Anhydrite screeds have become quite popular over the past couple of years as they have benefits over traditional sand and cement screeds which include low cost, minimal shrinking and self-levelling.
These screeds can cause problems for fixers if the floor isn’t prepared correctly, as an anhydrite screen dry’s a layer of laitance is formed on the surface. This layer of laitance needs to be mechanically removed before tiling as this layer is week and will debond from the base if it isn’t removed and tile will simply lift from the floor. The screed needs to fully dry before tiling can commence and removing the layer of laitance will help the screed dry quicker, a good guide to drying time is 1 day per mm up to 40mm in thickness in normal conditions, screeds thicker than 40mm allow 2 days per mm.
Due to the risk of a chemical reaction between the sulphate in the screed and cementitious materials in tile adhesive the floor then needs to be primed with an acrylic primer, the screed needs priming at least twice with the first coat being diluted 1:1 with water and the second coat applied neat. Tile adhesive needs to be applied when the second coat has dried tacky.
It is often hard to tell the difference between an anhydrite screed and a traditional sand and cement screed, often an anhydrite screed will appear lighter, sometimes almost white. We always recommend checking with who laid the screed first as to what type of screed it is before tiling commences.
Rees Tiles stocks all the products needed for tiling screeds and can offer advice if you have any questions.