Simple Screed Science

Self-levelling solutions: Tiling on to anhydrite screeds

By Mandy Searle, Head of Technical Services, Norcros Adhesives.

Anhydrite screeds, also referred to as calcium sulfate or gypsum-based screeds, are now increasingly popular for larger flooring installations. Their use has grown dramatically in the building industry over the last few years because they offer a number of benefits over sand:cement screeds.

The principal advantage is that anhydrite screeds are pumpable and can, therefore, be laid more quickly and easily than traditional sand:cement screeds, making them more cost-effective, especially for larger areas. They will self-level and offer minimal shrinkage, making them especially suitable for use with underfloor heating systems, where they will fully encapsulate all pipework.

Up to 2,000 sq,. metres of anhydrite screed can be laid in a single day; compared to 100 to 150 sq. metres which is the maximum possible with conventional sand:cement screeds, making them particularly suitable for use on large commercial contracts.

Additionally anhydrite screeds have a low manufacturing carbon footprint and can offer a saving in excess of 95% of the carbon emissions associated with the manufacture of traditional sand:cement screeds.

Standards relevant to the installation of anhydrite screeds
BS 8204-Part 7:2003 Pumpable Self-Smoothing Screeds – Code of Practice covering requirements including soundness and level tolerances
BS EN 13813:2002 Screed material and floor screeds
BS EN 13892 Methods of tests for screed material
BS 5385: 3: 2014 Design and installation of internal and external ceramic and mosaic floor tiling in normal conditions
NBS Publication Section M13

It is important to understand the differences between anhydrite screeds and more traditional sand:cement screeds.

Sand:cement screed
This is made up of Portland cement of strength class 42.5 Newtons, combined with a dry aggregate. The mix proportions for a levelling screed should be within the following range; 1 part cement to 3 parts sand through to 1 part cement to 4.5 parts sand by weight. If fine concrete is being mixed with the cement the total aggregate ratio should be between 1:4 to 1:5 by weight.

Wherever the levelling screed is required to be 50mm or greater, then fine concrete should preferably be used. The minimum water required to give sufficient workability for laying and thorough compaction should be used. The screed can either be bonded to the concrete substrate or separated by a membrane or insulation.

Anhydrite screed
These can also be known as a calcium sulfate screed, flow screed or a gypsum-based screed. Unlike a sand:cement screed, an anhydrite screed is a mix of fine and coarse aggregates with a calcium sulfate (gypsum) or calcium sulfate dehydrate binder. Conventional anhydrite screeds are not wearing screeds and are designed to receive a suitable floor covering, such as tiles. When mixed and applied correctly, anhydrite screeds will last a lifetime.

Like sand:cement screeds, anhydrite screeds can also be bonded, unbonded or floating. A bonded screed is laid on to a prepared concrete base which has been primed and sealed with an SBR or an acrylic, polyurethane or water dispersed epoxy primer. A bonded screed can be laid at a 25mm thickness. An unbonded screed is laid on to a separating membrane and can be used at a 30mm thickness. A floating screed is laid on to dense insulation, the minimum required thickness is 35mm in domestic installations and 40mm in commercial installations.

Extended drying time
There are however a number of disadvantages associated with this type of screed; the most significant of which is the extended drying time. Typically sand:cement screeds need to be dried for a minimum of 21 days before they are able to receive a tiled finish as per the recommendations contained within BS5385-3:2003.

However, in the case of anhydrite screeds, for the required maximum relative humidity of 75% to be reached, the minimum drying times before tiling can commence are 1 day per mm up to 40mm thickness, with an additional 2 days per mm thickness above 40mm, according to BS8204-7:2003.

In ideal drying conditions, therefore, a 40mm thick screed should be allowed to dry for a minimum of 40 days. At 50mm thickness the minimum drying time would be 60 days and at 60mm thickness a minimum of 80 days drying time would be required before the screed was sufficiently dry to receive a tiled finish.

These timings assume ideal drying conditions: cold or high humidity environments will require even longer drying times.

There are other factors which also have to be taken into account before tiling onto anhydrite screeds. After they have been pumped, a weak laitance/skin forms on the surface of the screed which needs to be mechanically removed around 4 to 6 days after screeding, using a suitable piece of equipment such as a sander or belt grinder. This opens up the surface of the screed and will therefore also aid the drying process.

Once dry, the screed should be primed and sealed with a suitable product such as an SBR (Styrene Butadiene Rubber) primer. An initial coat of SBR diluted 1:4 with water should be applied to the screed, as this will reduce the screed’s porosity and suppress dust. Once dry, a second coat of SBR, diluted 1:3 with water will need to be applied and allowed to dry. If the screed is still porous, then a third and final coat of SBR diluted 1:2 with water will be required. Once the primer has dried, tiles may be fixed in a suitable C2 or C2 S1 cement-based adhesive.

Failure to follow these recommendations will generally result in adhesion failure at the screed/adhesive interface.

Dealing with the issues on-site
The first issue the tile fixer has to deal with is to establish what type of screed has been used. This will not be immediately apparent. Where refurbishment work is being undertaken, any previous floor finishes, adhesives or smoothing compounds may hide the nature of the screed beneath. However there are ways to determine this.

