Ref. Ares(2023)944027 - 09/02/2023
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Funded by: Cementa AB and SIVL
Report summary , March 2019
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© IVL Swedish Environmental Research Institute 2019
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Report summary - International reporting and application of annual CO2 uptake in concrete – Executive summary
for greenhouse gas inventories in line with IPCC guidelines
Table of contents
Introduction and background ................................................................................................ 4
Theoretic background to CO2 uptake in concrete ................................................................. 4
International reporting of CO2 emissions, uptake and other applications ............................ 5
Calculation models for uptake of CO2 in existing concrete structures .................................. 5
Future development for application of CO2 uptake in concrete ........................................... 7
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Report summary - International reporting and application of annual CO2 uptake in concrete – Executive summary
for greenhouse gas inventories in line with IPCC guidelines
Introduction and background
The climate change issue is today very important both in an international and national perspective.
Many organizations and companies are actively working on climate change issues and greenhouse
gas reductions are often an important goal, as well as mapping and monitoring of greenhouse gases.
It is therefore of the utmost importance that such greenhouse gas calculations are correct.
Concrete is the single most important building material in society and is used for a variety
of important structures, such as houses, bridges, tunnels, roads, roof tiles, and other construction
products. After the product’s service life, the concrete is normally crushed and recycled as secondary
products e.g. in road base course or as fill material. In the production of cement and concrete, CO2 is
formed from combustion of (mainly fossil) fuels and from calcination of the raw materials in the
cement kiln. Depending on the CO2 profile of the fuels, about 36% of the CO2 emissions emanate
from the fuel combustion and the remaining part (64%) comes from the raw material, mainly
limestone. A very common estimate is that the use of cement today accounts for about 5-6% of the
world's greenhouse gas emissions. Roughly, one can thus estimate that about 3.5% of the global CO2
emissions are driven off from raw materials by calcination.
However, the calcination reactions in the raw meal/cement are not chemically stable but
are reversible in the final concrete. This means that CO2 in air reacts with hydrated cement phases in
the concrete and carbonates are regenerated. This process is usually called carbonation. Theoretical
estimations in combination with measurements show that about 75% of the CO2 from calcination can
be taken up in the concrete by carbonation. It is therefore of utmost importance, both for the
calculations of national and international emissions and removals, and of environmental
performance, that CO2 uptake in concrete is accounted for. Today, neither the national calculations
nor the international reporting to the UNFCCC includes any data for the uptake of CO2 in concrete.
This is a shortcoming that needs to be addressed and the present project is an important part of this
work.
Theoretic background to CO2 uptake in concrete
Portland cement is made mainly of various raw materials such as limestone, clay, marl, silica sand,
shale, etc. which are ground and mixed to a raw meal. The main raw material in cement is limestone
(CaCO3). In the cement kiln, the raw materials are heated up to about 1450°C and CO2 is driven off
to the atmosphere in the calcination reaction mainly from CaCO3 according to reaction (1).
CaCO3 + heat o CaO + CO2
(1)
In the manufacture of concrete, water is added to the cement to form the cement paste (hydration
process). The added water will then react with different substances in the cement such as tricalcium
silicates and dicalcium silicates to form hydration products such as calcium silicate hydrates (C-S-
H) gel and also Ca(OH)2. Thus, both C-S-H gel and Ca(OH)2 form a part of the hardened concrete.
CO2 in the atmosphere, in contact with concrete, will primarily react with Ca(OH)2 in the concrete
according to the principle reaction (2) but will also react with the C-S-H gel. These reactions represent
the uptake of CO2 in concrete, which is called carbonation.
Ca(OH)2 + CO2 o CaCO3 + H2O (2)
Concrete is a porous material that allows both CO2 in air and water to penetrate into the concrete.
The CO2 gas in the pores will dissolve in the water in the pores and carbonation can start. However,
if the pores are completely filled with water, the HCO -
2-
3 or CO3 ions have to diffuse in the water
phase into the concrete. This is a much slower process and will thus slow down the carbonation rate.
Mainly due to the formation of CaCO3 in the concrete, the carbonation rate will slow down with time.
