Calcium sulphate flowing screeds have been gaining rapidly in popularity over the last few years as their many benefits over and above the traditional sand and cement mix have become apparent. They are very easy to lay because instead of being trowelled on to the substrate by hand, they are pumped on to it using a long hose attached to the delivery truck. In fact, it is calculated that a flowing liquid screed can be laid as much as 20 times as fast as sand and cement.
The liquid screeds that we supply at UK Screeds are made using calcium sulphate as the binder instead of cement, and they are also known as anhydrite screeds because the calcium sulphate is anhydrous – i.e. dry. When water is added, the calcium sulphate becomes calcium sulphate dihydrate, more commonly known as gypsum, and so they are also called gypsum screeds!
Whatever the name, these screeds have many benefits, one of which is that they do not have to be laid so thickly as sand and cement. If a sand and cement screed is not going to be allowed to curl, it has to be laid a lot thicker than gypsum screeds. These do not curl in any case, and shrinkage is absolutely minimal.
When installing underfloor heating, a traditional screed needs to be at least 75mm thick, and may be as much as 100mm. By contrast, when using a flowing screed in Winchester, we only need to cover the heating pipes to a depth of 30mm, making 45mm in total. So, although the cost of the material is around double that of sand and cement, we don’t need to use nearly so much of it.
Reducing The Drying Time
This also reduces the overall drying time. Our screeds are calculated to dry at a rate of 1mm per day up to a depth of 40mm and ½mm per day thereafter, so a depth of 45mm will take 50 days to dry. That is a lot faster than a sand and cement screed of 75mm or more. However, in practice the drying times of both will take somewhat longer because the figures are calculated on the basis that the temperature is at 20°C and the relative humidity no greater than 65%. That is unlikely to be achieved in the British Isles for that length of time.
However, our flowing liquid screeds can be force dried after 7 days. First of all, the underfloor heating can be turned on and increased by 5°C each day until the maximum working temperature is achieved. The temperature should not exceed 55°C. It can be kept at the working temperature for at least a week, and then gradually reduced to about 15 – 20°C. Force drying equipment can also be brought into use in order to reduce the overall drying time before the flooring can be laid.
If there is one small disadvantage with our flowing screeds it is that as they dry they form a layer of particles on the surface known as laitance. These need to be removed before adding the flooring, so we do this by sanding between 7 and 10 days after the screed has been poured. Overall, our liquid screed will be ready to receive the final flooring a long time before a sand and cement screed would be ready.