Tiling to an anhydrite (calcium sulphate) screed

Jason Brunt, UK Ceramic Product Manager at Mapei UK

On the continent, anhydrite screeds have been used for a long period of time.  However they only started to gain traction in the UK approximately 10 years ago. During this 10 year period the use of specifications for this type of material has massively increased, both in terms of the number of projects and the square meterage involved.

In a traditional screed installation, the contractor mixes the cement and sand themselves on site.  This differs from anhydrite screeds which are manufactured and mixed at plant, ready to be pumped into the area with an installer monitoring the finish.

One of the major benefits highlighted by the manufacturers of these materials is the speed of installation.  It is said that an anhydrite screed can be laid to a good quality finish at a rate of 1,500 sq. metres per day compared to the install of a traditional screed at 150 sq. metres per day, saving precious time for the installer.  However, while time is saved in this part of the installation process, the material takes a long time to dry.

When comparing an average anhydrite screed to a faster drying proprietary screed, such as Mapei’s Topcem, there is a noticeable difference in the total time spent on the project from installation.
A screed containing Mapei’s Topcem can be tiled as it is being laid.  In theory from day two, the tiling can be started and, therefore, the job can be completed in 33 days from the start of installation, further reducing the estimated 52 days from start to finish.

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Drying of anhydrite
The norm for working out the drying of anhydrite screeds is to work on 1mm per day up to 40mm and two days per mm thereafter.  As an example, a 60mm anhydrite screed would take 80 days to dry based on good ventilation and conditions where temperature and humidity are consistent.  For UK climate and construction, this time frame is optimistic, with these favourable conditions rarely found.

So, why does this process take so long?  The screed has an extremely high water content to allow the product to be pumped and ensure that it reaches a consistent and even level.  The water content in the screed is prevented from escaping through the damp proof membranes beneath so the only way is up through the surface of the screed.

Some suppliers or manufacturers of anhydrite screeds will allow force drying of their screeds.  The force drying is normally carried out when the screed is poured over a water fed under floor heating system. The force drying of the screed involves a commissioning process which will be given by the supplier.  The force drying can reduce the waiting time before applying coverings, however a test must be carried out to ensure that the screed is dry enough.

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Testing the screed
When testing the screed the carbide bomb test is the preferred method.  In order to carry out this test, some of the screed should be removed and crushed.  The test result received are a water/weight figure and readings should be less than 0.5% before coverings are fitted.

Laitance
Anhydrite screeds will generate a laitance at the surface.  This is a weak friable layer which will impair the adhesion of following layers.  For a solid fixing of the ceramic tile the laitance should be removed.  This can simply be done by scraping or sanding, then vacuuming the surface.  Removing the laitance can open up the surface allowing further moisture to escape, so ideally this should be done approximately a week after installation.  The laitance is the finer particles being deposited at the surface as the water reaches the surface and evaporates.  It is vital when the laitance is removed that all dust residues are also removed.

Priming
Once the screed has had the laitance removed, been vacuumed and tested for moisture content, Mapei’s Primer G can be applied.  Dilute 1:2 with water for the first coat, followed by a neat coat for the top layer; dependent on the porosity of the screed.  If a screed has been polished to a point during the sanding process so that the surface is closed and acrylic primers will not adhere, Mapei’s Eco Prim Grip for non-porous surfaces can be used.

The use of primers has several benefits:

1.    Gives an even porosity to the surface of the screed allowing even drying of the following materials

2.    Protects the cement from sulphate attack

3.    Enhances the adhesion of the following products.

It is vitally important to prime the surface of the calcium sulphate screed when using cementitious adhesives.  Moisture in the adhesive, and remaining moisture in the screed and environment, can have a detrimental effect on the bond of a cementitious adhesive to the anhydrite screed.  In damp conditions with no primer present the cement particles in the adhesive can be attacked by the calcium sulphate in the screed leading to the formation of ettringite.  The formation of ettringite, a hydrous calcium aluminium sulfate mineral crystal, between the adhesive and screed can lead to a separation of materials and thus a failure of tile installation.

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Normal failure point
It is usually found that if there are surface failings, this normally occurs after fixing with a cementitious adhesive with lack of preparation to the screed surface.  The failure point is most commonly at the point between the adhesive and the screed.  This is the reason Mapei always advises a strict preparation regime.

Adhesive selection for application of tiles
Once it is confirmed that the screed is fully dry and suitably prepared the choice of adhesive can be made.  This can be determined by the size and type of tile used.  The majority of C2 classification adhesives are more than adequate.  If there is under floor heating contained within the substrate then a C2S1 may be more applicable and if the tiles are larger in size, such as 1,000 by 1,000mm, a C2S2 product may be more appropriate.

At this point it may be beneficial to consider installing  an anti-fracture and decoupling membrane such as Mapei’s Mapetex.  Mapetex is a special non-woven fabric designed to alleviate stresses within the substrate, thus preserving the integrity of the tiled finish. Used correctly, Mapetex can assist in dissipating stresses created within certain types of floor substrate, helping to prevent cracks appearing. Due to the construction of the Mapetex material there are no issues with any remaining moisture being trapped between the screed and tile. Anhydrite screeds, particularly those with under floor heating would benefit having the separation layer installed, especially if the finishing materials are that of a natural stone.  The British standard 5385 part 5 requires an anti-fracture membrane to be installed when  electrical under floor heating is present, so therefore the inclusion of such a material onto an anhydrite screed containing underfloor heating would definitely reduce any risk of failure with the stone material.

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When fixing the tiles it is imperative that a solid bed fixing method is adopted.  For larger format tiles this adhesive should be flat trowelled on the floor and then combed.  The tiles should then be back buttered.  This does not necessarily mean combing the back of the tile but is dependent on the size of the tile.

Mapei adhesives such as Keraquick, Granirapid and Keraflex Maxi are all S1 adhesives. If an S2 is required to improve the bond strength for larger tiles then the Mapei Keraquick can be mixed with Mapei Latex Plus.  Elastorapid, and Kerabond T mixed with Isolastic are also suitable S2 adhesives. If the tiles are larger than 1,000 by 1,000mm then we recommend the Mapei Ultralite range.

In short, anhydrite screeds are a suitable substrate to accept stone or ceramic tiles, as long as the following conditions are adhered to.

1.    Surface is flat

2.    Screed is dry

3.    Laitance is removed

4.    There is no remaining dust

5.    The surface is primed

 

Mapei UK Ltd
Head office, Mapei House Steel Park Road, Halesowen, West Midlands B62 8HD
0121 508 6970

Specification Centre, 6 Great Sutton Street, Clerkenwell, London EC1V 0BX
020 302 9610

info@mapei.co.uk
www.mapei.co.uk