Calcium sulphate self levelling floor screeds are all very similar for the end user with respect to the product properties. The differences are mainly in the aggregates, which vary in accordance with the method of application, for example, such as the mobile trans-mix vehicles or silos.

Binders that can be used compliant with the European standard EN 13454 are:

• Natural anhydrite
• Synthetic anhydrite
• Thermal anhydrite
• Alpha hemihydrate
• Mixtures of these “calcium sulphates”

These binders or binder compounds ware assigned to different strength classes.

Different binders

With cementitious screed the binder is cement; for calcium sulphate flowing screed the binder is calcium sulphate.

Application

• Cementitious screed is mixed on the building site and pumped onto the application surface using compressed air. It is then distributed using a shovel, and levelled using a timber batten, and in a third step it is smoothed with a smoothing edge or smoothed using a screed smoothing machine.
• Calcium sulphate flowing screed is delivered to the building site as a pre-mixed dry mortar in silo systems or bags, with mobile trans-mix vehicles or as ready-mixed self levelling floor screed in a truck mixer. It is then pumped in liquid form onto the installation surface.

The surface

In contrast to cementitious screed, calcium sulphate self-levelling floor screed does not need to be smoothed. It is practically self-levelling. The surface just needs to be equalized with a dappling bar. The result: A very even surface ideally suited as a substrate for laying large format floor covering such as board layers.

The surface treatment

Calcium sulphate flowing screeds on the basis of K-Sentials binders are optimized to produce a good surface finish. It is not necessary to additionally sand off the surface, if installed according to the application regulations. Please refer to the IGE Code of Practice No. 4 "Assessment and treatment of the surfaces of flowing calcium sulphate screeds" for this purpose.

The application thickness

A further difference is the application thickness, which is about 10 mm less with calcium sulphate flowing screed. Accordingly, thicker thermal insulation or footfall sound insulation can be laid, while maintaining the same application thickness for the floor construction. The thinner screed layer also offers benefits for underfloor heating systems, which as a result can noticeably deliver heat more quickly.

The bending tensile strength

Flowing screed has a higher bending tensile strength than cementitious screed resulting from its more dense structure, meaning that it can be loaded with a higher load at the same screed thickness. Accordingly, no “curling” occurs at the edges as the elastically sealed perimeter joints do not rupture after the floor covering is applied and do not form “dirt collecting joints”.

Sound insulation

Flowing screed features very good sound insulation properties. The screed has no direct connection to the perimeter walls as the surfaces intended for the application of the liquid mortar are prepared like a “tub” beforehand and all the perimeters are provided with an edge insulation strip. This ensures that any noise from the floor is not transferred to the walls.

Calcium sulphate flowing screed dries practically without deformation and is subject to very low stresses so that no joints are required. Only for larger or geometrically complex surfaces and with heated screed is it necessary to install structural joints and movement joints.

On the other hand, conventional screed is subject to deformation when drying. Shrinkage, expansion or curling are possible deformations that can occur and that can be transferred to the floor covering and cause damage. Joints can remedy the situation.

When the screed is installed without joints, it is the ideal substrate for laying large format and diagonally laid tiles or natural stone coverings. When not discontinued by expansion joints, the planning is simplified saving time and money for the investor, which in turn provides a more visually appealing room image.

Even when screed is applied as stipulated, cracks can appear in the screed due to a diverse range of causes: For example, a crack can occur with heated screed during the drying phase should very cold outdoor air coming through open windows or doors come into contact with the screed at this point. Cracks can also occur particularly where areas with contorted and complex geometries have too few movement joints. A flow temperature that is too high can also cause cracks.

Perimeter joints prevent the formation of cracks. They accommodate the stresses that result due to deformation and movement in the screed. The perimeter joints must also be implemented into the floor covering and also permit movement.

If the cracks are professionally remedied, the screed is deemed to be free of defects and cracks.

Flowing screed can be applied across the entire living space including kitchens and bathrooms. Wherever splash water is an issue, the screed construction must be protected from above, for example, using a brush applied sealant or an applied layer of elasticized adhesive with embedded joint tape.

Calcium sulphate flowing screed has been developed for indoor application. When subjected to moisture, Ettringite, a salt that damages the building substance can be formed. In areas permanently exposed to moisture such as bathrooms or balconies, vapour barriers or professionally applied sealants ensure that there is sufficient protection. In garages, the entrance zones where the moisture is at its greatest must be sealed.

Yes, on flowing screed all conventional floor coverings such as tiles and natural stone, carpet, PVC, linoleum, parquet, laminate or resin screed flooring can be used.

The floor covering can be applied when the screed has reached a point where it is ready for covering. The test to determine the readiness for floor covering performed by the floor covering specialist only permits the CM measurement method.

As with all mineral bonded building materials, all calcium sulphate flowing screeds must get rid of the surplus water that is not chemically bonded into the ambient air. The drying time will depend on the method and manner of drying as well as whether the drying process is supported by heating.

The optimum approach is by technical heating, e.g. a condensation drier unit in combination with the heating. For a heated screed the drying process can be accelerated by heating up the screed. Ventilation also plays a decisive role here. A further, very important criterion for drying of calcium sulphate flowing screeds is the observance of the required screed thickness: Exceeding this thickness will lead to a significant extension of the drying time.

