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Concrete Admixtures

Degussa believes it has developed a plasticizer that extends workability without compromising strength and durability, reports Margo Cole.

The versatility of readymix concrete is well known, but all too often the material that arrives on site may not be exactly what the engineer specified. The reality of delivering concrete from an off-site batching plant means it can easily be an hour between batching and pouring, during which time its characteristics could have changed considerably.

Construction chemicals specialists have long been working to develop admixtures that can ensure the concrete produced by ready-mix can meet its original specification when it arrives on site and is placed. Now, Europe's biggest admixture supplier Degussa believes it has solved the problem with a new superplasticizer, Glenium SKY.

The product is designed to combine accelerated strength development with extended workability, but without delaying the setting characteristics.

When added to a standard mix it has the effect of greatly increasing the slump - and maintaining those slump levels for up to two hours - without compromising the strength and durability of the mix.

Degussa developed Glenium SKY after the successful launch of Glenium ACE into the precast market two years ago. ACE combines high levels of workability with the durability needed by the structural engineering market, but with accelerated strength development that eliminates the need for steam curing.

Adding Glenium ACE makes concrete go off very quickly, which is fine in the precast industry, but unacceptable in ready-mix, which led to the development of Glenium SKY.

'When we started the research we wanted products for both markets, ' explains Roy Jones, admixture technical services manager for Degussa.

'The precast product came first, because we found it far more difficult to get the polymer to do what we wanted in the readymix sector. We wanted a longer working life, but with the same water reduction capabilities.'

Reducing water content is the main impact of plasticizers, giving improved workability without changing the strength and durability characteristics of the mix. The driver behind Glenium SKY was to develop an admixture that achieved this but at the same time extended the life span of the concrete to make it viable for most ready-mix applications.

Glenium SKY was developed at Degussa's laboratory at Treviso in Italy using polycarboxylate ether (PCE) polymers created by its German experts using nanotechnology. After the success of Glenium ACE the scientists spent a further two years engineering the polymers to extend the life of the concrete.

Superplasticizers work by causing the cement particles to repel each other. This is achieved by admixture molecules being adsorbed onto the cement particles and imparting a negative charge that causes electrostatic repulsion between them. But the hydration process works against the superplasticizer, because once molecules have been adsorbed they become ineffective, reducing workability.

The configuration of Glenium SKY molecules delays this adsorption, so dispersal takes place more efficiently over a longer period of time.

The molecular structure of the admixture is also important in the early development of strength. With conventional PCE superplasticizers, the molecules cover the entire surface of the cement grain and build a barrier against contact with water, so the hydration process takes place slowly. Glenium SKY molecules do not cover the surface of the cement completely, so hydration can start straight away, resulting in high early strength development.

Having developed the new admixture, Degussa believes the product could soon replace an entire suite of plasticizers, as it can be used in different doses for different requirements. For contractors, Glenium SKY's most obvious advantage is likely to be increased workability over a longer life, without delayed strength development. And its benefits of self-compaction mean the concrete does not have to be vibrated - the end result being a much smoother, defectfree finish requiring little or no remediation.

However, engineers are equally likely to appreciate the ability to specify higher strength concrete knowing they can increase the proportion of cement without creating a mix that is unworkable or goes off too quickly.

London Concrete currently gets all its admixtures from MBT - Degussa's admixture brand - and uses three generations of Glenium at its eight plants in Greater London. Technical manager Shaun Roche says concrete producers have been looking forward to 'the day when you can give us one admixture that can be used for different designs and different jobs.

'We will do our own trials with it, ' he says, 'part production trials and part lab trials, using various designs. Then we'll take it to one or two of our customers who are open to new things to get their view on it. MBT has to satisfy us that it works, then we have to satisfy ourselves, because when we put it on site we have to be sure it'll work.'

If the trials are successful, Roche says London Concrete could be using Glenium SKY within three months. It will be for specific applications at first, but Roche predicts it could be adopted as a single product within a year.

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