Ettringite formation over gypsum screed - TilersForums.com

[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