Calcium sulphate screed
Calcium sulphate screed (CE, CA)
Calcium sulphate screed (CA) is composed of calcium sulphate (anhydrite) as a binder, sand/gravel and water. From a chemical point of view, anhydrite is anhydrous gypsum that sets after the addition of water, resulting in the formation of calcium sulphate dihydrate crystals. However, these can form again when wet. Depending on the type, the screed contains additives such as synthetic resin dispersions or limestone chippings to increase its strength.
Calcium sulphate screed was previously also referred to as anhydrite screed (AE). However, this term is inaccurate, as there are different variants of calcium sulphate screed that are characterized by specific properties. These include
- Conventional anhydrite screeds
- Natural hydrite flowing screeds
- Alpha-hemihydrate flowing screeds (FGD anhydrite flowing screeds)
- Synthetic anhydrite flowing screeds
- Mixed forms
As a result of the Europe-wide harmonization of the binding agent standard DIN EN 13454, the binding agent anhydrite - along with other binding agents, most of which consist of calcium sulphate - was grouped together under calcium sulphate as a generic term.
Areas of application for calcium sulphate screed
Calcium sulphate screed is suitable for use in all dry indoor areas. Calcium sulphate screed is also suitable in combination with underfloor heating, as it has good thermal conductivity and heat storage properties. Thanks to its high early strength, underfloor heating can even be put into operation after around 1 week. However, calcium sulphate screed is only heat-resistant to a limited extent, which should be taken into account during application.
Caution: Calcium sulphate screed is sensitive to moisture. This is due to the fact that not all of the anhydrite turns back into gypsum when the screed sets. However, the absorption of ambient moisture produces gypsum, which swells as a result. For this reason, calcium sulphate screed should only be laid in dry rooms. If necessary, a seal or vapor barrier should be installed. Calcium sulphate screed cannot be used outdoors.
Laying techniques for calcium sulphate screed
In principle, calcium sulphate screed can be processed in the same way as cement screed. In addition to the screed in an earth-moist consistency, there is also the calcium sulphate flowing screed variant.
After mixing, setting / hardening takes place through crystallization. Depending on the building and requirements, calcium sulphate screed can be laid as
- floating screed
- Composite screed
- Screed on separating layer
- Heated screed
- Screed on hollow floors
What to look out for when laying calcium sulphate screed
Unhindered drying is very important for calcium sulphate screed. The drying time depends on the temperature and climate of the construction site. Under good conditions, calcium sulphate screed can be walked on after 3 days, lightly after 5 days and fully loadable after 4 weeks. It usually takes at least 1-2 weeks before the screed is covered with a surface covering. The prerequisite for applying the floor covering is that the moisture content is below 0.5 CM mass %.
Depending on the type, calcium sulphate screed achieves a medium to high compressive strength class (C16-C40).
Tip: If large-format tiles are to be laid on the screed, it should be sanded beforehand to improve durability.
The advantages of calcium sulphate screed
Calcium sulphate screed has a number of advantages that make this type of screed so popular. It is characterized by a very low shrinkage, is low-stress and dimensionally stable. This has the advantage that the edges neither bulge nor sag during drying. As a result, calcium sulphate screed has a low tendency to crack and can also be laid seamlessly over large areas. Reinforcement is not required.
Calcium sulphate screed is highly flexible and has good tensile and compressive strength. It can also be used as a heated screed. It also dries quickly after application and can usually be heated up after just 7 days.
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Questions and answers about calcium sulphate screed
Calcium sulphate screed essentially consists of anhydrite (which is why the screed was often referred to as anhydrite screed in the past), sand or gravel and water. Depending on the type, it may also contain additives such as synthetic resin dispersions, which make the screed stronger.
No, this screed does not tolerate moisture well. It should therefore only be used in dry indoor areas. In areas where moisture or vapor diffusion occurs, waterproofing or a vapor barrier is absolutely essential.
Yes, in addition to the conventional version, there is also a self-levelling calcium sulphate flowing screed. This has the advantage that it can be applied quickly and easily and an even surface can be achieved.