Tiling onto anhydrite screeds

Anhydrite screeds have become quite common as they offer benefits over sand and cement screeds. However, tilers must be aware of a number of potential anhydrite screed problems.

Common issues and queries you can face

Screeds have gypsum content

Anhydrite screeds have gypsum content

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.

Anhydrite has a weak surface layer

Anhydrite has a weak surface layer

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 may be difficult to identify

Anhydrite screeds may be difficult to identify

Firstly, what is anhydrite 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.

Preparation of the surface prior to tiling

If a screed is known to be anhydrite it must be thoroughly sealed before the application of a cement-based tile adhesive. If the screed type is not known and it is believed that it could be anhydrite, the screed should be thoroughly sealed as a precaution.

  1. Assess and prepare floor

    The cured screed will have a layer of anhydrite screed laitance that will need to be removed after 2-6 days (dependent on brand of screed used). This will provide a dense surface to tile onto and will aid drying.

    Ensure the floor is fully dry - the residual moisture level should be less than 0.5%. Drying times vary according to the brand of screed used. Some are designed for fast-track use, whilst others require the standard drying times. If no other information is available assume the screed will take 1 day per mm up to 40mm in thickness to dry in normal conditions. Screeds thicker than 40mm will require 2 days per mm.

    Seal the floor with weber PR360 — which can be used as an anhydrite floor primer — repeatedly until no more is absorbed and allow the primer to dry before tiling. An uncoupling membrane can be used to reduce stress on the tiling layer and to ensure that ettringite does not cause a failure.

  2. Lay tiles

    Fix tiles into a solid bed of weberset pro lite - rapid or weberset rapid SPF at least 3mm deep. Leave joints at least 3mm wide for grouting and make provisions for movement.

  3. Apply grout and silicone

    Fill the joints between tiles with weberjoint premium, and use weberjoint silicone sealant to fill perimeter movement joints.

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