Ettringite formation over gypsum screed - TilersForums.co.uk | Tile Forums

ecopaddy · 24-06-2008

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.

**whitebeam** · 24-06-2008

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

**Dave** · 24-06-2008

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 · 20-10-2008

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 · 20-10-2008

[quote=merlincoatings;133250]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?[/quote]
And the answer is?

merlincoatings · 20-10-2008

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.

**Sir Ramic** · 20-10-2008

Quote:

Originally Posted by 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.

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 · 20-10-2008

Opps.. should of checked the message again