diy dampmeter and it feels and looks damp still after 24 days down.
Screed is apparently 40 mm thick which should mean 40 days to dry.
Dont know what to do as committed myself after talking to the mapei tech guy on friday who said would be ok.
If i put this accrylic primer G down will this stop any moisture from screed coming up or just some?
I thought if it really seals in screed then any moisture would move sideways and up wall which would be fine would it not!

CLIFF IF YOU LOOK AT THIS I WOULD REALLY APPRECIATE MORE ADVICE OR CAN CONTACT ME DIRECT.
I AM DUE TO START MONDAY BASED ON MAPEI INFO BUT NOW IM CONFUSED.

THANKS JOHNNY

Colin Stanyard our Anhydrite Guru will call you first thing Monday !

MY Post just got tagged with a screedflo Anhydrite AD !!!!
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You probably all guessed that i'm not a fan of the "Devil's" screed !

But just to clear the confusion if you use Ditra matting , Mapelay or similar - you're not actually fixing cement based adhesives to the anhydrite - so the rules have changed.

PS if the screed was Topcem you only have to wait days not weeks !

Cliff

 

[Branty]

Does the screed still have to be below 2% moisture though Cliff?
Would recommend fixing with a contact adhesive?
I too hate this screed.

 

[bansko]

Brantys spot on, remove the laitance and check the moisture content, Thats a must. then prime thouroughly. both directions allowing to dry in between, one thing they do have going for them is they are flat which you dont get too often these days. Stick to brantys advice you wont have any problems.

 

[dock]

Johnny

You will find that Ditra matting is "breathable" albeit with a quite low vapour transmission rate.............but thats how the moisture gets out

dock


ps provided, as branty points out, the start point is 2% moisture as stated by schluter

 

[Branty]

Johnny

You will find that Ditra matting is "breathable" albeit with a quite low vapour transmission rate.............but thats how the moisture gets out

dock

But there website does recomend a moisture content no greater than 2%. I think.