Whether you're fixing natural stone Dijon limestone tiles, or architectural porcelain tiles, you need to assess whether your substrate is a calcium sulphate (anhydrite) screed and prepare it suitably to ensure your project is a success. Failure to assess and prepare your substrate correctly may result in the floor tiles cracking, coming loose, or the tile adhesive de-bonding from the substrate.
If the tiling surface you're faced with clearly isn't wood, it may be an anhydrite screed. These have become quite common as they offer benefits over sand/cement screeds. They are relatively easy to lay, low cost, fast-drying, pumpable, self-levelling and offer minimal shrinkage.
However, tilers must be aware that when floor tiles are fixed with a cement-based adhesive and applied directly onto an anhydrite screed, cement in the tile adhesive reacts with the gypsum in the screed resulting in a mineral called ettringite being formed at the where the two meet. The interface change is enough to cause a complete de-bond of the adhesive away from the screed.
To avoid this, it’s essential that a specialist gypsum-based floor tile adhesive suitable for anhydrite screeds is used. Alternatively, a suitable bonding agent such Kerakoll Primer A can be used to form a barrier between the substrate and the adhesive.
Where Are Anhydrite Screeds Used?
Generally there are four flooring screed constructions where anhydrite screeds are used:
- Bonded: This is a construction where the screed lays directly onto the floor base, only separated by a bonding agent such as Kerakoll Primer A.
- Un-bonded: A construction where the screed and floor base are separated by a damp-proof membrane (DPM)
- Floating: Where the screed and floor base are separated by a layer of rigid thermal or acoustic insulation material
- Wet underfloor heating: Where the screed and floor base are separated by a layer of rigid insulation material upon which underfloor heating pipes are fixed
Considerations Before Tiling Onto An Anhydrite Screed
The screed is generally defined as a self-smoothing 'levelling' screed, used solely to give a flat even surface to fix floor tiles onto. A 'levelling' screed doesn't contribute to the structural performance of the floor and tends to be thinner in depth than a thicker 'wearing' screed, which adds strength and stability to the floor. Key considerations before fixing floor tiles to anhydrite screeds:
- Is the screed dry? If the screed is new the general rule is allow a drying time of one day for every 1mm thickness. For new and old screeds alike it's prudent to perform a moisture test
- Is the screed damaged? Damaged screed should be replaced. Cracked screed should be filled, or a suitable anti-fracture matting such as Kerakoll Idrobuild Tex should be fitted
- Is the screed surface free from contaminants? Dust and dirt should be removed from old screeds. New screeds may have laitance, a surface layer of fine particles displaced as water evaporates
- Is the screed flat, level and smooth? An even and consistent surface will make fixing floor tiles easier and quicker
What Primer Should You Use On Anhydrite Screeds?
- The use of a bonding agent such as Kerakoll Primer A is key to ensuring adhesive can perform to its optimum. We don't recommend using PVR as a primer.