Approximately half of the CO2 emission from cement clinker production stems from the calcination of limestone, i.e. the process where limestone is burnt and CO2 gas is released to the atmosphere. Theoretically, the hardened concrete binds approximately the same amount of CO2 in the carbonation process as is released during calcination. The concrete ability to bind CO2 and the rate of the carbonation process depend on several variables, including the concrete quality application and its exposure to the atmosphere.
It is not documented in what way and to what extent the carbonation can be taken into account in assessments of concrete CO2 emissions, e.g. in life cycle assessments. Specifically there is a lack of knowledge about the carbonation of demolished and crushed concrete. Models for calculating the rate of carbonation exist, but they are simple, developed for a special outdoor type of concrete and they do not take into account that the concrete is crushed and recycled after use.
The consequence is that figures for the contribution of the cement- and concrete industry to net CO2 emissions are strongly overestimated. This will have a significant influence on the public CO2 policy, criteria for environmental labelling and for the selection of materials from principles of environmental correct design. A comparison of the environmental impacts from different building materials (e.g. wood and steel) is at present unfair because of the lack of documentation of the CO2 uptake in concrete.