What Is Calcium Sulphate (Anhydrite) Screed?
Also known as liquid or flowing screed, calcium sulphate screed is a self-levelling, pump-applied flooring material based on anhydrite (calcium sulphate binder), offering superior chemistry and performance characteristics compared to traditional cementitious options
Chemistry Matters: Why Anhydrite Works Better
Anhydrite screed forms via calcium sulphate chemistry—creating a free-flowing, self-compacting material that encapsulates underfloor heating pipes seamlessly. This flowable consistency means fewer joints, reduced curling or cracking, and a uniform density across the floor surface .
Plus, it’s environmentally advantageous: lower embodied CO₂, less material usage, and higher recycled content relative to cement screeds.
Cement-Based vs. Anhydrite Screed: A Direct Comparison
| Feature | Cement-Based Screed | Calcium Sulphate (Anhydrite) Screed |
| Drying Time | Slow (often 28+ days) | Faster (approx. 1 mm/day up to 40 mm) |
| Joints Required | Many | Few |
| Shrinkage / Cracking | Higher risk | Lower risk |
| Underfloor Heating Suitability | Thicker screed, longer thermal lag | Thinner depth, better heat transfer |
| Environmental Impact | Higher CO₂, embodied energy | Lower CO₂, recycled content |
Ideal for Underfloor Heating Systems
Calcium sulphate screed wraps tightly around heating pipes, increasing surface contact and heat transfer—reportedly delivering up to 2.5 W thermal transfer compared to conventional screeds.
Moreover, depths as shallow as 20 mm (or even 15 mm) are sufficient, significantly reducing floor build-up, thermal lag, and enabling faster response—and quicker project schedules.
Liquid-flowing or thin-section screeds can go from foot traffic in 48 hours to full loading in seven days, with the option for force-drying beyond 7 days to accelerate finish work.
Moisture Testing & Drying Guidelines
Moisture content is critical—most floor coverings require low residual moisture. For anhydrite screeds:
- Drying typically occurs at 1 mm/day up to 40 mm thickness, with slower rates beyond that.
- Herr the screed to the required dryness level before installing finishes. Most sensitive coverings demand strict moisture testing compliance per British Standards.
- Where needed, fast-track systems like Norcros Pro Gyp-Base allow tiling over 7-day-old screed up to 95% RH—reducing the typical wait time—when used properly with primer, suppressant, and gritted primer layers .
Usage Limits & Best Practices
- Thickness limits: Anhydrite screeds perform optimally in thinner layers—around 20 mm (even 15 mm in overlay scenarios) .
- Joint minimisation: Thanks to chemical stability and uniform density, jointing requirements are significantly lower compared to sand-cement variants .
- Structural considerations: They’re suitable across diverse build types—including timber frames, steel, masonry, modular units, or concrete substrates .
- Environmental design: Lower carbon footprint construction supported through high recycled content and reduced material use .
Conclusion: Why Choose Calcium Sulphate Screed?
Calcium sulphate (anhydrite) screed is a technically superior flooring solution—especially where underfloor heating, speed, and performance are priorities. With self-levelling, thinner depth, faster drying, reduced joints, and better thermal responsiveness, it far outperforms traditional cement-based screeds. Pair this with robust moisture testing compliance and optional fast-track coatings for tiling, and it becomes a highly efficient, future-ready system.
Accelerate flooring schedules, optimise thermal performance, and reduce environmental impact—with Midland Screed Floor’s anhydrite-based calcium sulphate screeds, you’re specifying success.
