Mandy Searle, Head of Technical Services at Norcros Adhesives, looks at the latest solutions for tiling on to anhydrite screeds in time-sensitive tiling installations.

Anhydrite screeds, also referred to as calcium sulphate or gypsum-based screeds, are now increasingly popular for larger tiling 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 can be laid quicker and easier than traditional sand:cement screeds and are 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 2,000m2

screed can be laid in a single day – compared to 100- 150m2

of anhydrite which is the maximum possible with conventional

sand:cement screeds, making them particularly suitable for use on large commercial contracts.

However, there are a number of disadvantages associated with this type of screed, mainly associated with the extended drying time. Typically, sand:cement screeds should be allowed to dry for a minimum of 21 days before they are able to receive a tiled finish, as per the recommendations contained within BS5385-3:2003. To achieve the required maximum relative humidity (RH) of 75%, the minimum drying times quoted for anhydrite screeds before tiling can commence are one day per mm up to 40mm thickness, with an additional two 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 thick the minimum drying time would be 60 days and at 60mm thick a minimum of 80 days drying time would be required before the screed was sufficiently dry to receive a tiled finish. Failure to follow these recommendations and tiling before the screed is sufficiently dry will generally result in adhesion failure at the screed/adhesive interface.

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

Reductions in drying time can be achieved with the use of a gypsum-based adhesive. This eliminates the formation of ettringite, a chemical reaction which occurs between a wet cementitious adhesive and the gypsum in the screed,

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causing tiles to loosen and debond. Additionally, tiling can be carried out when the screed reaches 85% RH, rather than the 75% RH, required when fixing tiles in cement- based adhesive. 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. The new preparation systems now coming on to the market make it possible to tile with a cement-based adhesive on to anhydrite screeds which have a relative humidity as high as 95%. This can normally be achieved in just seven days, at which point tiling can commence.

It can reduce the time required for the overall build-up of the floor surface by ten weeks, which clearly has a very significant impact on timescales and costs on-site.

These latest generation systems 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.

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.

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