A new international study indicates that cement structures may be a substantial but overlooked repository of carbon emissions, offsetting some of the emissions during cement production.
The study was conducted by the China Emission Accounts and Datasets group, an international team of researchers led by Professor Dabo Guan from the University of East Anglia (UEA). The researchers found that the natural carbonation process of cement materials acts as a large and growing "sink" for carbon dioxide.
While the Intergovernmental Panel on Climate Change (IPCC) guidelines and other environmental regulations consider the carbon dioxide emitted during the cement production process, they do not consider the carbon dioxide absorbed via cement carbonation.
Carbonation is a slow process, taking place throughout the life cycle of cement-based materials. As cement structures weather, carbon dioxide spreads into their pores, triggering a chemical reaction that starts at the surface and gradually moves inwards.
Using new data from field surveys in China and existing data and studies on concrete, mortar, construction cement waste and cement kiln dust during their service life, demolition and secondary use, the researchers modelled the regional and global atmospheric CO2 uptake between 1930 and 2013.
They found that cement stocks around the world absorb approximately one billion tons of atmospheric carbon dioxide annually, and estimated that 4.5 gigatons of carbon has been reabsorbed in carbonating cement material from 1930 to 2013, offsetting 43 per cent of the CO2 emissions from production of cement over the same period, not including emissions associated with fossil fuel use during cement production.
This is a significant finding because approximately 90 per cent of global carbon dioxide emissions from all industrial processes, and 5 percent of global carbon dioxide emissions from industrial processes and burning fossil fuels combined can be traced to carbon dioxide emissions from the cement production process.
According to Professor Guan, who is a professor in climate change economics at UEA's School of International Development, existing cement is a large and overlooked carbon sink and future emissions inventories and carbon budgets may be improved by including this.
"Efforts to mitigate CO2 emissions should prioritise the reduction of fossil-fuel emissions over cement process emissions, given that produced cement entails creation of an associated carbon sink," he said.
"If carbon capture and storage technology were applied to cement process emissions, the produced cements might represent a source of negative CO2 emissions. Policymakers might also investigate ways to increase the completeness and rate of carbonation of cement waste, for example as a part of an enhanced weathering scheme, to further reduce the climate impacts of cement emissions."
The researchers found mortar cement captured the most carbon, even though only approximately 30 per cent of cement is used in mortar. This is because it is frequently applied in thin decorative layers to the exterior of building structures, with higher exposure surface areas to atmospheric CO2, and therefore a higher carbonation capacity.
Despite a relatively smaller exposure area, and therefore lower carbonation rate, concrete cement is the second largest contributor to the carbon sink because approximately 70 per cent of all produced cement is used in concrete.
Demolition causes an increase in carbonation rates by exposing large and fresh surfaces. In countries where structures have a shorter service lifetime prior to demolition, the turnover of cement with respect to carbonation has been increasing over time, accelerating the uptake of carbon dioxide.
Between 1990 and 2013 the annual carbon uptake has been increasing rapidly by an average of 5.8 per cent a year, as the stock of cement buildings and infrastructure increases, ages and gets demolished and disposed. This is slightly faster than process cement emissions over the same period, on average 5.4 per cent a year.