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Portland cement substitutes gain ground

The benefits of alkali activated cements and geopolymers are finally getting noticed, even though they have been around for a while - possibly since ancient Egyptian times.

Alkali-activated cements and geopolymers represent a family of alternative binders for the full or partial replacement of Portland cements.

An alkali-activated cement or binder is essentially a material that gains strength by means of a chemical reaction between a source of alkali, commonly an alkali silicate solution, and an aluminate-rich material.

As well as being highly sustainable, largely comprising industrial by-products already familiar to the concrete producer such as fly ash and blast furnace slag, they can exhibit performance characteristics above and beyond those of conventional cement-based materials.

Enhanced chemical, wear and temperature resistance have all been shown to be achievable.

Bearing more resemblance to stone than cement, these materials promise low-carbon, high-performance alternatives to current binder technology.

Chemical resistance bonus

The chemical resistance of these materials is also often greatly superior to their conventional counterparts so their use in waste water applications should also prove attractive.

Unlike traditional Portland cement-based materials, the structure of the hardened geopolymer does not rely on hydrates, making it much more stable at high temperatures.

This makes them excellent for pure fire resistance, but possibly not for fire protection where the conventional concrete has the advantage.

As hydrates decompose they absorb energy, protecting embedded steel from the effects of the heat.

Unlike some earlier versions that required heating to cure, most alkali-activated cements and geopolymer formulations are ambient curing, allowing their use in most if not all forms of construction.

They can be placed, poured or sprayed as well as any conventional material and have been successfully trialled for precast applications, such as tunnel segments.

Many formulations remain alkali in nature and work well with conventional reinforcement, although it will be some time before we have all the data required to be able to predict long-term corrosion behaviour of embedded steel in more aggressive environments, such as marine applications or even coping with de-icing salts.

But while strength and resistance to temperature and chemicals are all attractive attributes, it is the sustainability credentials that are of greatest interest to many serious potential adopters of the technology.

For the main component in the binder, the energy commitment has already been made.

Energy and carbon savings

Virtually no more energy is required to convert them to useful material and so hefty savings of energy and carbon equivalent, up to 80 per cent, can be claimed over straight Portland cement equivalents.

To be fair, most conventional concrete contains a large proportion of the same materials for cement replacement, so sometimes the energy input comparison may be skewed.

Such arguments aside, the technology of AACs and geopolymers promises low-energy/low-carbon materials with enhanced temperature and chemical resistance properties.

They provide most of the attributes we require from conventional Portland cement-based materials, giving the levels of durability and sustainability that modern construction quite rightly demands.

Ancient Egyptian origin?

The term geopolymer describes binders produced by the polycondensation of alumina-silicates such as fly ash and slag with alkali activators to produce inorganic polymers. It was first coined by French materials scientist Professor Joseph Davidovits in 1979, although the technology is clearly much older than this.

At one extreme, Davidovits himself is firmly of the opinion that the Egyptian pyramids could not possibly have been built by hauling 15 tonnes of stone and were at least in part built from a limestone concrete manufactured by a geopolymer route, a view that has unsurprisingly raised many disbelieving voices.

The large-scale production and use of geopolymer-type materials can certainly be traced to the immediate post-World War Two Soviet Union where ‘soil cements’ were employed in the construction of high-rise apartment buildings and proved to be very durable.

While Davidovits is very clear about the distinctions between alkali-activated cements and geopolymers, for most people the terms are interchangeable. Wider take-up of such materials has to date been limited by the lack of extensive track record for current products and a dearth of nationally and internationally recognised standards to define their characteristics and performance. Both track record and standards should start to become available shortly but, until they do, these materials may be expected to appear mostly in niche applications where their special qualities can bring real benefits.

Paul Lambert is technical director, materials and corrosion technology at Mott MacDonald

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