What are calcium sulphate screeds and why are they used?
Calcium sulphate screeds, also known as anhydrite or gypsum-based screeds, use an anhydrite or alpha hemi-hydrate binder, whereas cementitious screeds use cement. Their lighter colour and finer aggregate can help distinguish them from traditional cement-based screeds, although misidentification remains common.
These pump‑applied liquid flow screeds are self‑compacting and self‑levelling. Their speed and ease of installation, compared with traditional semi‑dry sand and cement screed, have led to their increased popularity, and they are now widely used in both new‑build and renovation projects.
However, due to their specific preparation requirements and the ongoing challenge of correct identification, they remain one of the most common causes of flooring failures.
Why can tiles and floor coverings fail when installed on calcium sulphate screeds?
Failures typically occur due to:
Screed misidentification
Failure to remove laitance and surface contaminants
Excess moisture in the screed
Lack of priming or use of incompatible primers – resulting in an ettringite reaction with cement-based preparation or fixing products
How to prepare calcium sulphate screeds correctly before fitting floor coverings.
Following these steps will help ensure correct preparation:
Step 1 – Remove laitance and surface contaminants
Step 2 – Commission underfloor heating (heated screeds must be commissioned from 7 days after screed installation)
Step 3 – Moisture test to confirm the screed is sufficiently dry
Step 4 – Prime the entire surface using a compatible primer
Step 5 – Install the floor covering
Why using a specialist primer on calcium sulphate screeds can be critical
When using cementitious preparation or tile-fixing products, priming prepared calcium sulphate screeds is critical in avoiding failures. Without priming, tiles, vinyl, and other floor coverings may debond or fail. To remove uncertainty and reduce risk, we developed PrimeMore CS, a specialist primer designed specifically for calcium sulphate and anhydrite screeds.
Is ready to use straight from the pouch
Requires just a single coat
Penetrates and seals the screed
Forms a protective barrier to prevent adverse reactions with cementitious products
How do I know when the calcium sulphate screed is dry and ready for priming?
Drying times are affected by screed thickness, ambient temperature, ventilation and site conditions. While walking on the screed may be possible within days, full drying can take many months. In the UK, calcium sulphate screeds are considered dry when moisture readings reach 75% Relative Humidity (RH).
Moisture should be measured using appropriate equipment, with readings expressed in relative humidity (RH).
Key benchmarks:
≤75% RH – As most adhesives, along with some floor coverings, are sensitive to moisture, this level must be confirmed before installing resilient floor coverings.
≤85% RH – Ceramic and porcelain tiles can be fixed when using PrimeMore CS
Final recommendations
As misidentification remains a common issue, especially as cementitious flow screeds become more popular. Where screed type cannot be confirmed, following the five steps mentioned above and using PrimeMore CS provides a reliable and practical solution for avoiding floor covering failures.
Written by Craig Powell, Head of Technical at Kelmore.
Our Technical Support Team is available Monday – Friday, 8:00am – 4:30pm on 01257 830511 or technical@kelmore.co.uk. Please don’t hesitate to contact us for any advice or support.
