Concrete is one of the most widely used construction materials, comprising significant proportions of the built environment in many developed countries. It is considered to be an energy-efficient and eco-friendly building material, particularly when a life cycle assessment approach (LCA) is taken.
The attributes of concrete as a sustainable building material include its durability and longevity, thermal comfort, fire resistance, security, ability to recycle, acoustic properties and the potential to absorb carbon dioxide (CO2).
In New Zealand, the cement and concrete industry is well aware of its CO2 contribution and also of New Zealand's commitments under the Kyoto Protocol. Additionally, the continued focus on green building materials, environmental labelling and plans for an emissions trading scheme have highlighted to the industry the need to further understand and continue to reduce its impacts on the environment and have driven introduction of further sustainable practices in the sector. Production efficiencies have been introduced, cement additives and alternative fuels have been used and new technologies planned to ensure the industry understands and minimises its carbon footprint.
Of significance to the sustainability discussion is the ability of cement-based materials (typically concrete) to re-absorb / uptake CO2 throughout their life. Exposed to air, these materials will absorb CO2 over time in a process termed recarbonation.
Recarbonation is likely to occur during the service life of concrete, but more importantly will occur rapidly with demolition and reprocessing. The ability of concrete to reabsorb some, or all, of this carbon dioxide has important implications for the cement and concrete industry.
The primary recarbonation reaction in concrete is given by:
Ca(OH)2 + CO2 = CaCO3 + H2O
(Calcium silicate hydrates also contribute to the recarbonation of concrete to a lesser extent).
Research was conducted by Holcim in order to confirm whether recarbonation of concrete was a measurable phenomenon in a New Zealand context. Twenty samples of demolition and crushed concrete were collected from the Christchurch and Auckland areas late in 2007 and early in 2008. The samples were from buildings and structures dating between 0 and 84 years.
Tests determined the extent of recarbonation and the relationship that exists between carbon dioxide absorbed and age of the concrete. Recarbonation of New Zealand concrete appears to be occurring at a similar rate to that noted in previous Nordic research (Pade and Guimaraes, 2007).
The results from the New Zealand samples indicate that most of the carbon dioxide emissions from calcination of limestone during cement manufacture appear to reabsorbed (specifically when aged concrete is crushed for recycling).
While further research is needed to determine optimal conditions for carbon dioxide sequestration, and the timeframes over which this can be expected to occur for New Zealand concrete, it was concluded that with the wider use of crushed recycled concrete, the effects of recarbonation should be considered in any rigorous life cycle assessment of the sustainability of concrete structures.