What most people do not realize is that the release of carbon dioxide from calcination in the manufacture of portland cement may also be part of a cyclic process and is partially carbon neutral in smaller timeframes such as decades and may be fully carbon neutral in longer timeframes. It is easy to picture the organic portion of the carbon cycle with respect to plants as previously mentioned; carbon is absorbed through photosynthesis and released through respiration or decomposition. Inorganic forms of solid carbon such as rock are also part of the carbon cycle. Rocks and other minerals are by far the largest sinks of carbon on Earth and they can weather or decompose, either naturally or through anthropogenic processes such as in cement kilns. The carbon dioxide released to the atmosphere is naturally in constant flux with other large sinks, such as the oceans and other surface waters, where it dissolves and through a variety of both organic and inorganic calcareous processes, such as reef formation and precipitation, settles back into the Earth’s crust.

However, concrete can also absorb carbon dioxide and store it in a process commonly referred to as carbonation. This may be viewed simply as an additional, alternative loop of the complex carbon cycle. Carbon dioxide may be absorbed by concrete in its many forms such as buildings, bridges and pavements (Figures 1a, b and c). Concrete does not even necessarily have to be directly exposed to the atmosphere for this process to occur. Underground concrete piping and foundations can absorb CO2 from air in the soil, and underground and underwater applications might absorb dissolved carbon dioxide (carbonates) present in groundwater, freshwater, and saltwater.

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Figures 1a, b, and c. Various forms of concrete can absorb carbon dioxide

Figure 2 depicts the release of carbon dioxide from calcination, and then its cyclic re-absorption both by plants and concrete on the Earth’s surface. All the concrete sinks depicted in Figures 1 and 2 represent primary uses of concrete. In this age of sustainability, many people are also reusing concrete for secondary applications. Crushed concrete is being used as sub-base in roadways or construction drives, or as aggregate in both new concrete construction and asphalt pavement. Concrete might be reused for artificial reefs or in other novel ways. Each of these secondary uses might also be involved in the carbon absorption cycle, especially if they expose more concrete surfaces to carbon dioxide or carbonate ions in the atmosphere, underground or underwater.

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Figure 2

How Much Carbon Dioxide is Absorbed in Concrete?

So, how much calcium dioxide is absorbed in concrete and how long does it take? The answer is that we do not know. We have known for decades that concrete absorbs carbon dioxide, and in fact, there have been numerous studies on it. However, these studies mainly focused on only a portion of the carbon sequestration question, which is how carbon absorption might affect reinforcing bars within a concrete structure. When carbon dioxide is absorbed into concrete many chemical reactions take place. These usually result in a lowering of the pH in the portion of the concrete where significant amounts of carbon dioxide have been absorbed. The pH is a measure of how acidic (low pH) or basic (high pH) a solution is. Some reinforcing materials in contact with concrete that has a lower pH are not as well protected from corrosion as these materials in contact with concrete that has a higher pH. Carbon dioxide slowly absorbs into a concrete structure from the source of the carbon dioxide (usually the air around it), and the depth at which this absorption has resulted in a significant pH change is usually referred to as the carbonation front. Usually applications with reinforcing material which might be affected by a change in pH are designed so that the carbonation front does not reach these materials over its intended life.

So, carbonation depth or carbonation front really only refers to this specific phenomenon related to pH. These studies on carbonation depths are very useful though. Usually the carbonation front will move more quickly with adequate levels of moisture, higher porosity, and higher ambient CO2 levels, and with certain concrete mixes. Therefore we assume that applications with similar conditions will have absorbed more carbon dioxide than other applications in a shorter period of time. However, throughout a concrete structure there is some carbon dioxide absorption, usually more near the CO2 sources, and less in interior regions. What we really would like to know is how much carbon dioxide is at every depth, for every application and after how much time. Then we can determine the percent of carbon dioxide released from calcination that can be sequestered and provide more information about the extent of carbon neutrality in the calcination/concrete sequestering cycle. 

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Figure 5

Unfortunately the answers are not simple. Concrete is a very complex and dynamic material. It is made of many compounds, which change over time and vary with material mixes and exposure. Figure 5 shows some of the varied structure in hardened cement paste. Many of the compounds can react with carbon dioxide, but at different rates depending on various environmental conditions. It is not even certain if all the CaO in cement can carbonate, there are some researchers who believe that all the calcium species in cement have the potential to react with carbon dioxide or carbonate, and other experts who believe that only certain cement paste species will effectively carbonate under typical concrete applications and lifetimes. Therefore, many studies are needed to be able to understand how much carbon dioxide has been sequestered in current applications, and how much more can be sequestered in future applications.

What Do We Know and What Can We Do?

