Anhydrite screed: definition and characteristics

What is an anhydrite screed?

An anhydrite screed is a liquid screed that uses calcium sulfate as a binder instead of cement. It is pumped, poured, and self-leveling, which classifies it as a fluid screed, but its behavior differs significantly from a cement screed in three ways: shrinkage, drying, and water sensitivity.

‍

How it differs from a cement screed

First, shrinkage. A cement screed shrinks as it dries, which requires large surfaces to be divided by joints. Anhydrite has near-zero shrinkage, allowing for much larger continuous surfaces with fewer joints to plan for.

Next, thermal conductivity. It is more favorable than that of a standard cement screed, which improves the efficiency of an underfloor heating system.

Finally, water resistance, which is its weak point. Calcium sulfate degrades under prolonged exposure to moisture, whereas cement is resistant to it.

‍

Why it dominates in underfloor heating

The first two advantages are exactly what an underfloor heating system needs. Its fluidity coats the pipes without leaving any air pockets, thus eliminating parasitic thermal resistance. Its lack of shrinkage avoids the need for multiple joints that would otherwise require cutting through heating loops.

The result is a floor that heats up faster and more evenly. It is the most common configuration for this type of installation today.

‍

A point of caution: moisture

Two moments require attention. During construction, the fresh screed must be protected from drafts and any water ingress. After drying, in wet rooms, a waterproof membrane must be applied under the tiling, and the manufacturer's approval must be obtained.

Drying is the second critical factor. Anhydrite screeds can be walked on quickly, which creates a false sense of readiness. However, deep drying takes several weeks. Installing an airtight floor covering over a damp screed traps moisture, leading to peeling and blistering. Only a residual moisture test can determine when it is ready, never the calendar.

‍

Sanding the laitance

During the setting process, the finest particles rise to the surface, forming a brittle film known as laitance. This must be removed by sanding before the floor covering is installed; otherwise, the adhesive will bond to this weak layer rather than the screed itself. This is a mandatory step, not an option, and it is frequently overlooked in quotes.

We install anhydrite just like other self-leveling screeds, typically in a floating configuration. Detailed information on our solutions can be found on our screedpage.