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Calculation of CO2 uptake in concrete structures by carbonation

The uptake of CO2 in concrete occurs at concrete surfaces during all phases of a concrete product’s lifeime. There is thus uptake in concrete products during its service life, at end-of-life, as well as in secondary products such as crushed concrete aggregates. The conditions at the time of uptake also affect the uptake rate of CO2 in the concrete.

The CO2 emission model for the raw materials used in a cement kiln is presented in 2006 IPCC Guidelines for National Greenhouse Gas Inventories. The calculated CO2 emissions are based on the amount of clinker produced by a cement kiln. This amount of CO2 driven off from the raw materials can be considered as the maximum theoretical uptake of CO2 due to carbonation of cement containing products. The CO2 uptake model will thus estimate the CO2 uptake in different cement containing products during their service life, as well as in the end-of-life processes and when used as secondary products, such as crushed concrete in a road base or as landfilling material. The emission and uptake models are illustrated in the figure below.

Schematic figure showing the CO2 balance in cement containing products over a certain period of time

Schematic figure showing the CO2 balance in cement containing products over a certain period of time.

Relatively slow process

The CO2 uptake in concrete is a relatively slow process that takes place over many years. The first phase is uptake in e.g. concrete products or structures such as bridges, house frames, concrete tiles, concrete roads, railway sleepers, cement mortar etc. The carbonation process takes place from the surface of the concrete when CO2 in air diffuses into the porous concrete and reacts with Ca(OH)2 and other hydrated phases to form CaCO3 according to the reactions described in the chemistry section of these web pages. The surface area of concrete or the surface area/volume ratio of concrete products are important factors for a CO2 uptake model. By knowing the total yearly use of cement clinker and how much is used in different types of concrete categories (for example, in various strength classes, exposure conditions, and shapes and sizes of products and structures), the CO2 uptake surface areas can be estimated. The yearly use of cement clinker in a country can be calculated as (cement or clinker production-cement or clinker exported+cement or clinker imported). From the uptake surface areas, the yearly CO2 uptake over many years can be calculated. Data on the different types of concrete structures and products in which cement are used is developed in order to quantify the CO2 uptake over time. 

For large concrete structures, only a small part of the concrete, an outer surface layer, will be carbonated during its primary service life. Another factor that influences the CO2 uptake in the concrete is the moisture content. (See also the chemistry section of this web site). The surface can be located in different climates, be exposed to rain or located indoors/outdoors. These factors can affect both the carbonation rate and the degree of carbonation. Also, concrete additions, such as blast-furnace slag or fly ash from coal combustion, also hydrate, impacting both the permeability of the concrete and producing phases that also take up CO2, which is important to include in the calculations. CO2 uptake rate constants for different applications and conditions are available.

After the service life of a concrete structure, it will be demolished typically by crushing into finer pieces. This increases the specific surface area dramatically and increases the overall carbonation rate. The total carbonation in the entire concrete volume will also be increased when the concrete is crushed into smaller pieces. However, the use of the crushed concrete must be done in such a way that air and CO2 are allowed to access the increased concrete and CO2 uptake surface areas. This may require some active planning of the end-of-life/secondary use processes for concrete. Understanding of the end-of-life/secondary use CO2 uptake processes for concrete in different countries is lower and the uncertainties are greater. The CO2 uptake models for secondary use are therefore relatively uncertain. However, the CO2 uptake potential is generally large for the end-of-life/secondary use phase. The CO2 uptake process is illustrated in the figure below.

The image shows a graph of schematic CO2 uptake process

A schematic CO2 uptake process in concrete structures over time. At present (2020), assumptions for CO2 uptake during secondary use are conservative.

Total present CO2 uptake

As an example, it has previously been shown (Tier 1) that the total present CO2 uptake is estimated at about 23% (20% in use phase and 3% in end-of-life and secondary use) of the maximum uptake (calcination CO2 emissions). Assuming an additional 62% (giving a total CO2 uptake of 85 %) can be taken up by measures in end-of-life and secondary use, and that 60% of the CO2 emissions from cement production emanates from calcination, the global CO2 uptake potential can then be estimated at 8%*0.6*0.62=3.0% of the total global CO2 emissions if the cement/concrete global share can be estimated at 8%. If the total global CO2 emissions can be estimated at 40 000 Mtonne, the global potential CO2 uptake can then be estimated at 40 000*0.03=1200 Mtonne.

Knowledge of a possible reduction potential is only the first step in a development towards reduced CO2 emissions. An emission reduction also requires an active action to really reduce net emissions. This often requires both financial and technical incentives. One step towards this could be to include CO2 uptake in concrete in environmental product declarations (EPD) for concrete products.

Starts from surface

Carbonation of concrete starts from the surface of the concrete by transporting CO2 in air into the concrete and reacting it with e.g. Ca(OH)2 to form CaCO3. By measuring the carbonation depth in mm from the surface and the carbonation time, you can get a measure of the carbonation depth with time in mm/year for a certain concrete type and uptake environment (e.g. moisture content in the surface). Such measurements have shown that the depth of carbonation in the concrete is directly proportional to the square root of the number of years that the carbonation has been going on. Mathematically, this can be written as:

Carbonation depth = k*√year

where

Carbonation depth in mm

k = constant in mm/√year

√year = square root of number of years in carbonation

The value of k can thus be estimated by measuring the carbonation depth for one concrete object with known concrete characteristics, carbonation time, and exposure conditions. Measuring the carbonation depth for an array of concrete types and exposure conditions allows carbonation rates to be collected in tables for use in carbonation model calculations.These can then be published and used in uptake calculations for a wide range of concrete structures and products in a variety of exposure conditions.

Knowing the carbonation rate/depth and the exposed surface area, the carbonated volume of the concrete can be calculated. Multiplying the carbonated concrete volume by the clinker content in the concrete (kg clinker/m3 concrete), the calcination emission of the clinker (kg CO2/kg clinker) and the degree of carbonation, the CO2 uptake can be calculated.

Uptake throughout entire lifetime

A concrete product or structure usually takes up CO2 throughout its entire lifetime, but at a decreasing rate, and in addition, new concrete products are produced every year and a certain amount of concrete is also demolished every year. The uptake is limited to the maximum uptake based on the amount of clinker (used in the cement) in the concrete. The uptake can be decreased or even prevented by e.g. concrete surface coverage such as paint. This means that a complete calculation of the CO2 uptake in a country every year is a complicated calculation both in terms of numbers of variables involved and in terms of the collection of annual national data required for the models.

In a detailed model, CO2 uptake in concrete is calculated through complex models that take into account many of the variables that affect CO2 uptake. In order to perform such calculations, computer support with various developed models is thus required. As both the models and the data collection are so complex, there is also a need to be able to make simplified calculations so that all countries and organizations can perform such calculations even without special expertise and access to advanced data and many years of concrete statistics. This problem is also present in the usual emission calculations monitored by the UNFCCC and is thus described in the 2006 IPCC Guidelines for National Greenhouse Gas Inventories.

This guideline stipulates that different levels of calculation methods must be developed with different complexity and with different accuracy in the calculations. These levels are called Tier 1, Tier 2 and Tier 3, where Tier 1 is the simplest of the methods and must be able to be implemented without special competence and with access to general data. Tier 2 then represents a more developed calculation model, while Tier 3 can be a very advanced model that requires good access to data and calculation resources. These guidelines have been used here in the development of calculation models for CO2 uptake in concrete. At present, only methods and models for Tier 1 and Tier 2 are developed for CO2 uptake in concrete and presented on this web site.