From zero to zero – the carbon handprint of concrete

2020-06-30 Tommi Kekkonen

In the CO2ncrete Solution project, we are studying how concrete binds carbon dioxide. The purpose of this Canemure sub-project is to find out the amount of carbon dioxide bound by the existing concrete stock in Finland. We are also researching new ways of recycling concrete to enable the most efficient use of concrete as a carbon sink.

I personally joined this project without considerable experience with concrete, and I’m also sceptical by nature. As a foundation for the project, we compiled an extensive overview of the literature and research related to the topic, which helped clarify the concept.

Concrete as a carbon sink

Most of the emissions created in concrete manufacturing come from the carbon dioxide released at the cement plant when burning limestone. However, the product of this chemical reaction is not stable; the carbon dioxide is rebound to the concrete through a reaction known as carbonation.

The cement starts reacting with the carbon dioxide as soon as the concrete starts to harden. Although the reaction is slow, it binds considerable amounts of carbon dioxide in the concrete. The phenomenon has been studied for decades, but in a different context and in quite a different tone in the case of reinforcement corrosion. This comprehensive information and knowledge on carbonation chemistry can be found in several scientific publications.

This raises the question: why is this phenomenon not discussed more extensively and acknowledged in various carbon balance calculations?

We now plan to thoroughly research the existing concrete stock of Finland within the framework of the CO2ncrete Solution project. The survey will specify the age of the concrete and the conditions to which it is exposed. Relative humidity, for example, plays a huge role in the progress of carbonation. These statistics will enable us to calculate the amount of carbon dioxide bound by concrete structures, which has never before been calculated in Finland with this level of accuracy.

Making full use of recycling

Carbonation is a phenomenon working its way inwards from the concrete surface, which is why the reaction is quite sensitive to the surrounding conditions. This is where my extensive background in product development kicks in and my brain starts to dissect the facts; when a concrete structure reaches the end of its service life, it is demolished and crushed for reuse. At the same time, we reveal a vast amount of inert concrete surface, ready to receive carbon dioxide. There are also several concrete products that do not contain iron, which is why the pH value decreased through carbonation does not damage the structure.

We have invited experts and specialists of various industries to explore this theme of recycling concrete through the interest groups of the Canemure project. We are working with them to design applications that makes maximum use of the phenomenon for the benefit of the climate. These “workshops” will lead to various pilot projects that will yield new information, based on which we will compile instructions for the use of recycled concrete as carbon sinks.

Our journey is far from finished, but the start is promising and truly fascinating. The system we are exploring is extremely complex and full of causalities. Our sub-project is a tiny part of this huge, global challenge, but we believe that small streams make a mighty river.

Project manager Tommi Kekkonen, CO2ncrete Solution

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