THE use of anhydrite screeds has grown significantly in the flooring sector in recent years on account of the benefits they offer over sand-cement screeds. Their main advantage is that they can be laid more quickly and easily than traditional sand-cement screeds and are more cost-effective, especially for larger areas.

They self-level and suffer minimal shrinkage, making them especially suitable for use with underfloor heating systems, where they’ll fully encapsulate all pipework. Up to 2,000sq m of anhydrite screed can be laid in a single day – compared to 100-150sq m which is the maximum possible with conventional sand-cement screeds, making them particularly suitable for use on large commercial contracts.

Anhydrite levelling screeds are based on a calcium sulphate binder and require to be fully dried to 75%RH (0.5% w/w carbide test); abraded to remove laitance and efflorescence and then completely sealed with an effective and proven primer, so as to prevent the adhesive from coming into direct contact with the calcium sulphate in the levelling screed, which is likely to result in the formation of ‘ettringite’ expansive salts at this interface.

The minimum drying times quoted for anhydrite screeds before laying a floorcovering are one day per mm up to 40mm thickness, with an additional two days per mm thickness above 40mm according to BS8204-7:2003.

In ideal drying conditions therefore, a 40mm thick screed should be allowed to dry for a minimum of 40 days, at 50mm thick the minimum drying time would be 60 days and at 60mm thick a minimum of 80 days drying time would be required before the screed was sufficiently dry to receive a finish. Failure to follow these recommendations will generally result in adhesion failure at the screed/adhesive interface.

Palace Chemicals offers Pro Gyp-Base Fast Track Sealer within its range of preparation products. ‘This is a four-component solution, where two coats of Epoxy Primer are applied and over-coated with a moisture suppressant coating and finally an aggregated sealing topcoat, all of which combine to create an impermeable barrier, which is effective where the moisture content of the anhydrite screed is significantly higher than the traditional 75% RH target, (up to 95% RH after seven days) allowing for earlier application of floor finishes fixed with cement-based adhesives.’

A video is available which shows in simple detail how the system works:
www.youtube.com/@palacechemicals/videos

Says the company: ‘The system allows for fast-track installation of floorcoverings, without detracting from the overall screed strength. It also avoids the need for a special gypsum adhesive, which is normally required for fixing tiles and other floorcoverings on to anhydrite screeds in order to prevent them from debonding.

‘Pro Gyp-Base Fast Track Sealer can be used on either heated or unheated anhydrite screeds. A 15kg kit contains everything required to prepare and seal approximately a 25sq m area. In an ambient temperature of 20deg C, drying times are down to two hours for the first primer coat and 24 hours for the second primer coat.

‘The moisture suppressant and gritted primer require a total of three hours drying time before tiling commences. Screeds need to be a minimum of seven days old and have a maximum relative humidity of 95%.’
www.palacechemicals.co.uk