Carbonate or Recarbonate

The following article was first published in the December 2007 issue of the Cement & Concrete Association of New Zealand's (CCANZ) quarterly publication Concrete.


Does Concrete Carbonate or Recarbonate?

An important issue often overlooked in the sustainability debate is the reabsorption of CO2 from the atmosphere by concrete and other cementitious materials during their service life, and secondary life following recycling. Concrete technology ascribes the term ‘carbonation’ to this process. However, the term recarbonation is more accurate.

The carbonation of concrete is the reaction between atmospheric carbon dioxide gas (CO2), a weak acid, and calcium oxide (CaO), an alkaline product of hardened concrete, to form calcium carbonate (CaCO3):

CaO + CO2 = CaCO3

From the cement manufacturer’s perspective, the process can be correctly identified as recarbonation because the final product, calcium carbonate, is chemically the same as one of cement’s primary raw ingredients.

The manufacture of one tonne of cement typically generates approximately 0.85 tonnes of direct CO2 emissions. Some 40% of this is from the fuel used, and around 60% is from the thermal decomposition of calcium carbonate (CaCO3), a process known as calcination. Emissions of CO2 associated with calcium carbonate decomposition are not only distinct in terms of the process that generates them; they are also partly reversible through the recarbonation process1.

The mix design of structural concrete limits any recarbonation to the surface layer, helping to prevent corrosion of embedded steel reinforcement. There is, however, a greater degree of recarbonation 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, recarbonation 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.

A study by the British Cement Association shows about a 20 per cent take back of CO2 over the life cycle of cement2. In whole life terms 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, the Norwegian Building Research Institute BYGGFORSK has examined recarbonation in some detail3. As part of their research, the recarbonation of a number of concretes was examined under laboratory conditions. They found that 60 to 80% of the CO2 associated with calcination has the potential to be chemically reabsorbed by concrete mixtures with w/c of 0.6 or higher for the grain size of 1 to 8 mm within 20 to 35 days of exposure.

While the recarbonation 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 whole life performance it will ultimately reduce its environmental impact. Recarbonation is clearly an important concept, which CCANZ will be promoting as part of its Concrete3 initiative.

References

  1. Clear, C, & De Saulles, T.; BCA Recarbonation Scoping Study; Confidential report by British Cement Association, 2007
  2. Ibid
  3. Danish Technological Institute http://www.dti.dk/14460 [accessed 2007]