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Flowable calcium sulphate screeds, also known as anhydrite, hemi-hydrite and gypsum, have many advantages over traditional sand/cement screeds such as:
It is important that calcium sulphate screeds are identified before any tiling installations are carried out because they have different requirements. They may not be visually different from traditional screeds so always enquire, particularly if the screed contains underfloor heating. We recommend the use of a barrier primer on calcium sulphate screeds to avoid migration of moisture between adhesive and subfloor.
Unlike sand / cement and cementitious products, which can still have extremely high tensile and compressive strength whilst retaining a high level of moisture, calcium sulphate screeds need to reach a level of dryness to enable them to perform correctly underneath the tiling. The approved standard moisture test method is to use a surface hygrometer. This is an insulated box, fixed to the unheated floor for typically 4 days, after which the moisture in the air trapped in the box reaches equilibrium. This air is then tested using either an analogue or digital hygrometer. If the reading is less than 75%RH (relative humidity) then the screed is dry enough.
Other indicative test methods may be used to help identify if moisture is a concern or if the screed is close to dry. A simple test is to tape a piece of plastic to the floor for 48 hours. Moisture condensing on the underside of the plastic or a darkening of the screed indicates moisture levels are still significant.
The recommended drying times of calcium sulphate screeds, as quoted by the manufacturers, are usually based on drying conditions at 20°C, low air humidity and an open surface with no materials overlaid.
This does not represent a typical site scenario so they should not be relied upon. It is also important to remember that underfloor heating must have been fully commissioned. This does not mean a simple air pressure test but means a full cycle through the heating range. This is necessary to:
A. Identify if any weaknesses are in the screed by showing likely points of cracking and spalling (typically due to poor installation of the screed with heating)
B. Assist the drying of the screed
We recommend applying 2 coats of primer, ensuring consolidation and isolation of the calcium sulphate. This allows the standard classified cementitious tile adhesives to be utilised in the normal manner.