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Carbonation can be good for you Carbonation is usually seen as a long term threat to concrete durability. So why is one expert urging the benefits of deliberate early age carbonation - especially for

Compressive strength is said to more than double. Polymers and metals can be dissolved in supercritical CO2 and infiltrated into the concrete matrix.

'CO2 can transform the disposal of all sorts of waste materials - including those with high heavy metal contents.'

The slow, inevitable reaction between atmospheric carbon dioxide and hardened cement paste may actually increase compressive strengths - but it has a very big downside.

Carbonation ultimately reduces the alkalinity of the cement matrix, destroys the passivity of the surface of any embedded steel reinforcement and leaves the structure vulnerable to expansive rusting and destructive spalling. However, according to Kvaerner Technology's environmental consultant Dr Alan Maries, the reaction between CO2 and fresh cement paste is a very different matter.

'This can produce ultra-rapid curing and strength development in normal concrete products like roof tiles and blocks, as well as reducing long term shrinkage,' he says.

'And CO2 can transform the disposal of all sorts of waste materials - including those with high heavy metal contents.'

Maries points to the cement industry as one that could significantly reduce its CO2 emissions and waste disposal problems by utilising the carbonation reaction. Cement producers have always had problems disposing of surplus kiln dust, a highly alkaline, very fine material with no functional cementitious properties. This usually has to be dumped in nearby quarries, but is difficult to handle and its alkalinity can pose serious environmental problems.

'However, if waste CO2 from the kiln stack is passed through the fresh dust it can be solidified in minutes,' Maries says. 'This not only reduces its alkalinity and makes it easier to handle, but it cuts the amount of CO2 getting into the atmosphere.'

An even 'greener' development would be to link a low energy cement works with a precast concrete factory. Reduced temperatures in the cement kiln and modifications to the raw materials could produce a less reactive but 'self-pulverising' cement which would need no grinding. Concrete products made with this low quality cement would have their strengths boosted to normal levels by the recirculation of CO2 from the kiln flue.

'Waste products like furnace ashes and various metallic slags could be blended in as well. This would be a very appropriate set-up for developing countries,' says Maries.

But it is the use of CO2 to improve conventional concretes that first attracted Maries' attention. Research has shown that 25% of the cement in fresh concrete test specimens can react within a few minutes, with considerable heat evolution. CO2 consumption can reach 15% of the weight of cement - or approximately 25 times the volume of a typical concrete.

Maries says he is surprised that this reaction has not been better exploited by the commercial precast products industry, given the apparent benefits of higher strengths, much shorter curing times and the elimination of any need for extra heating,

He adds: 'Back in the 1950s some US block producers started recirculating the flue gases from their autoclaves during the curing process. But CO2 content of the gas was low, so the only real benefit was reduced long term shrinkage.'

More recently, a Swedish company patented a process called Quick-crete, based on gaseous carbonation under vacuum. This is said to be suitable for the production of high performance, lightweight, fibre reinforced and composite concrete products. A completely fresh approach was adopted by the Supramics Company of Reno, Nevada, which opted to accelerate the carbonation of hardened cement paste - using so-called 'supercritical CO2'.

This is CO2 at a pressure of 75bar and a temperature above 31degreesC, where it is as dense as a liquid but remains compressible and non-viscous. Inevitably the production process is somewhat complex and expensive, but the claimed improvement to concrete properties is nothing short of spectacular.

Compressive strength is said to more than double. Polymers and metals can be dissolved in the supercritical CO2 and infiltrated into the concrete matrix, creating flexible or conductive products. Waste materials can be converted into high quality concretes, and nuclear wastes can be made easier to handle.

'There is even the possibility of using supercritical CO2 to improve the durability of existing structures,' Maries reports. 'This would involve creating an impermeable skin of carbonated concrete to seal the surface against aggressive chemicals.'

Such a proposal might raise old fears about reinforcement de-passivation. Indeed, it might be thought that accelerated carbonation is a process that could only be applied to unreinforced forms of concrete such as tiles, blocks or bricks. Maries says research suggests otherwise.

'It turns out that although accelerated carbonation does temporarily reduce the pH of the fresh cement paste by three or more, in the longer term it actually produces a material which is little different to normal concrete in terms of microstructure or pH.

'So there is no need to be concerned over the durability of embedded reinforcement in treated concretes.'

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