Concrete pavement samples in engineer Liv Haselbach's laboratory at Washington State University. Click on image for full size (545 Kb) Courtesy of Alan Abdulkader, Washington State University

Many scientists currently think at least 5 percent of humanity's carbon footprint comes from the concrete industry, both from energy use and the carbon dioxide (CO₂) byproduct from the production of cement, one of concrete's principal components.

Yet several studies have shown that small quantities of CO₂ later reabsorb into concrete, even decades after it is emplaced, when elements of the material combine with CO₂ to form calcite.

A study appearing in the June 2009 Journal of Environmental Engineering suggests that the re-absorption may extend to products beyond calcite, increasing the total CO₂ removed from the atmosphere and lowering concrete's overall carbon footprint.

While preliminary, the research by civil and environmental engineering professor Liv Haselbach of Washington State University re-emphasizes findings first observed nearly half a century ago--that carbon-based chemical compounds may form in concrete in addition to the mineral calcite-now in the light of current efforts to stem global warming.

"Even though these chemical species may equate to only five percent of the CO₂ byproduct from cement production, when summed globally they become significant," said Haselbach. "Concrete is the most-used building material in the world."

Researchers have known for decades that concrete absorbs CO₂ to form calcite (calcium carbonate, CaCO₃) during its lifetime, and even longer if the concrete is recycled into new construction--and because concrete is somewhat permeable, the effect extends beyond exposed surfaces.

While such changes can be a structural concern for concrete containing rebar, where the change in acidity can damage the metal over many decades, the CaCO₃ is actually denser than some of the materials it replaces and can add strength.

Haselbach's careful analysis of concrete samples appears to show that other compounds, in addition to calcite, may be forming. Although the compounds remain unidentified, she is optimistic about their potential.

"Understanding the complex chemistry of carbon dioxide absorption in concrete may help us develop processes to accelerate the process in such materials as recycled concrete or pavement. Perhaps this could help us achieve a nearly net-zero carbon footprint, for the chemical reactions at least, over the lifecycle of such products."

That is the thrust of Haselbach's current NSF-funded work, where she is now looking at evaluating the lifecycle carbon footprint of many traditional and novel concrete applications, and looking for ways to improve them.

"This work is part of the portfolio of studies that NSF is funding in this vital area," added Bruce Hamilton, director of NSF's environmental sustainability program and a supporter of Haselbach's work. "Research relating to climate change is a priority."

Text above is courtesy of the National Science Foundation

Concrete pavement samples in engineer Liv Haselbach's laboratory at Washington State University. Click on image for full size (545 Kb) Courtesy of Alan Abdulkader, Washington State University

Making concrete releases carbon dioxide (CO₂), a greenhouse gas, into the atmosphere. The carbon dioxide from concrete, combined with greenhouse gases from many other sources, is causing Earth to warm. However, scientists have found that over many years concrete can absorb some CO₂, taking it out of the atmosphere. So the buildings, sidewalks, and other structures that are made from concrete are able to capture some of the greenhouse gas that was released when they were made.

Why does making concrete send CO₂ into the atmosphere? Carbon dioxide forms as cement, one of the major ingredients in concrete, is made from limestone. Also, it takes energy to make limestone into cement. The energy usually comes from fossil fuels which release CO₂ to the atmosphere when they are burned.

Concrete is the most-used building material in the world, so the amount of CO₂ created as new concrete is made really adds up. Many scientists think at least 5 percent of the CO₂ released into the atmosphere by humans comes from making concrete.

Yet scientists have found that over decades concrete absorbs CO₂ from the atmosphere. Elements in the concrete combine with CO₂ to form the mineral calcite. New research suggests that there may be other minerals forming within concrete from absorbed CO₂ too.

In Liv Haselbach’s lab at Washington State University concrete blocks like the ones in this picture are studied to help us better understand these processes. Haselbach, an engineer, does careful analysis of concrete samples to learn about the types of compounds, in addition to calcite, that may be forming in concrete.

Understanding how carbon dioxide is absorbed into concrete may help us find ways to make this happen faster. Someday we may be able to take as much CO₂ out of the atmosphere as we put in when making concrete.

Concrete pavement samples in engineer Liv Haselbach's laboratory at Washington State University. Click on image for full size (545 Kb) Courtesy of Alan Abdulkader, Washington State University

Lots of things are made of concrete - houses, apartment building, and even schools. Sidewalks are often concrete too. Making concrete releases greenhouse gas into the air, which causes Earth to warm. But scientists have found that over many years concrete can pull some greenhouse gas out of the air too, so it might not cause as much warming as we thought.

Concrete is made of sand and rocks mixed with cement, which holds it all together. When cement is made from limestone, it releases carbon dioxide, a greenhouse gas, into the air. Also, it takes energy to make limestone into cement. The energy usually comes from fossil fuels which release carbon dioxide to the atmosphere when they are burned.

However, scientists have found that over many years concrete can take carbon dioxide out of the air. So the buildings, sidewalks, and other things that are made from concrete are able to capture some greenhouse gas too. Liv Haselbach, an engineer at Washington State University studies how concrete blocks like the ones in this picture take in carbon dioxide.

Understanding how carbon dioxide is absorbed into concrete may help us find ways to make this happen faster. Taking more greenhouse gas out of the atmosphere would help slow global warming.