The floor covering installer determines the readiness for floor covering with a CM measurement test. This test involves taking a sample from the entire cross-section of the screed, then pulverising and weighing it. The prepared sample is then analysed in the CM measurement device to determine the moisture level. Electronic test devices are only suitable for preliminary testing.

Drying of calcium sulphate screed can be artificially accelerated by using a condensation drier or fans. A combination of both is ideal. The BEB Code of Practice “Accelerated drying of calcium sulphate screeds” (Date: January 2007) provides additional information on this topic.

A condensation drier operates according to the “refrigerator principle”: It consists of a compressor, refrigeration section and heater section. The moist room air is directed through the cooling section. The moisture condenses there on the cold surface. The cold dry air is subsequently diverted to the heater section and heated up. Then the air is released back into the room with a level of approx 30 to 40 % relative humidity.

Calcium sulphate screed conducts heat significantly better than cementitious screed. The good flowability ensures that the heating tubes are securely enveloped ensuring that the heat is transferred completely to the screed. Its low thickness ensures that the heat is quickly transferred to the surrounding air. Thus calcium sulphate screed is an ideal heated screed that is both energy-efficient and quick to respond to temperature changes.

Furthermore, heat up at an early stage accelerates the building phase and the calcium sulphate screed is possibly ready within 4 to 7 days. With cementitious screed, heat up can only commence after 21 days at the earliest.

The tubes for the underfloor heating system must be filled with water and anchored to the substrate. This will prevent them from floating when the screed is installed.

Flowing screed can be heated up after just 4 to 7 days after installation so that the screed dries even faster. Ensure that proper ventilation is provided.

Ventilation can commence after the second day.

Before you lay the floor covering, the residual moisture of the screed must be determined with a CM measurement device. With flowing screeds for underfloor heating systems, the level of residual moisture should be less than or equal to 0.3 %.

Important: Do not heat the screed to temperatures exceeding 55° C as otherwise it will be damaged. Use a floor thermostat on the heating element for temperature regulation.

General danger of overheating

With electrical underfloor heating, there is generally a danger of overheating when the screed is covered by an insulating material – such as a cabinet not mounted legs – and when the floor thermostat is not located directly underneath it. The user of the screed should be made aware of this and adapt their living behaviour accordingly.

No heating until dry

Should you apply electrical underfloor heating on a still new flowing screed, it is vital to ensure that the screed has achieved the required readiness to accept a floor covering before applying the floor covering, as it cannot be heated until dry like normal heated screed. Heating up prematurely can cause subsequent changes in the shape and result in damage.

Can I use flowing screed for cooling as a cooled floor?

Yes, this is possible. Due to the high thermal conductivity, flowing screeds for heating are also very suitable for cooling. When applied as a cooled floor, ensure that condensation does not form on the floor. For this purpose, the cooling must be regulated with continuous monitoring of the floor temperatures and the air humidity.

DIN EN 13813 and DIN 18560 describe the most important regulations and fundamentals

• for the properties
• for the intended use and
• professional installation of screed

In detailed processing and instruction sheets from the relevant associations such as the German BEB (BUNDESVERBAND ESTRICH UND BELAG e. V. - Federal association of screed and floor covering), there are numerous instructions concerning screed as a component and the ancillary trades.

CAF according to DIN 18560, the German standard for screed, represents the bending tensile strength.

CAF means calcium sulphate flowing screed, F 5 indicates a bending tensile strength of 5 N/mm². The F stands for the term Flexural.

The designations derive from differing standards:

• CA-C25-F5 is a designation according to DIN EN 13813 for conventional calcium sulphate screed as well as for calcium sulphate flowing screed and defines the material and technical properties of the screed.
  CA = calcium sulphate screed
  C25 signifies a compressive strength of 25 N/mm²
  F5 stands for a bending tensile strength of 5 N/mm².
• CAF-F5 is a classification according to the DIN 18560 part 2, the German application standard. It also differentiates between conventional CA calcium sulphate screed and CAF calcium sulphate flowing screed. It only demands the stipulation of the bending tensile strength, in this case 5 N/mm², as this is the most important factor on insulation layers.

You can determine this by the CE-marking on the packaging with the DIN EN 13813 standard. The CE-marking provides the basis for the marking of products within the EU and may only be used by the manufacturer if a declaration of performance is available.

Studies and gamma spectroscopic measurements indicate the gypsum originating from flue gas desuplhurization, abbreviated FGD, is not radioactive in accordance with radiation protection requirements. The tested FGD gypsum can be used without reservation for the manufacture of building materials. This also applies to calcium sulphate binders and calcium sulphate flowing screed made from FGD gypsum.

Scientific research carried out by the Fraunhofer Instituts für Bauphysik (Fraunhofer Institute for Building Physics) came to the following conclusions with regard to the calcium suplhates examined:

• no carcinogenic substances could be detected in the screed products it tested
• the measured emissions of volatile organic compounds were all within the permitted limits
• the calcium sulphate screeds examined complied with all requirements relating to the health assessment of construction products for use in indoor environments