Huge amounts of CO2 are released in the production of cement when limestone is essentially turned into lime, the figure is thought to represent 5% of global CO2 emissions from all industrial processes and fossil-fuel combustion.
A team of Chinese scientists led by Zhu Liu at the California Institute of Technology in Pasadena have found that, through a process known as carbonation, CO2 diffuses into the pores of cement-based materials, such as concrete and mortar. The process starts at the surface and progressively moves inwards, sucking in more and more carbon dioxide as time goes on.
Having compiled data on how cement is used around the world, they calculated the carbonation rate in mortar and concrete, whether underground, outdoors or indoors and have estimated that between 1930 and 2013, 4.5 gigatons of carbon has been soaked up by cement, thus offsetting 43% of the CO2 emissions from the production of cement over the same period.
Carbonation of cement products, therefore, represents a substantial carbon sink that is not currently considered in emissions inventories.
Read the full research paper at http://authors.library.caltech.edu/72406/