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Anhydrite screed and laitance

Anhydrite screed and laitance

Anhydrite screed is a type of floor screed that is made from calcium sulfate. It is often used in modern construction projects because of its quick drying time, but it can also pose challenges when it comes to tiling. One of the main challenges is the presence of laitance, which is a thin layer of weak and loosely bonded material that can form on the surface of the screed. Laitance can prevent proper bonding of tiles to the screed, which can lead to tiles coming loose or cracking over time. Here are some steps to remove laitance before tiling:

  1. Test the anhydrite screed: Before starting the removal process, it is important to test the anhydrite screed to ensure that it is fully dry and hardened. This can be done using a moisture meter.
  2. Remove any loose material: Use a scraper or wire brush to remove any loose laitance or other material from the surface of the screed. Be sure to wear protective equipment, including gloves, eye protection, and a mask.
  3. Use a mechanical scabbler: A mechanical scabbler is a type of tool that can be used to remove thicker layers of laitance or other material from the surface of the screed. This tool uses a series of rotating blades to chip away at the surface of the screed, leaving a roughened surface that is ideal for tiling.
  4. Vacuum the surface: After removing the laitance with a mechanical scabbler, use a vacuum to remove any dust or debris from the surface of the screed. This will help ensure a clean and smooth surface for tiling.
  5. Apply a primer: Finally, apply a primer to the surface of the screed to improve adhesion and ensure proper bonding of the tile adhesive. Be sure to follow the manufacturer’s instructions for application and drying time.

By following these steps to remove laitance from an anhydrite screed before tiling, you can help ensure a successful and long-lasting tile installation

Drying time for anhydrite

The drying time for calcium sulfate or anhydrite screeds before tiling can vary depending on several factors such as thickness, ambient temperature, humidity, and ventilation. Generally, a rule of thumb is to allow at least one day of drying time per millimeter of screed thickness up to a maximum of 40mm.

For example, if the screed is 30mm thick, it should be allowed to dry for at least 30 days before tiling. However, if the screed is thicker than 40mm, additional drying time may be required or alternative measures may need to be taken, such as installing a vapor barrier.

It is important to note that the screed must be completely dry before tiling to prevent moisture-related issues such as tile debonding and mold growth. A moisture meter can be used to test the screed’s moisture content before tiling.

Additionally, it is recommended to follow the screed manufacturer’s instructions for drying time and installation of the screed.


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Contact Flex Tiling with your project requirements. We offer a range of tiling services in and around BS16 and Bristol. See area covered below for details.

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