Liquid anhydrite screeds such as the ones we install at UK Screeds have many benefits compared with traditional sand and cement screeds, especially when used in conjunction with underfloor heating. Not the least of these is that when we apply the screed it is in liquid form and is as near as possible a self-levelling compound in Bristol – or wherever the contract is being carried out.
However, there are some things that you need to be aware of if the final flooring is going to be tile, stone, or anything else that is to be affixed to the screed with an adhesive. Our screeds contain anhydrous (dry) calcium sulphate and aggregates instead of cement as a binder. When water is added to anhydrous calcium sulphate it becomes calcium sulphate dihydrate which is more commonly known as gypsum. The reaction stops when the majority of the calcium sulphate binder has been used up, which is usually after a period of 3 – 7 days. The remaining water will then evaporate through the surface of the screed.
Before tiling, it is critical to ensure that the screed has dried out sufficiently. If the floor finish is moisture sensitive the typical value is less than 0.5% water by weight for an impermeable floor and less than 1% for flooring that is more permeable.
The drying time for a calcium sulphate screed is calculated as 1mm per day for screed to a depth of 40mm and 1/2mm per day for the rest of the screed if it is thicker than 40mm. So that means that a screed of 50mm depth would take 60 days to dry. However, it should be noted that this is under ideal drying conditions which are calculated as an air temperature of 20°C and relative humidity no greater than 65%. It is unlikely that these conditions would be met for the length of time taken to dry in the UK weather, so in practice the drying time is likely to be longer.
In the initial stages of drying the screed forms a laitance on the surface which is composed of particles that rise to the top of the screed, and this must be removed before the final flooring is fitted. This is too weak to tile on to and will lead to failure. We do this by sanding the screed between 7 and 10 days after it has been laid. Any longer than this makes the laitance more difficult to remove and also inhibits the drying process.
The drying process can be speeded up by commissioning any underfloor heating that has been installed and can also be assisted by using dehumidifiers. If underfloor heating is installed, it can be switched on as soon as we have finished the sanding process and can be increased by 5°C per day until the maximum working temperature is reached. The temperature should not exceed 55°C, but can be maintained at this level for a week to ten days, and then gradually reduced to a temperature of 15°C – 20°C.
Anything more than light foot traffic should be kept off the screed until it is fully dried. Ideally the screed would be protected by boards until the final flooring is ready to be fixed. The screed should also be protected from moisture because this could affect any underfloor heating and could also damage the integrity of the screed.
Before tiling commences it is advisable to apply a primer to the screed as this will create an effective barrier and make the surface more stable. The adhesive chosen must be one that is suitable for working with calcium sulphate screeds, as some adhesives are certainly not. One also needs to consider the type of tile being installed and also the type of cleaning that will be used when the floor is in service. This is because there may be water penetration.
Either slow or quick curing adhesives may be used. When installing the tiles there should be a minimum gap of 3mm between them and this must be filled using a suitable grout. It is also worth getting technical advice from manufacturers in order to ensure the longevity of the final flooring.