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.
right ... now to get bak to my bottle of wine....
