What is Calcium Sulphate Screed?
Calcium sulphate screed, also known as anhydrite screed, is a type of floor screed made from calcium sulphate hemihydrate and water. It sets through a chemical reaction that converts the hemihydrate into gypsum, a hydrated calcium sulphate.
Composition
The main ingredients in calcium sulphate screeds are:
- Calcium sulphate hemihydrate powder
- Water
- Aggregates like sand or crushed stones
- Additional chemicals like retarders or accelerators
How it Works
When water is added to the calcium sulphate powder, a chemical reaction called hydration occurs. This converts the powder into gypsum, which binds everything together to form the hardened screed. Special retarders can be added to slow down the hydration process. After it sets completely, it reaches very high strength.
Advantages of Using Calcium Sulphate Screeds
Here are some of the main benefits of using calcium sulphate rather than traditional cement-based screeds:
Fast Drying and Installation
A major advantage of calcium sulphate screeds is that they fully dry very quickly, often within 24-48 hours. This allows floor coverings like tiles to be installed faster without having to wait weeks for drying.
Low Tension and Cracking
Anhydrite screeds undergo very little shrinkage as they dry. This results in extremely strong flooring with very few cracks. There is also low tension between the screed and subfloor.
Excellent Thermal Conductivity
Calcium sulphate conducts heat better than cement screeds. This makes it suitable for underfloor heating systems, distributing warmth more evenly across room floors.
Lower Weight
These screeds are lighter than sand-cement screeds. At around 110 kg per cubic meter for a 35 mm thick anhydrite layer, it exerts less load pressure on structures.
Resistant to Deformation
Properly laid anhydrite screeds hold their shape well over years of use without hollows, indentations, or cracks developing through deformation.
Compatibility
Calcium sulphate screeds create a suitable alkaline base for most floor finishes like vinyl, wood, carpets, and tiles. It can be paired with many building materials.
Disadvantages and Considerations
While the pros often outweigh the cons with anhydrite screeding, there are still some downsides to weigh:
Surface Treatments Needed
The surface needs to be treated before laying floor coverings. Primers, sealants, or dismantling fluids assure proper bonding between layers.
Moisture Sensitivity Before Drying
Though they dry quickly, moisture during installation can disturb the hydration process. Any leaks or excess water exposure needs prevention.
Limited Thickness
Anhydrite screeds should not generally exceed 60mm thickness for proper structural strength. Thicker sections may crack or crumble over time.
Specialized Tools and Pumps Needed
Proper installation requires specialized pumps and tools to pour and spread the flowable screed before it hardens too much. This adds complexity.
Long-Term Performance Questions
While used for decades in mainland Europe, calcium sulphate screeds are still somewhat new in regions like the UK. Their long-term performance is still being confirmed.
Is a Calcium Sulphate Screed Right for Your Project?
If you’re wondering whether you should choose an anhydrite rather than cement screed, consider these factors:
Use Case Requirements
What floor finishes do you want to install, and what performance needs do you expect from the slab? Anhydrite suits lightweight tiles, underfloor heating efficiency, and quick turnaround projects well.
Subfloor Type and Environment
Moisture exposure risks factor into whether calcium sulphate’s hydration will work properly. What’s the climate exposure, site drainage, concrete type and moisture content?
Installer Expertise
Proper pumping, spreading, hydration control, curing conditions and connections take skill to avoid surface issues or weakened areas later on. Review crew experience.
If your needs align with calcium sulphate screed’s strengths for resilience, fast drying, heat conductivity and strength, it can be the right choice over cement. Carefully review the weak points as well when planning your particular floor system.
Installation Process for Calcium Sulphate Screeds
Here is an overview of installing and finishing an anhydrite flowing floor screed:
Surface Preparation
The base concrete floor surface needs proper cleaning, potential repairs, and priming before screed application.
Levelling
If surfaces are highly uneven, self-leveling compounds may be used to create a flatter profile.
