The anhydrite or calcium sulphate floor screeds that we install at UK Screeds have very many advantages over sand and cement screeds. They are much quicker to lay, as we pour them in by a pump, and this means that a far greater area can be laid in a day than the manual laying of sand and cement. It is estimated that a liquid screed is around 20 times faster to lay.
Our screeds are also very quick-drying, attaining a walkable state in 48 hours or less, so they don’t hold up other contractors on the site. They are ideal for use with underfloor heating, since they fill every gap and void around the pipes which makes for maximum heat transfer into the room. The screed itself is almost twice as good as sand and cement at heat transfer, so it is environmentally friendly too. Furthermore, a liquid screed can be laid far thinner than sand and cement, so uses less material.
On top of that, our liquid screeds at UK Screeds are far easier to level than sand and cement and can easily achieve level SR2 and very often SR1, and it doesn’t get any better than that.
Nevertheless, despite the many benefits, there can be potential problems. Instead of a cement-based binder, these screeds contain anhydrous (dry) calcium sulphate and aggregates and when water is added the binders form calcium sulphate dihydrate which is gypsum. When most of the calcium sulphate binder is used up the reaction stops and this can be anywhere between three and seven days, which leaves any remaining water to dry through the screed surface.
Before undertaking any tiling, it is essential to ensure that the moisture content is at the correct level. Typical values for floor finishes that are moisture sensitive are 0.5% water by weight or less than 75% RH (Relative Humidity) for impermeable flooring, and less than 1% water by weight for more permeable floors. If it is necessary to tile quickly, the contractor can use a damp-proof membrane.
In ideal drying conditions – 20°C and RH less than 65% – the drying time is calculated at one day per mm of screed up to 40mm thick, and two days per mm thereafter. The drying time will increase for screeds that are thicker, and also if drying in poorer conditions than above.
Anhydrite screed in Bristol, or anywhere else for that matter, can be force-dried after seven days, and at this point any underfloor heating that has been installed can help. The heating system should be increased by 5°C per day until the maximum required working temperature is reached or following the heating manufacturer’s instructions but must not exceed 55°C. This temperature should be maintained for a week, and then gradually reduced to 15-20°C. Dehumidifiers will also help to speed up the process.
However, there is one more job that we need to out, as anhydrite screeds do have one small drawback and that is that they produce a layer of fine particles on the surface as they dry which is known as laitance. This needs to be removed before any tiles are laid. Laitance is too weak to tile on to and can also hinder the drying process. Many floor failures are the result of the adhesive being contaminated by laitance.
At UK Screeds we remove the laitance by light abrasion using a floor sanding machine and a 60’s grit sandpaper. The floor surface should first be cleaned in order to remove any objects such as nails which might damage the sander. The sanding should be done from seven to ten days after the screed has been laid. It can also help with the drying out process. The excess dust produced must be removed with a vacuum cleaner.
Before carrying out the tiling, it is recommended that a primer should be used before the adhesive is applied. Before selecting the adhesive, you should consider the tile type and the method of cleaning the floor when in use, such as the possibility of the floor becoming wet and possible limited water penetration. Depending on the type of tile and the environment, either slow or quick-setting tile adhesives may be used.
Finally, when the adhesive has cured, a suitable grout should be used to fill the joints between the tiles. A minimum gap of 3mm between tiles is recommended.