Ettringite formation over gypsum screed

[ecopaddy]

Just updating a previous post that I made in February about using cement based adhesive over an anhydrite calcium sulphate (gypsum) screed with no primer.

Took four months for the problem to develop but tiles are a write off and will all now have to be ripped up and replaced. Symptoms start with hairline cracks in the grout and a hollow sound under the tiles. When you pop up the tile, the cement remains attached to the tile but a fine layer of power crystal breaks the bond between the cement adhesive and the gypsum screed. Yes, the dreaded Ettringite.. see attached photo. Note that the break across the ettringite was a chalk line.. must have prevented the reaction in some way

I have to say that I am not a fan of anhydrite screed at all after this. There really is a lot to be said for sand/portland cement based screed especially in terms of stability and durability in wet areas. I also think that the so-called fast drying time of anhydrite was not something that I found to be true at all. In fact I would say that is was extremely difficult to dry.

What have I learned from all of this.. Don't mix cement and gypsum products if at all possible. If you are going to use cement and gypsum together, make sure that they are well separated.. i.e. prime, prime and if in doubt, prime again. A £20 bottle of Bal Prime ADP would probably have prevented this..

Paddy.

 

[White Room]

Hi ecopaddy, I've hav'nt laid on that screed yet but dread the day I do, Great information though, Thanks.

 

[DHTiling]

This just goes to show that using the correct primer is an absolute must......Cement ( portland) and gypsum DO NOT mix what so ever....and this case just shows the result of this.....thanks very much for sharing this with us......

will you keep up up to date with the re-tiling of it....?

 

[merlincoatings]

Food for thought, when chosing a primer it is impotant to consider the following;

Age of the screed, has it reached full strength.

Moisture content, most primers are only suitable on dry screeds.

Curing time, there is no point priming if you then have to abrade the surface back to get a good key, so plan carefully.

What are you putting over the primer, is it compatable.

More importantly what do you do if the screed still has a high moisture content and the drying time is running out with the client demanding access to the area?

 

[Branty]

Food for thought, when chosing a primer it is impotant to consider the following;

Age of the screed, has it reached full strength.

Moisture content, most primers are only suitable on dry screeds.

Curing time, there is no point priming if you then have to abrade the surface back to get a good key, so plan carefully.

What are you putting over the primer, is it compatable.

More importantly what do you do if the screed still has a high moisture content and the drying time is running out with the client demanding access to the area?

And the answer is?

 

[merlincoatings]

Hi Branty,

Did post the product details in the previous post, but (slap hand) didn't read the forum rules in terms of advertising products and had to remove the information. I am waiting for Dave? (site owner) to get in touch.

 

[Gazzer]

Hi Branty,

Did post the product details in the previous post, but (slap hand) didn't read the forum rules in terms of advertising products and had to remove the information. I am waiting for Dave? (site owner) to get in touch.

Sorry if this breaks to rules again, a google search for my username should provide you with a little more information about the barrier coat product.

You mean Dan.

 

[merlincoatings]

Opps.. should of checked the message again :embaressed_smile:

 

[Ajax123]

Hi Eco paddy

You are quite right that the anhydrite screed should not ghave been mixed with the cement based adhesive. This does not mean that the screed is a problem but simply that product advice was not followed i.e. no primer was used. You could also have solved the issue by using a Calcium Sulphate based tile adhesive. These are much more commonly used in France than in England. France uses something like 3million square meters of anhydrite each year and around 80% of this is tiled. There are unfortunmately not many gypsum based tile adhesives in the UK. my info about France is that they have very few if any failures when using this type of adhesive on anhydrite screed.