Experiments and studies of real structures have shown that the carbonation rate can be assumed to
be proportional to the square-root of time (t), √ . Other factors that will influence the carbonation
rate are porosity of the concrete, w/c ratio, cement type and additions, and surface treatment of the
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Report summary - International reporting and application of annual CO2 uptake in concrete – Executive summary
for greenhouse gas inventories in line with IPCC guidelines
concrete products, which are factors that can readily be estimated, calculated, or are known at
national level.
At the end-of-life of concrete products, they are demolished and normally crushed. When
the concrete is crushed, new surfaces are created and smaller concrete pieces are formed. This can
dramatically increase the carbonation rate if access to CO2 in air can be maintained. To estimate the
uptake of CO2 in concrete, it is thus important to include both the use stage and end-of-life stage of
the concrete products as well as the secondary use of the crushed concrete.
Carbonated concrete is chemically stable and increases the strength of the concrete. The
carbonation process is associated with lowering of the pH in the concrete, thus reducing the
corrosion protection of built-in steel reinforcement. Therefore, international construction codes have
design rules for protection of the reinforcement by, for example, a protecting concrete layer covering
the steel rebars, which, in this way, can meet certain criteria based on, for example, climate exposure,
material properties, and the designed service life.
International reporting of CO2 emissions, uptake and other applications
Accurate, robust, and reliable information on anthropogenic greenhouse gas emissions and removals
are an essential part to follow-up the national and international emissions, reduction targets, and
mitigation measures. Countries report such information to the UNFCCC1 by annual inventories.
Estimated emissions and removals in the inventories are based on methodological guidelines2
prescribed by the IPCC3. The latest version of the IPCC guidelines for national greenhouse gas
inventories (from 2006) does not consider CO2 uptake in concrete and, so far, no data on uptake has
been reported by any country to the UNFCCC but similar methods to those proposed here do exist
for forestry.
However, the mineral carbonation process has been mentioned briefly in the IPCC
guidelines, but also that further work was needed before carbonation could be included into national
inventories. Even if the IPCC guidelines does not prescribe methods for carbonation, the reporting
to the UNFCCC may include estimations based on other methods, as long as they are properly
documented and validated. Today, much more information about carbonation of concrete are
available and proposed calculation methods have been developed4. CO2 uptake calculations cannot
only be used for international reporting but also for improved calculations of the environmental
performance of specific concrete products or in LCA5 and EPD6 applications. However, the
calculation methodology has to be somewhat modified, for example, in handling of export and
import of cement and concrete products in relation to the CO2 uptake.
Calculation models for uptake of CO2 in existing concrete structures
Calculating the uptake of CO2 in all existing concrete during a given year for a specific country
(which is necessary for greenhouse gas reporting) is a very complex task if accurate results are to be
obtained. Methods for such reporting have been proposed in footnote 4. These methods are based on
previous international research projects for the calculation of national CO2 uptake and on the IPCC
guidelines for emission calculations and removals2. Seven studies from different countries have been
compiled representing Ireland, the Netherlands, Norway, Spain, Sweden, Switzerland, and
“Global”.
1 United Nations Framework Convention on Climate Change
2 2006 IPCC Guidelines for National Greenhouse Gas Inventories
3 Intergovernmental Panel on Climate Change
4 Stripple H., Ljungkrantz C., Gustafsson T., Andersson R., CO2 uptake in cement-containing products - Background and calculation
models for IPCC implementation. IVL report B2309 (2018).
https://www.ivl.se/english/startpage/pages/publications/publication.html?id=5656
5 Life Cycle Assessment
6 Environmental Product Declarations
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Report summary - International reporting and application of annual CO2 uptake in concrete – Executive summary
for greenhouse gas inventories in line with IPCC guidelines
The IPCC has classified its methodological approaches in three different tiers, according to
the quantity of information required, and calculation accuracy. Tier 1 represents a general and
simplified approach based on national statistics together with standardized assumptions (factors
and parameters) from the IPCC. Tier 1 is also to be used as a starting point to make an early emission
or removal estimate of the level of magnitude. Tier 1 can also be used when national information is
lacking or if a specific source is considered to be of minor importance/magnitude. The CO2 uptake
values are proposed to be related to the reported calcination emissions from the clinker consumed
in the country of concern. For more significant sources, IPCC encourages Tier 2 or Tier 3 with a
greater level of granularity. Tier 2 comprise more calculations compared to Tier 1 and applies
statistics, factors and parameters, which are more detailed and specific to the country. Tier 3
generally refers to complex models or monitored data (e.g. emission measurements).
