Opinion

By Patrick McGuire – Cement & Concrete Association of New Zealand (CCANZ)

An important issue often overlooked in the sustainabi lity debate is the absorption of carbon dioxide (CO2) from the atmosphere by concrete and other cementitious materials.

Concrete technology ascribes the term ‘carbonation’ to this mechanism. Research into carbonation has been ongoing for decades. However, in light of growing concern over greenhouse gases and climate change this research
has taken on a new emphasis. As a result, it is emerging that concrete is
being unfairly blamed for a larger share of CO2 emissions than is warranted.

The carbonation of concrete is the reaction between atmospheric CO2
and calcium oxide, which is an alkaline present in hardened concrete. The product of this reaction is calcium carbonate, which is of the same chemical composition as a primary raw ingredient of cement – limestone.

Only ten-15 per cent of the concrete mix is cement. The manufacture of one tonne of cement typically generates approximately 0.85 tonnes of direct CO2 emissions. Some 40 per cent of this is from the fuel used, and around 60 per cent is from the thermal process that occurs in the cement kiln, known as calcination.

Emissions of CO2 associated with calcination are not only distinct in terms of the process that generates them, they are also partly reversible through the carbonation process.

Structural concrete is generally designed to limit any carbonation to the surface layer, helping to prevent corrosion of the embedded steel reinforcement. There is, however, a greater degree of carbonation at the end of concrete’s structural life when it is typically crushed for reuse as an aggregate.

This results from the significant increase in surface area, allowing CO2 to be more readily absorbed, even when used in ground works.

In low strength concrete such as blocks, and cementitious materials such as mortar, carbonation is much more rapid during the service life, as CO2 can permeate the material more easily. This does not affect durability because there is no steel reinforcement.

Concrete roads and pavement with significant exposed surface area also offer the potential for carbonation, and the absorption of CO2. A study by the British Cement Association shows about a 20 per cent take back of CO2 over the life cycle of cement. In life-cycle analysis it can be argued that this significantly reduces the impact of one tonne of cement to approximately 0.65 tonnes of CO2.
This reduction is an average based on the various applications and markets for cement and concrete in the UK, and is an important factor when considering the environmental impact of cementitious materials.

Furthermore, a recent Nordic R&D project has examined carbonation in some detail.1 As part of their research, the carbonation of a number of concretes was examined under laboratory conditions. They found that up to 60 – 80 per cent of the CO2 associated with calcination has the potential to be chemically reabsorbed by certain concrete mixes.

While the carbonation process cannot be said to diminish CO2 emissions resulting from cement manufacture (the main contributor to the embodied CO2 in concrete), when viewed in terms of life-cycle analysis it will ultimately reduce its environmental impact.

There is currently a lack of data surrounding the carbonation capacity of demolished and crushed concrete in a New Zealand setting. However, preliminary research recently undertaken by CCANZ and Holcim (NZ) Ltd, suggests that carbonation is occurring here at a similar rate to that measured in Europe.

Tests on samples of historic crushed concrete from Auckland and Christchurch locations show that carbonation increases with the age of concrete. CO2 uptake is at its highest when fresh concrete surfaces are exposed to the atmosphere and reduces with time. Further research conducted by CCANZ and industry partners will focus on the optimal conditions for carbonation, and gain a better understanding of the timeframes over which it takes place.

The ability of concrete to absorb CO2 highlights the huge importance of conducting a thorough lifecycle analysis of construction materials, in which post-life applications of recycled materials are taken into account. Not to do so leads to the overestimation of CO2 emissions, and so incorrectly influences material selection.

At a time when New Zealand has undergone the worst net deforestation in its modern history, it is crucial to examine further whether our concrete infrastructure has the potential to be the largest man-made carbon sink.

1 Danish Technological Institute. www.danishtechnology. dk/building/14460