 

[Ajax123]

There is no need to dread it just seek the right advice from the manufacturers before commencing tiling. There are simple steps to follow in preparing anhydrite screed and some useful products out there to make it easy for example if its a gypsum based screed why not use a gypsum based tile adhesive. This completely eliminates ettrignite reaction. If you use cement based adhesive then maybe consider an epoxy primer which will also avoid ettrignite reaction. It is important to make sure the screed is dry and that any underfloor heating is comissioned before tiling.
Hope this helps
Alan

 

[Brindle]

Hi folks,
I have a job starting this week over a gypsum screed. 100m2 Travertine. I normally use an SBR to prime floors (and walls where appropriate). The customer already has bought cement based adhesive (white rapid).
Is SBR or APD the correct primer for this combo? I was planning on speaking to the adhesive manufacturer first thing.
Cheers

 

[Ajax123]

Hi folks,
I have a job starting this week over a gypsum screed. 100m2 Travertine. I normally use an SBR to prime floors (and walls where appropriate). The customer already has bought cement based adhesive (white rapid).
Is SBR or APD the correct primer for this combo? I was planning on speaking to the adhesive manufacturer first thing.
Cheers

Is there underfloor heating. If so it deffinitely needs to be comissioned before tiling.
Do you know whose gypsum screed it is as this will potentially affect surface prep.
You need to double check if the screed is dry. I would avoid Acrylic and SBR primers with cement based adhesives (although if the manufacturer offers a gurantee then so be it) because any residual moisture in the screed will move upwards with the action of heating and 2 things can happen.
1. the primer can emulsify weakening the bond at the screed interface encouraging delamination.
2. if the primer emulsifies it can allow the transimission of sulphates from the screed through the primer and this will interact with the cement in the adhesive causing ettrignite formation and ultimately delamination.
I would go for a water dispersible epoxy primer. e.g. Laybond Screedmasters Epoxy or similar.
Ensure there is no loose surface laitance - this should only be very thin skin on the surface of the screed. the screed under the laitance should be hard and sound and look in texture like a sand cement screed but a much lighter colour. However if it is one of the new generation anhydrite screeds it should be very hard and smooth. This will need to be lightly sanded to give you a key but you do not need to grind into the surface generally. If the screed looks like neither of these I would seek advice from the manufacturer or installer before proceding.
Have you thought about telling the client to change the adhesive to one based on calcium sulphate..maybe you could buy the stuff he has bought off him for another job..just a thought but calcium sulphate removes all of the chemical reaction risks, it removes risks from differeintial thermal expansion and contraction and you can use an acrylic primer with it. Additionally generaly speaking it will tolerate higher moisture levels. Lastly if the screed is too wet and not been tested the tile will tend to delaminate from the adhesive making the tile reusable rather than the adhesive coming away from the screed on the back of the tile as happens with cement adhesives.

 

[Brindle]

A lot to take in there!
The product that was used was supplied locally and pumped in, the only info I have to date is that it needs to be left for 40 days before tiling. It's had 50 now. If I apply 2 or 3 coats of APD prior to tiling, will that not suffice? I'm worried now...eek

 

[grumpygrouter]

If I was laying trav on a heat floor I would def be considering a decoupling membrane........

 

[Brindle]

Yeh, it was recommended by me, along with a couple of other suggestions, but customer decided not to. I will be taking a disclaimer along, probably best!

 

[Prem Tiler]

can I ask guys how can you tell what the surface is your about to fix to? as a new tiler is there a way of knowing what surface is what. I've never been in to the building trade before taking up tiling and am learning every day. is it something you just pick up with exsperiance?

 

[Andy Tiler]

Hi premier i think this might help you



Hhttp://www.netweber.co.uk/tile-fixing/weber-products/problem-solutions/internal-floor/tiling-onto-anhydrite-screeds.html

 

[Andy Tiler]

try again

Tiling onto anhydrite screeds - Weber, world?s leading manufacturer of industrial mortars for the facade, tile fixing and construction markets

 

[Brindle]

can I ask guys how can you tell what the surface is your about to fix to? as a new tiler is there a way of knowing what surface is what. I've never been in to the building trade before taking up tiling and am learning every day. is it something you just pick up with exsperiance?

When it's a new project, you can find out from the customer/ builder when pricing up the job what the substrate is going to be. If it's an older floor, It's likely to be concrete or sand/cement screed, or one of these with a scoat of self level on top. With stuff like vinyl, the best bet is to use the technical helplines such as BALs to check out what's what.

 

[Brindle]

Nice one Andy, the Weber info very good

 

[Prem Tiler]

cheers guys I might get my self of a few building site to do a Pepsi challenge and see the difference. it says on the Weber site that anhydrite cant be used in wet conditions. dose this mean it wouldnt not be found in bathrooms?

