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Are anhydrite screeds still causing you a headache? by Instarmac’s Martin Cummins

May 2017

The use of calcium sulphate screeds offers extensive benefits from both a design and application viewpoint, so they are here to stay.  A great number of installations of flooring and tiling products are carried out on a daily basis, so why, I wonder, do I still get calls asking, how do I know if it is one of “those” screeds?   Am I responsible for getting the suitable products?  What needs to be done to make sure the installation is a success?

I would like to take this opportunity to answer these questions without talking too technically about the different types of screeds available, the sub-base requirements underneath them or definitive test methods to assess them.  This is more about practical awareness for the fixer.  If you ever feel that you need further detailed information, please feel free to email or call me personally, or contact the screed manufacturer directly.

Before arriving on-site you should have already asked what the subfloor is.  Now assuming this information hasn’t been forthcoming you can make a visual assessment to see if indeed you have a calcium sulphate screed.

If laid properly, calcium sulphate screeds are very smooth and level.  The surface is not granular in appearance, as can often be found 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 and does not need any stress joints introducing.

If there is an opportunity to see the cross section of the screed, maybe at a doorway or a plinth, measure the depth and if it is laid at less than 50mm (or less than 70mm above underfloor heating pipes) there is a good chance it is a flowing calcium sulphate product.  If it is laid above 70mm deep then be concerned as it will take an age to dry out.

Take a look around for radiators or heating systems.  If there are none, it is likely that underfloor heating has been incorporated.  Calcium sulphate application and technical performance works very well with underfloor heating, so it is a good indicator that a flowing screed has been used.  Ask where the manifold is and request that you need to be assured that the underfloor heating has been commissioned and brought up to full working temperature.  If it hasn’t, you cannot guarantee what will happen to the screed once it is tiled over and the heating is switched on.

It is also advisable to question if there is a shiny ‘skin’ type material on the surface.  Is so, this is referred to as laitance and it can pose problems.  Fortunately this is now much less of an issue since low laitance calcium sulphate screeds have become more prominent.  If there is laitance then it needs to be mechanically abraded away to ensure you can fix to the actual screed (not a surface skin) and also because it will inhibit the drying, this is your responsibility.  When calcium sulphate screeds first entered the market it was part of the installers’ package to return and abrade away laitance.  This is not the case now, so make sure that you budget and give appropriate timeframes that will enable it to be done.  Do also make sure that it is strong, sound and dust free after completing your preparation.

Finally, does it look a bit lighter in colour than a traditional screed?  The first products on the market were distinctly different but this isn’t always the case now, so if it doesn’t look different to a normal screed still check the other aspects as discussed previously.

A bone of contention with such screeds is the level of dryness and the importance of being dry.  In the tiling world we are used to fixing to cured screeds – these are hard, strong and we can get adhesives to bond to them.  Are you aware however that these screeds may still contain a significant amount of moisture.  Cured does not mean dry.  When completing flooring installations, retained moisture is a nightmare whether it is in cement or calcium sulphate subfloors; it affects adhesives, floor coverings and smoothing compounds.  A wet calcium sulphate screed can cause similar havoc with tiling installations.  The basic chemistry is that moisture movement between calcium sulphate and cement can cause expansion and tiles lifting.

A further problem is that calcium sulphate doesn’t really have an inherent surface strength under damp conditions unlike cement based screeds (think of plasterboard compared to tile backer boards under damp conditions).  It is therefore important to get the screed to a required level of dryness.  Technically this requires full testing apparatus or samples being sent to a lab, so how can you make a valued judgement?  My recommendation is to fix an impervious material (polythene or rubber mat for example) over the screed for approx. 3 days, then remove it and compare to the uncovered area.  It is always a good idea to do this in view of the client as it can give a dramatic picture.  A darker appearance and a surface that can be readily scratched by a coin indicates a screed still with significant moisture.  Do not lay.  Turn the underfloor heating on, get dehumidifiers in and come back a week or two later and re-assess.

The stage is reached where you know or think it is calcium sulphate screed, the underfloor heating has been commissioned, the floor is dry, and so what is next?  Use a CE certified cementitious adhesive, and create a barrier primer to ensure initial cure of the adhesive without significant interaction with the calcium sulphate.  The Ultra Tile process is to apply 2 coats of an acrylic resin primer, Prime IT AR, which is specifically designed to film form and offer the barrier requirements.  A 3:1 dilution on day 1 is required.  This is then left overnight to film form before applying a 1:1 dilution as a standard bonding aid.  Allow to dry for a few hours.

The selection of the cementitious adhesive then depends on other requirements such as whether underfloor heating is involved (if so use an S1 grade) and also on the type of tile.  We would advise S1 for vitrified tiles, whilst a C2 is sufficient for ceramics.  For natural stone tiles always speak to your adhesive supplier.

I trust this is informative in guiding you through the anhydrite minefield.  They are good products – we just don’t give them the opportunity to perform due to our rapid installation programmes.

Martin Cummins is Technical Sales Manager, Ultra Tile
T: 01827 871871  W:

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