E
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.
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.