The debate between contractors about the pro’s and con’s of anhydrite screeds and newer alternatives looks set to run and run. Here, Ron Dainter, Technical Director of maxit UK explains why he believes alpha-hemihydrate screeds are the way to go.

The old adage about “you get what you pay for” generally holds true in most things.

This is certainly true when comparing the relative merits of alpha-hemihydrate screeds and their anhydrite counterparts.

The paradox is that for all they complain about the perceived deficiencies of anhydrites, contractors invariably plump for them despite the availability of a superior alternative.

Why’s this? Well, primarily because they are cheap, which has advantages for both the contractor and is an “easy sell” to the client who, after all, doesn’t have the bother of pumping the stuff and likes the perceived “bottom line” advantage.

“Whole life cost” has become a buzz-phrase in lots of areas lately, whether you are deciding on the relative merits of similar pieces of equipment, buying a truck or especially when constructing a building. Generally speaking, if you go for a more advanced, albeit more expensive, product it’s more likely to last longer and give good “whole life” value.

When it comes to flooring screeds there’s a quality gulf that makes comparing anhydrites and alpha-hemihydrates a bit like chalk and cheese.

Put simply, the Alpha hemihydrate binder is subject to a sophisticated and highly controlled factory production process to produce a more reactive and technically superior product. When combined with dry graded aggregates, a consistently high quality screed is achieved.

The production of hemihydrate compounds requires more time, more care in selecting and preparing raw materials, and greater controls are necessary during blending. They are supplied as pre-blended dry powder containing one molecule of water for every two molecules of calcium sulphate (CaSO4).

Because the binder is blended with graded aggregates and there is already water within its molecular structure, when water is added to an alpha hemihydrate screed prior to application, it mixes better and more quickly, reacts faster and more completely, is easier to apply, dries faster and forms a flatter surface which requires minimal preparation for final floor coverings, without the need for grinding, which is often necessary with conventional calcium sulphate anhydrite screeds.

Alpha-hemihydrate holds many advantages over anhydrite screeds, which have no water within their molecular structure and are generally delivered to site as a wet mix, which can vary in consistency from batch to batch. For convenience Alpha-hemihydrate is available as a dry compound in silos, enabling contractors to mix the quantities they need on demand.

Key differences between the two alternatives are summed up in the following table: