Over the course of the last few years, anhydrite screeds have grown enormously in popularity, and with good reason. They have so many advantages over the traditional sand and cement screeds that have been used for years. However, when you are going to use tile as the finishing floor surface there are a few things that you need to take into consideration.
Anhydrite screeds use anhydrous (dry) calcium sulphate as the binder for the screed instead of cement. When water is added to it, the calcium sulphate turns into calcium sulphate dihydrate, otherwise known as gypsum. This helps to explain why these screeds are known by different names such as anhydrite screeds, gypsum screeds, calcium sulphate screeds and, because they are in liquid form, they are also liquid screeds, flowing screeds, self-levelling screeds (any liquid is self-levelling), and more. Suffice to say, they are all the same thing.
Yet another name is a rapid cure screed, so if you need a rapid cure screed in Newbury, for instance, at UK Screeds we can help you. When we say “rapid cure” we are referring to the fact that, although the screed is in liquid form, nonetheless it will be dry enough to walk on in 24 – 48 hours after installing. This is of great benefit to any construction project because it means that the other contractors who are needing to work on the site will not be unduly delayed because of the screed drying time.
However, although the screed is dry enough to walk on, it is still not fully cured, and this will take longer before it is possible to lay the tiles – or whatever else the final flooring is going to be.
The drying time for the screed is calculated as 1mm per day for the first 40mm of depth, and half a mm per day thereafter. However, this is based on an air temperature of 20°C and relative humidity of no more than 65%. These conditions are unlikely to be met in the UK, so the drying time could be longer.
There is a very important point to notice here, and that is that our liquid screeds can be laid much thinner than traditional sand and cement. So, where sand and cement may need to be laid to a depth of at least 75mm our anhydrite screeds only need to be 45mm deep which includes enough to cover underfloor heating pipes. If underfloor heating is not being installed, then 25mm is plenty. As you can see, this means that the overall curing time of our liquid screeds is going to be considerably less than sand and cement under any conditions.
Not only that, but if underfloor heating is fitted you can decrease the curing time even further. Once the screed has been laid for 7 days you can commission the underfloor heating and increase it by 5°C per day up to a maximum of 55°C. It can be left at this temperature for at last a week and then gradually decreased to about 15 – 20°C.
Of course, not only can you commission the underfloor heating, but you can also speed up the process by using dehumidifiers as well. By doing this you can reduce the overall curing time down to around 28 days, after which you can lay the tiles.
One thing to note is laitance. When the screed is poured it will produce a fine layer of particles on the surface as it dries. This is called laitance, and it needs to be removed by sanding between 7 and 10 days after the screed has been poured. Left any longer and the laitance will become harder to remove.
Before adding the tiling, it is best to prepare the surface by applying a primer to it before applying the tile adhesive. This creates an effective barrier and helps to make the surface more stable. Slow or rapid-setting adhesives can be used according to the type of tile and the environment. One should also consider the cleaning method of the tiles when in use and whether or not any minor water penetration may occur.
Once the adhesive has cured, a grout can be used to fill in the gaps between the tiles, and these should be a minimum of 3mm.