You may have heard the term anhydrite screed before, and wondered what it was or what it meant. You might have an idea what a concrete screed is. In this article we’ll explain some of the more important aspects of these terms, and how they pertain to tiling.
Concrete and Cement Based Screeds
The ingredients in concrete and a screed are basically the same - cement, aggregates and water - but there are differences in mix ratios, the size of the aggregates, the mix consistency, and of course the intended use.
Concrete is an exceptionally hard substance, which can be used for structural purposes. The strength of the cement binding with the aggregate means that entire buildings can be constructed from it.
A screed, however, is a much thinner layer which is composed of finer aggregate components. A screed is used to cover over structural layers (such as a concrete floor), insulation boards, and / or underfloor heating systems. The intention is to provide a smooth level surface upon which decorative building materials, such as floor tiles, can be installed. If renovating an old farmhouse and installing a dream bathroom, for example, the homeowner might wish to install underfloor heating on an old floor, cover the heating pipes with a smooth screed, and then top it all off with beautiful bathroom tiles.
Screeds are also used to increase the height of a floor. Lately, there's been a rise in popularity for leaving the screed raw and untouched.
The video below gives a great demonstration of tradesmen in a house laying down ground insulation, then underfloor heating pipes, and then covering both with a screed, which is produced with an incredibly smooth finish.
These screeds have broadly the same purpose as their cement based counterparts. However instead of cement, the binding agent is Calcium Sulphate.
When this substance is exposed to water during the mixing process, it reacts with the water and begins to form a substance known as Gypsum. This reaction can last up to a week as the screed cures and dries out. It ceases when all the Calcium Sulphate binder has been used up. Any water or moisture which remains after this chemical process eventually evaporates
Advantages and Disadvantages of Concrete and Anhydrite Screed
There are advantage and disadvantages to either type of screed. These will determine which one a builder or contractor is likely to use. Let’s look at some of the reasons for choosing one type over the other:
- Concrete can be used as an exposed flooring surface. There may be occasions when this is desirable, perhaps in a warehouse, in which case the anhydrite screed is not an option, as it is not designed to be left exposed. So in essence, concrete has an advantage in that it can be used in all situations.
- For a small area, it is more economical to use the sand and cement mix. However, as soon as a medium or large area is being laid, the economies of scale greatly favour the anhydrite system.
- Because the anhydrite mix is pumped, and it can cover an area up to 2,000 square metres in a day, a contractor can install 5 cubic metres in under half an hour. There’s no need for tamping down or vibration to clear the mix of trapped air. A little dappling will do. Installing a concrete screed, by comparison, is back breaking work.
- Drying time for concrete is slow, estimated at a millimetre of depth per day. Anhydrite screeds can be significantly quicker, as often they have a thinner bed, especially compared to a cement based screed that is set onto damp course or insulation boards, where it needs to be substantially thicker.
- Anhydrite screed is the preferred method of choice when installing underfloor heating. The heating elements are usually pinned to insulation boards, and then covered with screed. The anhydrite screed can function at depths as low as 25mm, which is the minimum required to cover the elements - the pipes are closer to the surface and therefore more effective.
- Anhydrite screeds do not shrink or warp. Concrete screeds are prone to “curling” near walls, due to non-uniform drying speeds, or cracking.
- In large areas there is less need for expansion joints when using anhydrite screed.
- Cement screeds have a much large carbon footprint. And thinner layers mean there is less material used with anhydrite.
Implications for Tiling
While visually these screeds are very similar, it’s important that tilers know what they’re looking at before tile installation work is commenced
A thin powdery layer can form on the top of an anhydride screed as it cures. It is resultant from a mix that may be slightly too watery, and the smallest, finest particles of aggregate can rise to the surface. It’s crucial that this layer is removed before tiling. A light sanding is usually all that’s required.
Anhydrite screeds are not compatible with cement based adhesives, which is the majority of them. The chemicals in the screed (the sulphates) can react with those found in cement (the aluminates). This results in the production of a powder (ettringite) which can critically damage the screed itself, as well as weaken the bond to the floor tile adhesive.
This reaction is exacerbated by moisture. In fact, it’s important that anhydrite screeds are not subjected to water at all. For this reason, they are not suitable to remain exposed or outdoors or in wet areas such as shower rooms or saunas, nor indeed anywhere there may be spillage or leaking of rain water.
The solution to this problem is to use a primer on the floor, as recommended by the manufacturer of the adhesive. Other, more expensive ways of dealing with the issue are the use of gypsum formulated adhesives, or decoupling membranes, which are thin mattings that offer a barrier between the substrate and the adhesive.
When performed correctly, a beautiful tiling job can be created on a once upon a time uneven floor:
To Wrap Up:
In most situations, the benefits of anhydrite screeds outweigh those of concrete screed. When tiling, however, caution must be taken that the correct products and procedures are used so that failure of the screed or adhesive bond is avoided.