ANHYDRITE screeds, also referred to as calcium sulphate or gypsum-based screeds, are nowadays a popular solution for contract flooring installations. Their use has grown dramatically in the building industry over the past few years because they offer several benefits over sand-cement screeds.
The principal advantage is that anhydrite screeds 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 will 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.
However, there are several disadvantages associated with this type of screed, mainly associated with the extended drying time. To achieve the required maximum relative humidity of 75%, 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.
These extended drying times compared with those for sand-cement screeds, which should be allowed to dry for a minimum of 21 days before they’re able to receive a flooring finish.
Palace Chemicals has recently added Pro Gyp-Base Fast Track Sealer to its range of flooring adhesives and preparation products, which includes fast-track levellers, thick-bed levelling compounds, smoothing and levelling compounds, surface preparation aids and DPMs.
This product comprises a three-stage preparation system for fixing ceramic and porcelain tiles on to screeded surfaces. It’s sold as a kit containing the elements necessary to achieve a successful bond, including a primer, moisture suppressant and gritted primer.
Recently Palace has created a video which shows in simple detail how the system works: www.youtube.com/@palacechemicals/videos
Says the company: ‘Used together, 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 acts as a barrier in the overall build-up between the screed and the tiles. This makes it suitable for use with anhydrite screeds. Pro Gyp-Base Fast Track Sealer can be used on 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%.
‘After drying, tiles may be fixed with the Palace Multi-Flex S1 Adhesive. This is a grey or white cement-based, flexible, fibre-reinforced, cementitious S1 adhesive specially formulated for fixing fully vitrified porcelain, ceramic and stone tiles. It has a setting time of three hours at 20deg C permitting early trafficking of the tiled installation. The adhesive has excellent thermal ageing properties making it suitable for use on heated floors. It’s sold in a 20kg pack.’
‘This system is designed to tackle one of the key issues associated with the increasingly common anhydrite screeds, namely extended drying times, which can be up to 80 days for a 60mm screed,’ says Steve Ball, commercial director at Palace Chemicals. ‘This new addition to our range should prove a real benefit onsite for flooring contractors.’
www.palacechemicals.co.uk