Laying Screed
The anhydrite mix is pumped or poured onto the prepared surface at needed depths up to 60mm, then flattened.
Curing Process
Moisture exposure is limited during an initial 24-72 hour curing period while hydration occurs and strength builds.
Drying Time
Before adding floor coverings, adequate drying out of excess moisture completes hardening while monitoring humidity levels.
Surface Work
The top gets sanded or grated, then sealed/treated to assure proper bonding with additional floor layers. With the right precautions for base preparation and installation conditions, contractors can achieve rapid, crack-free floor building using calcium sulphate screeds.
Common Applications of Calcium Sulphate Screed
Some typical applications where anhydrite screeds offer benefits include:
Underfloor Heating Systems
The improved thermal conductivity distributes underground heating efficiently across room floors. Drying speed also allows faster installation.
Wet Rooms
Water exposure won’t damage correctly sealed anhydrite surfaces, making it suitable for bathrooms, laundry rooms, and kitchens.
Renovation Projects
Calcium sulphate screeds can be applied over old cement floors. The limited weight and depth ease building above existing slabs during renovations or extensions.
Office Buildings
The lightweight physical load and resilience against indentations make anhydrite ideal for installation below vinyl or carpets in office spaces with rolling loads.
Apartments and Flats
Thinner screeds allow maximizing room height in multi-resident spaces. Noise transmission is also lower between floors compared to other underlayment types.
Historic Buildings
Limited weight loading suits building additions on existing structures with minimal foundation adjustments or risks. Crack and shape resilience preserve structural aesthetics long-term.
Cost Analysis
Installing a 100m2 GypFloor quartz anhydrite flowing screed from industry leader Gyvlon in a typical UK residential application would compare with cement-based screeds as follows:
Anhydrite Screed
- Materials: £6000
- Labor: £2000
- Total Installed: £8000
Cement Sand Screed
- Materials: £800
- Labor: £2000
- Total Installed: £2800
While calcium sulphate screed material costs about 3 times more than cement materials, professional installation is around the same labor price. The anhydrite screed allows floor coverings in 1-2 days rather than 1-2 weeks though, offsetting extended site time costs.
When accounting for project schedule impacts, anhydrate screeds become even more economical, with the 7-14 day flooring installation acceleration justifying the higher material expense. The enhanced crack resistance, strength and thermal properties also assure long-term performance value.
For many clients, the turnaround speed and reliability make calcium sulphate well worth the investments over traditional sand cement.
Conclusion
Calcium sulphate screeds offer excellent strength, thermal conductivity, and rapid drying for quality underfloor builds. While costing more upfront, professional installation can accelerate turnaround schedules by weeks compared to cement alternatives.
Careful base preparation, controlled hydration, and surface treatments remain vital to avoid weakness issues. When properly executed, anhydrite screeds provide lasting performance and resilience suitable for most flooring types.
If your upcoming residential, commercial or renovation plans demand quickly reliable, crack-free slab builds, modern anhydrite solutions present an enticing option over dated cement methods. Review leading suppliers’ offerings to choose a formulation suited to your site requirements.
FAQ
Can you put underfloor heating on an anhydrite screed?
Yes, calcium sulphate’s improved thermal conductivity compared to cement actually makes it an ideal base for efficient underfloor heating installation.
Is anhydrite screed waterproof?
Once fully cured and sealed appropriately, anhydrite resists water exposure damage very well. This enables uses in bathrooms, kitchens and other wet zones.
How thick should an anhydrite screed be?
A typical calcium sulphate screed thickness ranges 35mm to 60mm. Thicker sections risk delayed drying and potential strength issues long-term.
Can you go straight onto anhydrite screed?
No, the surface needs proper cleaning, potential sanding/grinding, and sealing treatments before adding floor coverings like vinyl or tiles.
How much does screed per m2 cost?
Installed anhydrite screed costs around £80-100 per m2 typically, 3-4 times more than basic cement sand screed. But drying speed offsets overall project schedules significantly.