Tiling onto anhydrite

Calcium Sulphate is the name of the binder that is used in the screed. Traditionally, screeds are cement-based however this type is an exception and uses calcium sulphate rather than cement. It’s a by-product of the chemical industry in the UK that gets milled for reuse rather than landfilled.

These types of screeds contain anhydrous (dry) Calcium Sulphate and aggregates instead of a cement-based binder. The binder comes as either an alpha hemi-hydrate (a stronger and harder crystalline form) or anhydrous Calcium Sulphate. When water is added, the binders will form calcium Sulphate dihydrate – more commonly known as Gypsum. This reaction stops when the vast majority of the Calcium Sulphate binder is used up and is generally complete after a period of 3-7 days, leaving the remaining water to evaporate through the surface of the screed.

The screed has low shrinkage, has a lessened risk of cracking, is quick setting making it fast to install and no reinforcement is required.

It is vital to ensure that the moisture content in the screed is at the correct level for the required floor finish, the screed has to be dry before tiling.

Typical values for moisture sensitive floor finishes are less than 0.5% Water by Weight (w/w) or less than 75% relative humidity (RH) for impermeable floors and less than 1% w/w for more permeable floors.

As a general rule of thumb, the drying time for a Calcium Sulphate screed, so the moisture content reaches 0.5% w/w, is approximately 1mm per day up to 40mm in ideal drying conditions (i.e. an air temperature of 20°C with a relative humidity no greater than 65%).

Drying times will significantly increase for thicker screeds or those in poor drying conditions. If you do not have a screed moisture meter, many adhesive or screed manufacturers offer a free moisture testing service.

The speed of drying out of the screed can be improved by a) increasing the ventilation in the room b) the application of external heat i.e. space heaters, combined with the use of industrial dehumidifiers in order to remove water vapour as the higher temperature encourages the natural evaporation of excess moisture.

Accelerated drying of Calcium Sulphate screeds may be used once the screed is at least 7 days old. It’s at this point that any underfloor heating system should be increased by approximately 5°C per day until the maximum required working temperature is reached or as per the underfloor heating manufacturer’s recommendations (do not exceed a temperature of 55°C).

Maintain for a minimum of one week, before being gradually reduced to around 15-20°C.

Once tiled do not turn on the underfloor heating for a minimum of 14 days, it is at this point that any underfloor heating system should be increased by approximately 5°C per day until the maximum required working temperature is reached or as per the underfloor heating manufacturer’s recommendations. If the heating is turned on before the 14 days has passed, there is a high chance the tiles will lift from the floor.

Further advice should be obtained from The Tile Association Technical Document “Tiling to Calcium Sulphate Based Screeds” and BS 5385-3.

Laitance is a weak, intermediary layer of fine particles deposited on the surface of the screed, generated by water migrating through to the screed surface as drying occurs and can often appear as a wax-like covering. This layer is too weak to tile onto and can also inhibit the screed from drying.

Before laying tiles onto a Calcium Sulphate screed you must ensure that all laitance, dust, dirt or any other loose surface material incompatible with the tile adhesive must be removed. Remove laitance by light abrasion using a suitable sanding machine i.e. a rotary floor scarifier with a 60’s grit sandpaper.

This abrasion should be carried out 4-6 days after application and can aid in the drying process. Scarifying the surface, even if you are tiling onto a low laitance anhydrite screed, will provide a key for the primer and this aids adhesion for the adhesive. Seek further advice on the correct method to achieve this from the screed manufacturer.

Ensure to remove any excess dust with a vacuum cleaner.