Over the last 15 years or so, calcium sulphate, or anhydrite, screeds have grown considerably in popularity and today account for about one quarter of all screeds installed in new buildings.
There are several advantages that they have over and above traditional sand and cement screeds, not the least of which is that they are very quick and easy to lay. At UK Screeds our anhydrite screeds are delivered to site pre-mixed and we then lay them using a pump and a long hose. This means that our teams can pour and lay up to 2,000 square metres in a day, which is anything up to 20 times as fast as laying a sand and cement screed by hand the traditional way.
Once the screed is poured, all it requires is running over the surface with a dappling bar in two directions and it can then be left to dry. It will be dry enough to walk on in 24 – 48 hours which means that it will not delay any other contractors who need to work on the site.
There are other advantages over sand and cement screeds as well, one of which is that, because the screed is liquid, it is also pretty much self-levelling and will certainly achieve a more level surface than sand and cement. Sand and cement screed can sometimes struggle to achieve measurement SR3, while our liquid screeds will always achieve SR2 and usually SR1 which is as good as it gets.
Our anhydrite liquid screeds are perfect for use with underfloor heating, too. When you use a sand and cement screed which is laid by hand, it is very difficult to totally cover the heating pipes. What happens is that there are voids and air pockets and these will affect the thermal heat transfer into the room. With a liquid screed it completely covers the heating pipes and leaves no voids, so the heat transfer is perfectly even.
Furthermore, our liquid screed is laid far thinner than a sand and cement screed which means that less energy is required to heat the room to the desired temperature. Not only that, but the gypsum screed itself has thermal transfer properties which are nearly twice those of sand and cement, again adding to the energy saving.
When the water is added to anhydrous (dry) calcium sulphate it becomes calcium sulphate dihydrate which is gypsum. When the bulk of the calcium sulphate binder is used up, the reaction stops, and this is usually after somewhere between three and seven days. This leaves the remaining water to evaporate through the surface of the screed.
As the screed dries it brings particles to the surface which form a layer known as laitance, and it is essential that this is removed before the final flooring is laid. We do this using floor sanding machines between seven to 10 days after pouring the screed. However, if you choose, you could do this yourself, and we can let you hire floor sanding machines in Bristol, or wherever you happen to be, for that purpose.
Whether you carry out the floor sanding or we do, this must be carried out carefully and the floor surface vacuumed afterwards so that no particles are left. It is essential not to leave the floor sanding for too long because the laitance will harden and become more difficult to remove. It can also interfere with the drying process.
Before laying the flooring on to the calcium sulphate surface it is necessary to ensure that the required level of drying has been achieved and this can be assisted by turning on the underfloor heating, if fitted, and gradually increasing the temperature. It can be increased by 5°C per day, but should not exceed 55°C. It can then be left at this temperature for a week, and then gradually reduced again to 15°C to 20°C.
It is also possible to use force drying with the use of dehumidifiers to help speed up the drying process, and by using them in combination with the underfloor heating, the overall drying time before the final flooring can be laid can be reduced to as little as 28 days.