okay a bit of a hypothetical question, we all are aware of the potential pitfalls of using a cement based adhesive on an anhydrite screed, but what about using an anhydrite adhesive on a concrete screed?, I was mulling this scenario over the other day when I was skimming a sand and cement rendered wall...just thought I'd throw it in for general discussion
I phoned TileMaster technical about this very subject a month or so ago & they said a big No No.
I asked the question about fixing Hardi with AnhyFix & while I was at it I asked if I primed the surface of a cement screed to create a barrier then could I in theory use AnhyFix.
They said we would rather you didn't.
I honestly don't see a problem with it but it would cost more as a gypsum based addy is more expensive than a cement based one.
its all to do with the passage of moiture and concentration of sulphates.
If you have an anydrite screed at say 40mm deep with a moisture content of 0.8% moisture so alost dry and the you plonk a cement based adhesive on top the excess moisture in the screed will tend to equilibrate and rise to the surface where the moiture borne sulphates will attack the cement products in the adhesive causing the formation of ettringtite. you need a flipping good primer to stop the moisture borne sulphates from migrating into the intrface region. An Epoxy primer will do this well but an acrylic primer will not.
If you have a thick cement based screed with an anhydrite adhesive on top there is no moisture borne sulphates in the screed so there is little to react at the interface point and an acrylic primer is likley to suffice. the use of anhydrite or gypsum based smoothing compiuds on sand cement screeds has happened for years without issue.
In other words the use of cement on anhydrite is high risk wheras the use of anhydrite adhevie on cement based screed is lower risk.
If both screeds are properly dried then the chance of reaction is eliminated.
On the basis that cement is compatible with cement why would you want to use a gypsum based adhesive on a sand cement screed.
hahha that's what I love about you Ajax............... soooo technical... for a moment I thought I was a tiler !! then I remembered its just science !!
its all to do with the passage of moiture and concentration of sulphates.
If you have an anydrite screed at say 40mm deep with a moisture content of 0.8% moisture so alost dry and the you plonk a cement based adhesive on top the excess moisture in the screed will tend to equilibrate and rise to the surface where the moiture borne sulphates will attack the cement products in the adhesive causing the formation of ettringtite. you need a flipping good primer to stop the moisture borne sulphates from migrating into the intrface region. An Epoxy primer will do this well but an acrylic primer will not.
If you have a thick cement based screed with an anhydrite adhesive on top there is no moisture borne sulphates in the screed so there is little to react at the interface point and an acrylic primer is likley to suffice. the use of anhydrite or gypsum based smoothing compiuds on sand cement screeds has happened for years without issue.
In other words the use of cement on anhydrite is high risk wheras the use of anhydrite adhevie on cement based screed is lower risk.
If both screeds are properly dried then the chance of reaction is eliminated.
On the basis that cement is compatible with cement why would you want to use a gypsum based adhesive on a sand cement screed.
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