It’s strong, it’s temperamental and if you treat it wrong there could be consequences. Yes, this month, we’re talking about the hotheaded anhydrite screed. A product more confidently used by the professionals in the trade, however, with tradesmen having to adapt their skills more, Debi Bailey, Brand Manager of Dunlop Adhesives, casts her beady eye over the use of this tricky substrate and shares some trade secrets with merchants to encourage better practice in the building environment.
Why use anhydrite screed?
So, let’s start with the benefits. Anhydrite screed possesses properties that make it an extremely attractive option in the trade. Whereas traditional sand and cement screeds can be slow and labour intensive to install, anhydrite screed is substantially quicker and easier. In addition, it is also cheaper, self-levelling, offers minimal shrinkage and is convenient for use with underfloor
heating. It is also fast setting, and can be ready to be worked upon four or five days later.
The overall effect is that anhydrite screed, at least in the laying phase of usage, is less reliant on the quality and experience of the installer. However, when it comes to tiling on top, the expertise and knowledge of the tradesman is paramount, which is where merchants can step in and share their inside knowledge to help tradesmen avoid potential pitfalls and ensure repeat custom.
The Dangers!
The reason for this screed’s perceived ‘ill-behaviour’ is that if it is not correctly prepared, the combination of moisture and a potentially weak surface can react negatively with the tile adhesive, terminally disrupting the bonding process. As with ceramic/porcelain tiles as well as with stone, there are a number of specific effects to be wary of.
The first involves the direct application of the cement-based adhesive. The adhesive can react with the gypsum in the screed resulting in a mineral called ettringite being formed at the interface. The subsequent chemical changes are sufficient enough to cause de-bonding of the adhesive away from the screed base. Secondly, the presence of a weak layer of laitance is created, slowing the drying time and making the screed weak to tile onto. As with traditional cement; sand screeds, it is always good practice and important to mechanically abrade and remove any laitance from the surface of the screed using suitable vacuum equipment as per the screed manufacturer’s recommendations.
Finally, and an issue that applies regardless of the tiling material, is the simple fact that whilst on site it is extremely difficult to tell an anhydrite screed apart from a traditional screed.
When any of the above issues are not fully considered, the effects can be disastrous for not only the tradesmen but the homeowner as well, with tiles failing to bond or becoming detached from the floor.
Solutions
To avoid the above problems, the key is in the preparation. Once the anhydrite screed has been laid, the problematic layer of laitance should be removed approximately 4-6 days after application (although the precise time depends on the brand of screed used), which will not only help the drying out process, but will also provide a stable surface on which to tile onto. If the finished floor may be exposed to moisture, a watertight primer should be used, to seal the anhydrite before the application of a cement-based tile adhesive. However, it is also extremely important to be aware of the residual moisture content of gypsum-based screeds. For priming and direct fixing to the screed with a cement based adhesive, the moisture content must be less than 0.5% water by weight .
The use of a primer is crucial in order to help to stabilise the screed surface further and to act as barrier/sealer between the Anhydrite screeds and the adhesive.* The tiles can then be installed onto the screed using a solid 3mm bed of cement-based adhesive, however tradesmen should also take into consideration the tile type they’re working with when selecting an adhesive. When the adhesive is set, tradesmen can then safely begin grouting, ensuring that the grout completely fills the joint.
And that’s it! The characteristics of anhydrite screed have been laid bare; the secrets of the substrate have been revealed. Go forth and floor.