 

[Ajax123]

cheers guys I might get my self of a few building site to do a Pepsi challenge and see the difference. it says on the Weber site that anhydrite cant be used in wet conditions. dose this mean it wouldnt not be found in bathrooms?

No it does not mean that anhydrite is not found in bathrooms in the UK. in France the building regs are different and Anhydrite is not allowed in domestice bathrooms or kitchens.. they tend to use cementitious flowing screed. This type of product ha its problems as well but that's a different forum. Anyway in the UK there is no real reason why it should not be used. What Weber are talking about is permanently wet environments such as swimming pools and the like. Ths is because it is virtually impossible to stick a floor covering to a wet screed and as anhydrite is designed to take a floor covering it should not be used in those areas which will not dry out. If you can protect the dry screed from water getting into it it is fine but no one would be willing to guarantee any structure where there is always water that it will not find it's way into the screed. The other place you should not find anhydrite is where it has been laid to falls.

 

[Ajax123]

Yeh, it was recommended by me, along with a couple of other suggestions, but customer decided not to. I will be taking a disclaimer along, probably best!

Sorry did nto mean to worry you. I work with anhydrite a lot and as log as you follow the rules it need not be a problem. If your customer won't listen to sensible suggestions like a decoupling mat or epoxy primer etc then I don't know what to suggest. If you are confident that the screed is dry the APD primer may work ok but bare in mind that the drying time of 40 days applies in good drying conditions. We have not had good drying conditions generally over the last few months so unless the client has employed some sort of forced drying it may not be dry yet. I would deffinitely insist on a proper moisture test using hair hygrometers as per BS8203. a good indicator test is to lay a small sheet of polythene (1m square) on the screed and leave it there for around 3 days. If the screed does not sweat under the polythene it is appropriate to do the moisture test properly. If any sweating or condensation on the underside of the polythene the screed is not dry and it would be fool hardy to lay the tiles with a cement based tile adhesive unless the screed is dry.

 

[Brindle]

Cheers Ajax.
They've had a huge dehumidifier going in the property to help take out moisture. After reading up a bit and everyones advice I am thinking that If I prime with undiluted BAL APD in one direction, then prime again at 90 degrees, then see if it needs another coat? The BAL in fo claims it's for this purpose..?
I saw somewhere that a water dispersible epoxy primer is advisable, but I haven't been able to find anything described as such. Can you tell me of one?
Thanks for your help here

 

[Ajax123]

Try Laybond Products - part of the Bostik Group they do a product called Screedmasters Epoxy - their number is on line

 

[Brindle]

Is there underfloor heating. If so it deffinitely needs to be comissioned before tiling.
Do you know whose gypsum screed it is as this will potentially affect surface prep.
You need to double check if the screed is dry. I would avoid Acrylic and SBR primers with cement based adhesives (although if the manufacturer offers a gurantee then so be it) because any residual moisture in the screed will move upwards with the action of heating and 2 things can happen.
1. the primer can emulsify weakening the bond at the screed interface encouraging delamination.
2. if the primer emulsifies it can allow the transimission of sulphates from the screed through the primer and this will interact with the cement in the adhesive causing ettrignite formation and ultimately delamination.
I would go for a water dispersible epoxy primer. e.g. Laybond Screedmasters Epoxy or similar.
Ensure there is no loose surface laitance - this should only be very thin skin on the surface of the screed. the screed under the laitance should be hard and sound and look in texture like a sand cement screed but a much lighter colour. However if it is one of the new generation anhydrite screeds it should be very hard and smooth. This will need to be lightly sanded to give you a key but you do not need to grind into the surface generally. If the screed looks like neither of these I would seek advice from the manufacturer or installer before proceding.
Have you thought about telling the client to change the adhesive to one based on calcium sulphate..maybe you could buy the stuff he has bought off him for another job..just a thought but calcium sulphate removes all of the chemical reaction risks, it removes risks from differeintial thermal expansion and contraction and you can use an acrylic primer with it. Additionally generaly speaking it will tolerate higher moisture levels. Lastly if the screed is too wet and not been tested the tile will tend to delaminate from the adhesive making the tile reusable rather than the adhesive coming away from the screed on the back of the tile as happens with cement adhesives.

