Capturing CO2 within the material

Science topics January 2019 InnovationMaterials and structuresEcology

By Mickaël Thiéry and updated by Jean-Michel Torrenti, Director - Mast Department

The november 5, 2018 Update

 

Every kilogramme of cement produced in a cement factory generates an average of between 0.6 and 0.7 grammes of CO2. In view of the enormous amounts of cement that are produced worldwide, the cement industry is responsible for between 5 and 7% of anthropic CO2 emissions. Almost half of these emissions are due to the chemical nature of the cement’s principal ingredient, clinker, which is obtained by decarbonating limestone1.

 

The carbonation of concrete

Hydrates are formed when water is mixed with cement, and these are what give concrete its strength. Atmospheric CO2, even though it is present in very small quantities (0.038%), causes slow irreversible carbonation2 of these hydrates.Example pathologies (spalling) of reinforced concrete due to reinforcement corrosion induced by carbonation (photo credit Ifsttar Hugh Delahousse). Over geological timescales, carbonation is thus capable of chemically trapping some of the CO2 that is emitted at the cement factory when the limestone is calcinated. Carbonation nevertheless has various consequences depending on whether it occurs when the concrete is in service or after demolition.

IFSTTAR has been working for many years on the carbonation of in-service concrete in the case of reinforced concrete structures. Atmospheric CO2 diffuses within the porosity of the concrete and dissolves, forming acids on contact with the interstitial solution. The main consequence of this is to lower the pH of the medium and corrode the reinforcing steel. IFSTTAR has developed models to predict lifetime in relation to corrosion risk. These can be used during concrete mix design or when dimensioning structures, etc.

 

A godsend for improving the carbon balance of concrete

Although carbonation has adverse impacts on the durability of reinforced concrete structures, the process can be beneficial as far as the concrete itself is concerned. In the case of members that contain no reinforcement, carbonation can help to capture CO2 and improve the carbon balance of the concrete. IFSTTAR has been working on the positive aspects of carbonation in work that has been funded by the National Research Agency (ANR) – the CRAC project (Carbonation of Recycled Aggregates of Concrete) which ended in 2013 and which was awarded the Eugène Freyssinet Prize in 2011.
Concrete has a natural ability to capture CO2, but in a structural member in a building the reaction takes place on a very small surface area and is therefore extremely slow. However, when concrete is crushed during demolition, the surface area that interacts with the atmosphere increases and the reaction that traps CO2 takes place more quickly. 

Since 2018, the national FastCarb project has been studying ways to further accelerate this CO2 recovery, in order to recover about 20% of the CO2 initially released during the manufacture of a given concrete, i.e. 40 to 60 kg of CO2 per m3 of concrete. In collaboration with many partners, IFSTTAR is studying how the process can be optimised. The project will also provide an industrial-level demonstration and applications on sites using recycled aggregates.

 

The use of demolition concrete aggregateSite demolition concrete storage (photo credit Ifsttar Hugh Delahousse)

Carbonation also has another advantage: it improves the microstructural and mechanical properties of the concrete. Consequently, when CO2 capture has been optimised, the recycled aggregate is of better quality and can be used to manufacture more concrete. This is an undeniable benefit in view of the large number of buildings in France that are reaching the end of their service life and which will have to be destroyed. IFSTTAR is therefore working on the use of carbonation to improve the properties of demolition concrete aggregate with a view to recycling it to remanufacture concrete.

 

 


1 Limestone (CaCO3) is, with clay, the raw material from which clinker is made. The manufacture of clinker requires the limestone to be decarbonated at around 900°C in a kiln in the cement factory (CaCO3 → CaO + CO2). Its manufacture therefore produces CO2 chemically, to which is added the CO2 that is generated by the combustion required to heat the kiln.
2 Carbonation involves the transformation of the hydrates in the cement into limestone (calcium carbonate CaCO3) due to the chemical action of atmospheric CO2.

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