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Advice from The Tile Association
Calcium sulfate screeds have seen a huge rise in popularity over the last few years. It's easy to see why, given they have a number of benefits over sand:cement screeds.
Whereas traditional sand:cement screeds can take longer and be labour intensive to install, calcium sulfate screed – also known as liquid, anhydrite or gypsum-based screed – is substantially quicker and easier.
However, these screeds have caused some issues in the tiling industry over recent times. Caution must be exercised when tiling on them and the following guidance from The Tile Association will help to avoid costly mistakes…
Preparation is especially important when it comes to calcium sulfate screeds. Good practice at this stage cannot be overstated.
It doesn't help that it's not always obvious that a screed is – or is not – calcium sulfate based.
Often, a fixer arriving at a job may not know what type of screed has been used. Although calcium sulfate screeds tend to appear lighter than a sand:cement screed, sometimes almost white, it can often be difficult to tell them apart. When they have been laid properly, calcium sulfate screeds appear smooth, not granular in appearance as is often the case with sand:cement screeds. There may be very few, if any, joints in the floor span because the product would most likely have been pumped in one pour, without the need for stress joints.
However, it is vitally important to know if the screed is calcium sulfate-based as there can be a reaction between normal cement-based adhesives and the calcium sulfate. If you have any uncertainty at all, then chemical analysis will have to be carried out. Calcium sulfate screeds are not suitable for tiling onto in areas which may become wet.
There are three steps to follow for successful, trouble-free tiling installations on calcium sulfate screeds:
• Correct drying out
• Appropriate surface preparation
• Priming
Correct drying out
Before ceramic floor tiles are laid, the moisture content of the screed should be checked to make sure it is adequately dry. Unlike cement-based materials, which can remain strong even when wet, calcium sulfate-based materials gain strength by a process of crystallisation and must be fully dry before covering.
The residual screed moisture must not exceed 75%RH (Relative Humidity) when using a hair hygrometer in accordance with BS8203-1.
Surface preparation – including sanding
During surface preparation (and subsequent floor covering installation) the area should be kept clear of all of the trades to avoid any contamination, or damage to the floor covering.
ALL calcium sulfate screeds must be buffed or ground. As calcium sulfate screeds dry, a layer of laitance forms on the surface. This is a weak layer which, if not removed, will result in tiles debonding, with adhesion failure generally occurring at the interface between the laitance and screed.
All traces of dust, laitance and loosely bound particles must be fully removed by using abrasive sanding equipment, and then vacuumed. Some suppliers are now stating that their product does not need to be buffed – but The Tile Association recommends that: -
The main contractor, project manager or responsible person should ensure that the surface of the screed has been correctly prepared to ensure removal of all laitance and loosely bound particles.
This can be achieved by using abrasive sanding equipment and vacuum cleaning. For a more effective, dust-free and environmentally friendly technique, a contained shot blasting equipment with vacuum extraction is recommended. The tiling contractor should ensure this has been carried out before tiling begins.
The screed should be swept and vacuumed to remove all dust and friable material so the surface to be tiled is clean, dry and sound.
Priming
When cement-based adhesives and/or levelling compounds are used, surface priming is necessary to avoid adverse chemical reactions between Portland cement and the calcium sulfate screed (i.e. ettringite formation). The primer application needs to provide complete coverage and must be adequately absorbed in the calcium sulfate surface in accordance with manufacturers’ instructions.
Specifically designed calcium sulfate adhesives that eliminate the need for primers are also available.
