Find out all you need to know about specifying the correct products for tiling to calcium sulphate/anhydrite screeds to avoid installation failure.
Anhydrite screeds are gypsum (calcium sulphate) based flow screeds made of a gypsum binder, aggregate and mixing water that are normally pumped and are self-levelling. They are becoming more prevalent in the building industry as they are easy to form and a lot less labour intensive than traditional sand and cement screeds. They are also quicker to lay and can be a lot thinner than a traditional screed. They are often used on top of under floor heating systems, particularly water based ones as they conduct heat better than a sand & cement screed. As a result of this they are now used in all types of situation from large & small scale commercial projects to domestic ones.
They do however have some disadvantages: they are prone to cracking if poorly installed and take up to 6 weeks to dry out completely. A layer of laitance forms on their surface which needs to be removed before tiling and they react with cement based adhesives if used without a primer.
Drying the Screed
Some manufacturers offer screeds designed to dry out and be ready for use more quickly than the standard ones, and these screeds will come with instructions on drying times. However the rule of thumb for normal screeds is 1 day drying for each 1mm depth of screed up to 40mm thick. Above that and you need to calculate 2 days for every 1mm of thickness. The screed should be protected for the first 24 hours from drying too quickly and should then be dried in a well-ventilated room at ambient temperature. Where time is at a premium, it is possible to speed up the drying process one week after laying with the use of de-humidifiers or by initialising the underfloor heating. If the screed needs force drying and contains heating pipes, it can be speeded up further according to the schedule below.
| Drying Times for Anhydrite Screeds in Normal Conditions | |
| Thickness | Drying Time |
| 25mm (minimum thickness required over underfloor heating and conduits) | 25 days |
| 40mm | 40 days |
| 50mm | 60 days |
| 60mm | 80 days |
| Force Drying of Heated Screeds | |
| Action 1 | Wait 2 days after laying at basic temperature of 20oC |
| Action 3 | Increase heat by 5oC per day until the maximum input temperature is reached. |
| Action 4 | Keep the input temperature constant for at least 7 days without reduction |
| Action 5 | Reduce the temperature by 10oC until a screed surface temperature of 15-20oC is reached. |
| Action 6 | Get the screed checked by the installer before tiling. |
Tiling should not commence until there is less than 0.5% residual moisture in the screed. This can be tested in various ways but the most common is to use a hair hygrometer as this is a non-destructive test. BS8203 – 1:2001 sets out the correct method for doing this. The other commonly used method is a carbide bomb which is recommended by the European Standard for testing calcium sulphate screeds.
Electric moisture meters are not suitable for use to test the residual moisture content as they are not accurate enough.
| Moisture Testing Methods | |
| Hair Hygrometer
British Standard BS8203-1:2001 method |
A non-destructive test method for sand: cement screeds may also be used for pumped calcium sulphate
based screeds. This figure equates approximately to 75% relative humidity (the required limit for floor finishes). For correct results, the BS8203 method must be strictly adhered to, including the use of a correctly sized and insulated box sealed to the floor, a sufficiently long test for equilibrium to be reached and the use (where appropriate) of an impervious sheet around the instrument. |
| Carbide Method
European Standard for testing calcium sulphate screeds |
A ‘Speedy’ moisture tester is used, preferably a model which has a scale reading of 0% – 2% on the recording dial. Typical requirements will be 0.5% water by weight for impermeable coverings (e.g. ceramics). This figure equates approximately to 75% relative humidity. |
| Oven Drying of a Sample | The sample is weighed before and after the oven drying to determine the weight loss as a percentage of the dry weight. The oven temperature should be 40oC (higher temperatures will give false results). Samples should be of full screed depth and are to be wrapped in plastic immediately after sampling to prevent them drying during transport to the testing laboratory. |
Surface Regularity
Anhydrite screeds are supposed to be self-levelling, however conditions when laying can affect the flow and the floor will not always be perfectly level. A surface regularity test should therefore be carried out and any deformities that are over the norm should be corrected. This is particularly important across screed joints.
