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Cement Can React With Calcium Sulphate

Stuart Whiteley on surface preparation for calcium sulphate screeds

THIS month I will discuss the challenge when working with calcium sulphate screeds, and offer a simple, effective solution.

The development of cement technologies has helped overcome many of the difficulties a flooring contractor may face on site.

From lightweight smoothing underlayments, for projects where structural loading is a consideration to fast-track solutions that enable floors to be installed in a single day, manufacturers of subfloor preparation products have worked hard to create products that promote ease and convenience for the industry.

An increasingly common issue for contractors is dealing with calcium sulphate screeds, which are becoming more popular. Speed of application over large areas, perceived quicker drying times and the ability to be forced dried after seven days, allows for installations to be completed within a shorter time compared to traditional screeds.

However, despite the time saving benefits for the contractor, there is some concern over the compatibility between calcium sulphate screeds and some smoothing underlayments that may cause a new floor to fail, especially if it is not properly prepared beforehand.

n Reaction between cement and calcium sulphate: When working with a calcium sulphate subfloor, contractors should take note of the dryness of the substrate.

It is known that the application of a cement-based levelling compound on to a calcium sulphate screed in a damp environment can result in a reaction taking place.

The reaction product is a crystalline material known as ettringite, which is an expansive crystal and has been known to cause the floor to fail.

Some calcium sulphate screed manufacturers therefore recommend a calcium sulphate based self-levelling compound, to avoid any potential reaction occurring.

n Calcium sulphate based solution: Product development has led to the formulation of a smoothing underlayment for use in such situations. F Ball recently launched a calcium sulphate based smoothing underlayment.

Unlike most cement-based products, the gypsum smoothing underlayment is virtually tension-free and does not shrink during the drying process, which limits the amount of stress at the bond interface.

As well as providing a specialist solution for the levelling of suitably prepared calcium sulphate screeds, the smoothing underlayment can be effectively used over a wide range of surfaces including waterproof surface membranes, concrete and sand/cement screeds.

n Specialist subfloor preparation: Much like any other flooring installation, it is essential that a calcium sulphate screed is suitably sound, smooth and dry before it is ready to receive a new floorcovering.

Contractors should remove all laitance and contaminants and conduct a moisture measurement test. If a subfloor is suitably dry to continue (RH readings below 75%), the surface must then be primed.

Priming a calcium sulphate screed is imperative to the overall appearance and performance of the floor. This is to help promote the adhesion of the smoothing underlayment to the screed, and prevent the rapid drying of the product.

Advances in smoothing underlayment technology have been of real benefit to flooring contractors. No longer are difficult or challenging projects avoided, thanks to the excellent product range now available.

Calcium sulphate based smoothing underlayments that are easy and convenient to use offer a guaranteed high performance floor finish.

Stuart Whiteley is section leader, cement technology at F Ball and Co
T: 01538 361633

This article has been reproduced from the Contract Flooring Journal website. You can find them at