There has been a considerable increase in the use ofpumpable calcium sulphate screeds over the last few years because of the many benefits which they provide over and above traditional sand and cement screeds. They consist of anhydrous (dry) calcium sulphate and aggregates rather than a cement-based binder.

When water is added to the mix, the binders will form calcium sulphate dihydrate which is known as gypsum. The process stops when most of the calcium sulphate has been used up which is generally after somewhere between 3 and 7 days depending upon the weather, and then the rest of the water will evaporate through the surface of the screed.

The drying time before the final floor surface can be laid will vary according to the thickness of the screed, and this is generally calculated as 1mm per day up to a thickness of 40mm and ½mm per day for any thicker screed. This is dependent upon the weather conditions and is based on an average temperature of 20°C and a relative humidity no greater than 65%. Since it is unlikely to achieve these conditions for that length of time in the UK, the actual drying time will often be longer.

If you are using a calcium sulphate screed for floor preparation in Aylesbury, or for that matter anywhere else, it is critical that the moisture content is at the right level for the required floor finish. For moisture-sensitive floor finishes, the values are about 0.5% water by weight or less than 75% relative humidity for impermeable floors and 1% water by weight for flooring that is more permeable.

Laitance Removal

Calcium sulphate screeds do need to be sanded before the final flooring is laid because as they dry, they form a layer of laitance on the surface, and this must be removed. This should be carried out between 7 and 10 days after the screed is laid. Left any longer, the laitance will harden and become more difficult to remove. It will also delay the drying of the screed.

It is possible to speed up the drying process once the laitance has been removed. This can be done by turning on any underfloor heating. It can be increased by 5°C per day until the maximum working temperature has been reached and then left at this level for a week. After this, the temperature can be gradually decreased to about 15 – 20°C. The temperature should not exceed 55°C.

It is also possible to speed up the drying process by the use of dehumidifiers. On any building contract, time is money,so by using force drying in conjunction with underfloor heating where it is installed, it can cut the overall drying time considerably. If the screed is 45mm thick, at the temperatures and relative humidity quoted, it will take 50 days to dry. In practice, depending upon weather conditions, it is more likely to be around three months. Using force drying can bring that down to around four weeks, which represents a considerable saving.