Problem
When a cement-based adhesive is applied directly onto the floor, cement in the tile adhesive reacts with the gypsum in the screed resulting in a mineral called ettringite being formed at the interface.
The associated structural change is sufficient to cause a complete debond of the cementitious adhesive away from the screed base.
As anhydrite cures, a weak layer of laitance is formed on the surface.
This layer is too weak to tile onto and also slows the drying time of the screed.
Anhydrite screeds are made from inert fillers such as sand, with a binder system based on calcium sulphate. Consequently they can look very similar to a sand/cement screed.
Anhydrite will tend to appear lighter, sometimes almost white, but in practice it is difficult to identify an existing anhydrite screed from a traditional one.
Solution
Preparation of the surface prior to tiling
If a screed is known to be anhydrite it must be sealed before the application of a cement-based tile adhesive. If the screed type is not known and it is believed that anhydrite is possible the screed should be thoroughly sealed as a precaution prior to tiling.
Weber products
Main products
weberPR360
Styrene acrylate dispersion for use with tile adhesives, levelling compounds and weber.sys protect
weber.setrapid
A rapid-setting, cement-based wall and floor tile adhesive, available in light grey or natural white, for interior and exterior use