Anhydrite screeds tend to appear lighter in colour than a sand:cement screed, sometimes almost white. When they have been laid properly, anhydrite screeds are smooth, not granular in appearance, as tends to be the case with sand:cement screeds. There may be very few, if any, joints in the floor span because the product would most likely have been pumped in one pour and does not need any stress joints.

However, in very large pours of unbonded or floating construction, with a dimension exceeding 40m without interruption (such as dividing walls), suitably formed bay joints of compressible material should be evident; and these have to be continued through the tiled finish to prevent tiles cracking or debonding.

If still in doubt, a chemical test can be carried out which will detect a high sulfate content, if it is an anhydrite screed.

Having established that the screed is anhydrite, the specifier has to be aware of the risks associated with using a normal cementitious tile adhesive. If a cementitious adhesive is used prior to the screed being sufficiently dry, a chemical reaction occurs between the gypsum in the screed and the cement in the adhesive, causing the formation of an expansile crystal, known as ettringite. This results in tiles loosening and debonding, and failure occurring at the screed/ adhesive interface.

This risk can be reduced by using a gypsum-based adhesive. In this case, tiling may commence slightly earlier when the screed has achieved a relative humidity of not greater than 85%, instead of the 75% required when fixing with a cement-based adhesive.

In an attempt to reduce the number of failures experienced in the industry, Norcros Adhesives worked together with both a major national house builder and screed manufacturer Gypsol to prepare a generic specification for the industry. This provided a safe method of tiling to anhydrite screeds using cement-based adhesives, but did not greatly improve the drying times required before tiling could commence.

A number of tile adhesive manufacturers are now offering a tile to gypsum product.

However the latest innovation is a fast-track system which is a complete game changer in terms of the drying times required. These new preparation systems, such as Norcros Pro Gyp-Base Fast Track Sealer, make it possible to fix tiles safely using a cement-based adhesive on to anhydrite screeds; even if the relative humidity is as high as 95 per cent, which can normally be achieved in just seven days.

These latest generation products work by taking a system approach, comprising all the required elements – a primer, a moisture suppressant and a gritted primer to provide a ‘key’ on to which the ceramic tile adhesive can form a strong bond.

In summary therefore, the following are the three options that are available for tiling on to anhydrite screeds. For all options the screed will require sanding. However this only needs to be a light sanding when a gypsum-based adhesive or the fast track option is used.

Using traditional cement-based adhesives, the screed must achieve a maximum relative humidity of 75%. In ideal drying conditions (20ºC/65% RH) the minimum drying times are 1 day per millimetre thickness up to 40mm with an additional 2 days per millimetre above 40mm.

So a 40mm screed will have a minimum drying time of 40 days, increasing to 80 days for a 60mm thick screed. It should be noted that ideal conditions are rarely encountered on building sites. Additionally anhydrite screeds will absorb moisture from the atmosphere, and it is not uncommon for the RH of a screed to increase during the plastering process or when ventilation within the room is limited, such as when windows and doors are closed. It has in fact been known for anhydrite screeds to be too wet to receive a finished covering for some 6 to 12 months after laying.

Once dry the screed needs to be both primed and sealed to both reduce absorption and prevent ettringite formation.

When it is intended to fix tiles using a gypsum-based adhesive, the screed will be suitable to tile once it has achieved a maximum relative humidity of 85%, offering a small improvement in the time it should take to achieve this. The screed only needs to be primed to suppress dust and reduce its porosity. Because the adhesive contains gypsum, and not cement, the potential for failure due to ettringite formation is removed, meaning that it is not necessary to seal the screed.

However the best improvements in speed are obtained by using one of the new fast track systems, such as Norcros Pro Gyp Base FastTrack Sealer. This enables the screed to be primed just seven days after it has been laid, once it has achieved a relative humidity of not greater than 95%. After the two coats of primer have been applied and dried, two coats of moisture suppressant are applied by roller and allowed to dry before the gritted primer is applied to provide a key for the recommended C2 cement-based adhesive to bond to. This means that around eight days after installing, the screed is ready to receive a tiled finish.

This reduction in drying time can reduce the time required for the overall build-up of the floor surface by at least ten weeks, which clearly has a very significant impact on timescales and costs on-site. The adoption of one of these new systems will allow contractors to derive all the benefits of rapid and easy pouring of an anhydrite screed, without involving delays on the job caused by having to wait for an extended drying period.

To improve product knowledge and understanding of the issues around anhydrite screeds, Norcros Adhesives is now offering Solutions for Tiling to Anhydrite Screeds, within its portfolio of RIBA-approved CPD seminars, aimed at architects, specifiers and contractors. The company is aware that there is a lack of published detail readily available on this subject and customers frequently contact the Norcros technical department seeking information.

The Norcros Adhesives CPD presentation examines all the issues, so that construction specifiers have the unbiased information that they need when working with these types of screed. Like the company’s existing seminars, the new presentation can be delivered in a variety of different ways. As well as traditional seminars, it can also be viewed online via YouTube or the company’s website.

Norcros Adhesives
Harewood Street
Tunstall
Stoke-on-Trent
Staffs ST6 5JZ
T: 01782 524140
W: www.norcros-adhesives.com