Surface Regularity of Pumped Self-levelling Screeds
| Class | Maximum permissible departure from a 3 m straightedge resting in contact with the floor | Application |
| SR1 | 3 mm | Ceramic floor tiles bedded in adhesives directly to the prepared and primed screed |
| SR2 | 5 mm | Ceramic floor tiles bedded after pre-smoothing the screed |
| SR3 | 10 mm | Ceramic floor tiles bedded in an unbonded semi-dry cement and sand bed |
Movement & Stress Relieving Joints
Joints in the tiling should coincide with the joints in the screed whether these are for stress relief or movement. Wherever possible these joints should have been formed as straight joints that intersect at right angles to make it easier to set out the tiling scheme and thus avoid stresses to the tiles. In heated screeds the width and spacing of movement joints should take into account any thermal movement between the minimum and maximum operating temperatures. It can be helpful to establish the tiling scheme prior to the installation of the screed so that it can be taken into account when planning movement joints.
Good Preparation
The secret to any successful tile installation is good preparation allowing you to achieve a clean, sound base and this is particularly so with anhydrite screeds. Any cracks should be primed where necessary and filled and any surface damage made good; again priming is almost always necessary before using smoothing or repair materials as otherwise the sulphate in the screed is likely to react with them.
Loose material and any laitance present should be fully removed and a moisture test as detailed above carried out before tiling.
Laitance Removal
All anhydrite screeds produce a layer of laitance. It is imperative that this is properly removed ideally 2 – 6 days after laying. The exact timing depends on the brand of screed used. Removing it allows the screed to dry more quickly and gives a dense layer for tiling on to. Not removing the laitance fully will cause the installation to fail as the tiles will debond. The floor should be sanded smooth and then vacuumed to ensure that it is clean and sound.
How to Identify an Anhydrite Screed
An anhydrite screed will tend to appear lighter than a sand and cement one, sometimes almost white. But in practice it is very difficult to tell an anhydrite from a traditional screed as sand is used as a filler in the former, and the only way to be sure is to ask the installer. If in doubt treat the screed as anhydrite: doing so will not cause the tiling installation to fail on a sand & cement screed, whereas not doing so will almost certainly result in tiles debonding if the screed is anhydrite.
Cement Based Adhesives
The reaction caused between the cement in cement based adhesives and the gypsum in the anhydrite screed forms a layer of crystals called ettringite. This is calcium sulphoaluminate. It is formed by the sulphate in the gypsum reacting with the calcium aluminate that is present in the cement based adhesive within the first few hours after adding water. This reaction also occurs with sand & cement screeds and helps the adhesive to bond in that situation. However with anhydrite screeds the presence of water causes the reaction to continue until the gypsum is depleted, weakening the screed, causing the surface bond to break and hence the installation to fail. The solution to this is to use a primer that will prevent the reaction from occurring.
Kerakoll produce Primer A Eco to prime gypsum based screeds. This EC1 classified water based product acts as a barrier between the anhydrite screed and the adhesive, preventing the chemical reaction which leads to the formation of ettringite. The primer is applied neat to the floor and is a distinctive green colour, making it easy to see whether the whole floor has been covered. The primed floor will be ready for tiling after 4 hours. The moisture content of the substrate should be checked after the floor has been primed.
Biogel Adhesives
Kerakoll’s Biogel No Limits (standard set C2 TE S1) and Biogel Revolution adhesives (rapid set C2 FTE S1) can be used on gypsum-based screeds that have been primed with Primer A Eco. These are both gel adhesives that are easy and pleasant to work with, with excellent performance values and low environmental impact. Their excellent spreadability and wettability properties makes it easy to ensure that there are no cavities between the tiling and the screed, even for extra-large tiles.
Grouting
Kerakoll’s hybrid resin-cement grout range, Fugabella Color is perfect for finishing the installation. It is formulated with a new mix of pure, natural binders which have been hybridised with resin binders and other additives, giving them very low VOC content, lower environmental impact than traditional cement or epoxy-resin grouts, and making them safer for people’s health.
It is suitable for use in a wide range of temperatures and environmental conditions meaning that you can use the same grout for everything rather than having to choose one that fits the on-site conditions. The rich, intense colours maintain the depth of colour and don’t fade as resin-cement grout is not subject to salt and lime efflorescence.
The purity of the materials employed along with the use of specific natural additives gives reduced water absorption, lower levels of joint staining and complete protection from common acid pH substances. A revolutionary bonding system gives superior resistance to abrasion of 425 mm³ for Fugabella Color after 72 hours compared to 970 mm³ for a cementitious grout.