Accordingly, three different methods for calculating the annual CO2 uptake in a country
are suggested in footnote 4. This knowledge is set in line with the IPCC system view, which generally
opens up for future inclusion and reporting to the UNFCCC. A total national CO2 uptake, in the use
stage and end-of-life/secondary use stage, of 23% of the national calcination emissions (adjusted for
export and import), is proposed as a value for use in Tier 1 today. The uptake of CO2 in end-of-
life/secondary use is today small (~3%) due to the age structure and lifespan of the concrete products.
The amount of demolished concrete is however expected to increase, which will increase the
potential uptake of CO2 in the future, see figure 1.
For Tier 2 and 3, methodologies are recommended in footnote4 for more advanced and
accurate calculations. For significant sources, IPCC specify Tier 2 or Tier 3 with a greater level of
accuracy. For Tier 2, longer time series of cement consumption data are used (at least 10 years),
concrete products covering at least 65 % of the concrete use are identified and specific surfaces are
calculated (m2 surface/m3 concrete), and the uptake is calculated according to a formula given in
standard EN 16757 Annex BB. In Tier 3, computer models are used, which can be based on uptake
in all concrete surfaces historically produced with different carbonation condition and an uptake rate
reduction proportionally to the square root of time.
In the Swedish study (footnote 7), a methodology has been used, qualifying for Tier 3. In a
specific concrete product, such as roof tiles, or concrete structure applications, such as apartment
buildings, there is a dynamic yearly input of new constructions as well as an outflow of constructions
that is at the end of its service life. Therefore, a yearly input has to be calculated for each year the
products and structures have been used, as well as when they are no longer in use. The annual
uptake of CO2 may be described by Eq. (3) where the annual uptake
ΔCO2 in year t is equal to the
difference between the accumulated uptake at the end of year t and (t-1), respectively.
ΔCO
uptakein year t
CO
CO
CO
2
2
¦
uptake,(t)
2
¦
uptake,(t
1)
2
(3)
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Report summary - International reporting and application of annual CO2 uptake in concrete – Executive summary
for greenhouse gas inventories in line with IPCC guidelines
Figure 1. Calculation of a future scenario for CO2-uptake in Sweden7.
Future development for application of CO2 uptake in concrete
In order to speed up the improvement work on the reporting of greenhouse gas emissions and
removals, it is proposed that a selected number of countries start calculating and using the uptake
of CO2 in concrete on a national basis and for reporting to the UNFCCC. Even if no calculation
methods or reporting structure for uptake of CO2 in concrete is provided by the IPCC or the
UNFCCC, countries can choose to include such information in their National Inventory Reporting
(NIR). The NIR contains detailed descriptive and numerical information on anthropogenic emissions
and removals. The UNFCCC can then process this data material and include it in the future climate
work.
In this project, the above proposed calculation methods could be tested and evaluated in
the selected countries. Based on the results of these tests, a broader international implementation
with different calculation support can then be planned. Future support may also include the
development of a common software to simplify the calculations, especially related to Tier 3
calculations. The work in this project is to be carried out during 2019 (possibly prolonged to spring
2020), in close cooperation with the national cement industries and the national greenhouse gas
emission inventory teams.
During the project, bilateral and/or multilateral discussions and meeting between IVL and
the different countries will be held through emailing, Skype and/or telephone. If needed, multilateral
discussions will be held on the different national circumstances and data availability in relation to
the different proposed methods. In this initial project, the goal is to develop at least Tier 1 calculations
and, if data and human resources are available, possibly Tier 2 calculations. At the end of the project,
if needed, the tier methods will be modified based on the discussions during the different national
case studies.
Based on the outcome of the country case studies, IVL will also aim at including Tier 1
factors to the IPCC emission factor database ( https://www.ipcc-nggip.iges.or.jp/EFDB/main.php ).
Inclusion in the database will enable researchers to include methodologies on CO2 uptake in concrete
in future updates of the IPCC guidelines.
7 Andersson R., Fridh K., Stripple H. and Häglund M., Calculating CO2 Uptake for Existing Concrete Structures during and after
Service Life, Environmental Science & Technology, 2013, 47 (20), pp 11625–11633.
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