Concrete Buildings Emerge as a Major Storehouse of Carbon from the Atmosphere

 Concrete buildings are quietly absorbing carbon dioxide from the atmosphere, so much so that the built environment represents one of the world’s largest storehouses of carbon that is excluded from current greenhouse gas accounting rules.

This is the main message from peer-reviewed scientific studies summarized in a paper released today by researchers at CalPortland, a founding member of the Pacific Northwest Building Resilience Coalition.

Exposed concrete in the built environment absorbs carbon dioxide through a process called carbonation, a term commonly used to describe any reaction of CO₂ with concrete compounds resulting in CaCO₃, the main component of limestone. This reaction removes CO₂ from the atmosphere and binds it in a stable state in the concrete.

Concrete, the world’s most popular building material, constitutes a large portion of the global built environment. The production of Portland cement, which is used to make concrete, is commonly identified as a major contributor to greenhouse gas emissions.

CO₂ emissions occur when calcium carbonate, the main component of limestone, is exposed to intense heat as part of the cement production process. These emissions come from both the fuels used to heat the cement kiln and from the CO₂ released from the raw carbonates used to make cement. This process is called calcination.

A widely quoted estimate is that cement and concrete use accounts for about 5 % of the world’s carbon dioxide emissions. Often the story ends there, but in fact, the process of calcination is not chemically stable but reversible. This means that CO₂ in the air reacts with hydrated cement in the concrete and carbonates are regenerated. This process is called carbonation.

“We have long been aware of the effects of carbonation in concrete,” noted William Larson, Vice-President of Marketing, CalPortland. “It has been explored extensively with regard to concrete durability; however, in the past few decades, there have been many studies that have examined factors affecting the capacity of concrete to absorb carbon dioxide from the atmosphere.”

One of the more recent studies from the Swedish Environmental Research Institute (SERI) examined data from several European countries and globally to develop practical models to gauge the extent of CO₂ uptake by concrete in the built environment.

The researchers developed several approaches and statistical models of increasing complexity to obtain valid estimates of the overall CO₂ uptake by concrete.

The Tier 1 model provides a simplified approach for use nationally, based on annual emissions from cement production in the same year.  It offers two options for calculating CO₂ uptake. Option A uses the mean value of 20% for estimating uptake of CO₂ over the life of concrete structures. Option B uses the mean value minus a standard-deviation factor to estimate CO₂ uptake, resulting in 15% uptake.

The standard deviation adjustments are designed to account for various factors that could affect the rate of carbonation. Such factors include the duration of exposure to the atmosphere. humidity, porosity of the concrete, cement type, and water to cement ratios.

Water is essential for the carbonization process. Painting or covering the concrete with any layer through which CO₂ must pass will reduce the rate of carbonation.

Many organizations and companies are actively working on measures to reduce greenhouse gas emissions in the built environment, notes Larson. “Having a better appreciation of the level of CO₂ uptake by concrete can help us develop better strategies to mitigate the impacts of climate change. As this paper and several of the studies reviewed clearly show, carbonation in cement products represents a substantial carbon sink that is not currently considered in emissions calculations.”

The paper, Incorporating the Effect of Carbonation in Concrete Life Cycle Assessment, is available here http://bit.ly/2XBQybZ

For more information, contact

Tel: (626) 852-6200 or info@calportland.com

or

Frank Came, Communications Director, Pacific Northwest Building Resilience Coalition

c/o franktcame@gmail.com

Tel: 778-229-7461