Anhydrite Screeds: Preventing Sulphate Attack & Key Considerations
Anhydrite screeds — also known as calcium sulphate screeds — have become increasingly popular in UK construction, particularly for underfloor heating applications. They offer genuine performance advantages over traditional cement-based screeds in many situations. However, they also have specific vulnerabilities that you need to understand and manage. One of the most important is the risk of sulphate attack.
What Is Sulphate Attack?
Sulphate attack occurs when the calcium sulphate in anhydrite screed reacts with certain cement-based materials applied on top of it — particularly Portland cement-based tile adhesives and levelling compounds. The chemical reaction produces ettringite, an expansive compound that causes the interface between the screed and the overlay to swell and eventually fail. The result is debonded tiles, lifted levelling compound, and costly remedial work.
This is not a theoretical risk — we see it regularly on projects where standard cement adhesives have been applied directly to anhydrite screed without proper preparation. The damage typically appears weeks or months after installation, making it particularly frustrating because the floor initially looks perfectly fine.
Preventing Sulphate Attack
The good news is that sulphate attack is entirely preventable with the right preparation sequence. Here's what we recommend.
Surface Preparation
Anhydrite screeds develop a thin surface laitance layer during curing that must be removed before any overlay is applied. This laitance — a weak, milky-coloured film on the surface — prevents adhesion and traps moisture. It should be removed by light sanding or grinding once the screed has sufficiently cured, typically at 7-14 days depending on the product.
Removal by light sanding is usually sufficient. Avoid aggressive mechanical preparation that damages the sound screed beneath the laitance. After sanding, vacuum thoroughly to remove all dust.
Priming
This is the critical step. After removing the laitance, the anhydrite surface must be sealed with a specialist anhydrite primer. This primer creates a barrier between the calcium sulphate screed and any cement-based products applied above, preventing the sulphate reaction from occurring.
We stock specialist primers from Ardex and Mapei specifically formulated for anhydrite surfaces. These are not optional — they're essential. Standard PVA primers or diluted bonding agents are not suitable substitutes. The primer must be specifically designed for anhydrite applications.
Apply the primer according to the manufacturer's instructions — typically two coats with a drying time between coats. Do not skip the second coat, and do not dilute the primer beyond the recommended ratio.
Compatible Products
After priming, use only products that are confirmed compatible with anhydrite screeds. Most major manufacturers now offer anhydrite-compatible ranges — tile adhesives, levelling compounds, and floor finishes that are specifically formulated to work over calcium sulphate substrates. Check the product data sheet for anhydrite compatibility before purchasing.
Moisture Sensitivity
The other key vulnerability of anhydrite screeds is moisture. Unlike cement screeds, which are relatively tolerant of temporary water exposure, anhydrite screeds can be damaged by prolonged contact with water. If water penetrates the surface — from leaks, flooding, or wet trades working on the screed — it can soften the binder and weaken the screed.
This means anhydrite screeds are generally not recommended for wet rooms, shower areas, or any location where regular water exposure is expected, unless specific waterproofing measures are in place. During the construction phase, protect anhydrite screeds from water — keep them covered and ensure wet trades (plasters, painters) don't allow water to pool on the surface.
Moisture Testing Before Finishing
Anhydrite screeds require moisture testing before any floor finish is applied, just as cement screeds do, but the target values and testing methods differ. For anhydrite screeds, the calcium carbide test should achieve a reading of 0.5% or below (compared to 0.5% for cement screeds measured by the same method, though the behaviour is different). Hygrometer testing should show 75% RH or below.
Commissioning the underfloor heating system before moisture testing accelerates drying and gives more representative results, as the heating drives moisture to the surface. Follow the commissioning protocol in our UFH screeding guide.
Why Choose Anhydrite Despite These Considerations?
With proper preparation, anhydrite screeds offer significant advantages. Their thermal conductivity (typically 1.2-1.5 W/mK) far exceeds cement screeds (0.4-0.8 W/mK), making them the premium choice for underfloor heating efficiency. They're self-levelling, achieving SR1 surface regularity without trowelling. They cure quickly and can accept foot traffic within 24-48 hours. And they generate virtually no shrinkage cracking.
The preparation requirements aren't onerous — they simply need to be understood and followed. We stock Tekcem anhydrite screeds alongside all the compatible primers, adhesives, and levelling compounds needed for a complete system. Call us on 0118 370 2060 for product recommendations for your anhydrite project. Free delivery on orders over £600 ex-VAT.
