CO2 uptake in cement-containing products - Background and calculation models for IPCC implementation
Concrete is one of our most important building materials and is of great importance for the construction of our modern society structures. However, the production of concrete is energy intensive and emits CO2 during the manufacturing process. CO2 is mainly derived from the cement kiln, both from the use of fossil fuels and as CO2 from raw materials to the cement through calcination. The latter process, however, is reversible so CO2 can be taken up in the concrete. This uptake of CO2 is not normally included in the CO2 calculations for climate analyses, which the present study analyses.
The climate issue is today a very important issue both globally in an international perspective and nationally. At the international level, the different countries report greenhouse gas emissions to the United Nations Framework Convention on Climate Change (UNFCCC). The international monitoring of net greenhouse gas emissions to the atmosphere is an important part of the mapping and monitoring of global climate effects. The guidelines for how emission calculations are to be carried out are presented in the framework of the Intergovernmental Panel on Climate Change (IPCC).
The CO2 emissions from cement production are calculated from both the combustion of different fuels in the cement kiln and from the discharging of CO2 from the raw materials, mainly due to various incoming carbonates. This process is usually referred to as calcination. However, the calcination reactions in cement are not chemically stable but are reversible. This means that CO2 in the air reacts with hydrated cement phases in the concrete and carbonates are regenerated. This process is usually called carbonation. Carbonation can take place during the lifetime of the concrete product, but also in, for example, crushed concrete as a secondary product (e.g. base course of a road or as landfilling material). This uptake of CO2 in the concrete thus reduces the net emission of CO2 from the raw material part. This uptake of CO2 can be significant, which is why it is important to take this effect into account in emission calculations. The current version of the “2006 IPCC Guidelines for National Greenhouse Gas Inventories” does not take into account the effects of carbonation. This could lead to reduced accuracy in both national and global emission calculations. The present study analyses and develops methods and model for the calculation of CO2 uptake in concrete to be used in greenhouse gas inventories.