Hi Ajax,
I've been to inspect the job this morning....
The laitance is present and comes off with grinding action rather than scraping, but with it being a big floor I've hired a grinder; have then spoken to the screed installers/ manufacturers and UFH people who are all happy for the tiling to commence prior to commission (the screed people are coming to test the moisture levels on Wednesday); then the adhesive manufacturers have advised on their own SBR primer be used 2 or 3 coats, varying ratio for use with Anhydrite screed.
I have gone through all of this with the customer and he is wanting the job to proceed as above- if the floor is within permitted levels of moisture. As you say if the adi manufacturers advise and specify a particular primer and will guarantee that info?!
Blimey

 

[Ajax123]

Hi Brindle
I would strongly recomend the UFH to be comissioned prior to tiling but obviously it's up to the client to decide that and if you have the ok from all other parties I guess there is not a lot else to say. I'd be interested to know how you get on with this one. I presume its BAL Primer but you have not said.
Best of luck

 

[Brindle]

Cheers. I have pointed out to customer but he spoke to everyone else and they happy to proceed. Nightmare.
Could you remind me again why it should be commissioned before tiling and I'll speak to him again. Is it just this anhydrite screed that should be commissioned before, I only ask as I've followed manufacturers procedures before with UFH and cement screeds - i.e left for 2 weeks then put on slowly.
They're saying the screed was laid with the pipes etc all under pressure.
I've told him the heat put on could cause problems, if the screed hasn't been used to being heated up beforehand.
Thanks for your help, it's not easy sometimes :/

 

[Ajax123]

Cheers. I have pointed out to customer but he spoke to everyone else and they happy to proceed. Nightmare.
Could you remind me again why it should be commissioned before tiling and I'll speak to him again. Is it just this anhydrite screed that should be commissioned before, I only ask as I've followed manufacturers procedures before with UFH and cement screeds - i.e left for 2 weeks then put on slowly.
They're saying the screed was laid with the pipes etc all under pressure.
I've told him the heat put on could cause problems, if the screed hasn't been used to being heated up beforehand.
Thanks for your help, it's not easy sometimes :/

Hi Brindle

I am all for risk management and the best way to manage the risk is to remove it.
Comisioning the underfloor heating prior to flooring is considered best practice and is endorsed by the tile association in its guide to tiling on calcium sulphate screeds.

If anything untoward is going to happen to a heated screed e.g. thermal cracking etc it is better to have it happen before the tiles are placed so that the screed can be easily repaired prior to tiling. It is most likely that if this type of cracking is going to happen it will do so when the heating is commissioned. It may not happen of course as anhydrite is particularly thermally stable. If the tiles are in place the consequences of the screed cracking are fairly obvious.
Commisioning the screed also "destresses" it so that when the three elements are bought together i.e. screed, tile and adhesive there is less mechanical pressure on the interfaces between them due to their expanding and contracting at different rates so less risk of delamination. I beleive travatine is particularly prone to cracking from tensile stresses.

When the heating is switched on the vapour pressure inside the screed increases and any residual moisture is forced upwards. It cannot escape from the screed because of the tiles. If this moisture level is too high and softens the primer the bond at the interface with the screed will potentially be disrupted. Additionally it could allow the penetration of sulphates which can attack the adhesive and cause ettrignite formation.

Hopefully the screed will be found to be well below the recomended minimum moisture content in which case you probably have little to worry about. However the screed is only 50 days old I think you said earlier. It is likely to be 50mm to 55mm deep on Underfloor heating and the drying time is actually 1mm per day up to 40mm and then 2 days per mm thereover i.e. 60 days at 50mm. I know they have had a dehumidifier in place so this will have made the difference.

I know this all sounds a bit scary but it is really just common sense and highlights that the rules are important and if followed there is little to worry about. Anyway - forwarned is forearmed as they say and I would rather the customer was aware of the risks so he can make an informed judgement.

 

[Brindle]

It all makes total sense mate, and I am very grateful for your knowledge. The screed is 40mm, and the is going to be checked for moisture on Wednesday, so fingers crossed it's under and I can proceed!
Not paranoid, one step ahead ?