We do know that the carbon dioxide released in the calcination process is not a good estimate of the carbon footprint of concrete based the inorganic carbon cycle. We do know that carbon dioxide is everywhere around us and therefore may be absorbed even in fresh concrete. We do know that most concrete applications will continue to absorb more carbon dioxide over their primary lives and perhaps even continue to do so in secondary or end-of-life applications. We do know that the amount of CO2 sequestered in concrete is a significant percentage of the amount released during cement calcination. We do know that increased carbonation does not necessarily negatively impact other important characteristics of concrete. For instance strength might be improved since carbonated concrete is a denser material than un-carbonated concrete. And, we do hope that more definitive information on the ranges of the percent of carbon dioxide absorbed for various concrete applications, and under different environmental conditions, will make us more knowledgeable about the carbon footprint of concrete and therefore allow us to make better informed environmental decisions.

So what can we do? Where appropriate, we can encourage applications and activities that improve the carbon footprint of concrete prior to and during its primary life. Where lower pH is not a concern, or even a benefit, processes, mixes and additives which increase the rate of absorption of carbon dioxide should be preferred and encouraged. Novel additives and mixes should be developed with higher initial carbon dioxide content and which promote more rapid carbon absorption over a product’s primary life. 

End-of-life and secondary use applications can further improve the carbon footprint of concrete, particularly uses or disposal options with increased exposed surface area and favorable moisture conditions. It is estimated in countries such as Denmark where concrete is frequently crushed prior to disposal or reuse, that the absorption of carbon dioxide will more than double over the amounts sequestered during primary life applications. Even considering where, and for how long, recycled concrete is staged or stored between its primary use and secondary use/disposal will improve the carbon footprint of concrete. Figure 6 is a photo of concrete rubble stored outside for several months exposed to ambient carbon dioxide and moisture levels prior to reuse. 

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Figure 6. Concrete rubble exposed to ambient carbon dioxide and moisture for several months prior to reuse.

Most importantly, we should be open-minded to the concept that concrete may also serve as a carbon dioxide sink with the potential to balance some of the releases in its manufacture. We should consider employing many of the options that might improve the sustainability of this material, and continue to gain knowledge and expertise in this area.

Carbon Dioxide (CO2) and Global Climate Change

Liv Haselbach, associate professor, Civil and Environmental Engineering, Washington State University

The topic of global climate change is frequently in the news. The International Panel on Climate Change (IPCC) reports that the increase in the concentration of many compounds in the atmosphere will impact global climate. These compounds are commonly referred to as greenhouse gases. Greenhouse gases might be emitted from natural sources or manmade (anthropogenic) sources. Some of these greenhouse gases are long-lived, i.e., they are stable in the atmosphere and may impact climate for many years. The most notable of the long-lived greenhouse gases are carbon dioxide and methane. Figure 7 depicts the estimated average concentrations of carbon dioxide in the atmosphere over the past 20,000 years as taken from the Intergovernmental Panel on Climate Change (IPCC) report. 

Figure 7. Estimated average concentration of CO2 in the atmosphere (Solomon et al. 2007)

The emission rates of carbon dioxide to the atmosphere from anthropogenic sources such as the burning of fossil fuels are much smaller than the total emissions from other natural sources such as respiration and decomposition of plants and animals. However, the IPCC believes that the increase in the burning of fossil fuels as the world has industrialized in the last two centuries is one of the main causes of this increased atmospheric concentration of CO2. (Fossil fuels are organic based carbon compounds and burning them provides energy and breaks these compounds down into simpler molecules such as water and carbon dioxide.) Other important contributions to this increase include changes in land use such as deforestation, which might reduce the dynamic sink of carbon dioxide found in tree and other vegetation growth. Forests and other vegetative land covers are considered to be carbon dioxide sinks (or reservoirs in that these plants take CO2 out of the air and through photosynthesis store it as organic carbon as they grow. Therefore they store carbon, until these plants are eaten, used as fuel or decompose and carbon dioxide is again released to the atmosphere.

In response to the concerns about the impacts of global climate change, there are international, national, and regional efforts to reduce the manmade emissions of carbon dioxide over the next few decades, or to compensate for the emissions by providing a sink for them, or to encourage processes with cyclic carbon cycles commonly referred to as carbon neutrality. Using bio-based organic fuels is considered to be carbon neutral with respect to the fuel source. Plants grow, are used for fuels, release carbon dioxide and then re-sequester the CO2 when they grow again. (Of course, all the processes involved in growing, harvesting, transporting, and manufacturing these bio-based fuels also use energy and may not be carbon neutral.) Fossil fuels are not considered to be carbon neutral, since, even though they are thought to be derived from plants and other living creatures, the process might take millions of years and we are effectively using the reservoirs of these fuels much faster than they can be replenished.