20 May 2009
Understanding concrete's carbon footprint
At least 5% of humanity's carbon footprint comes from the concrete industry, from energy use, and the carbon dioxide (CO2) byproduct from the production of cement, one of concrete’s principal components, scientists said.
Yet, several studies show small quantities of CO2 later reabsorb into concrete, even decades after installation, when elements of the material combine with CO2 to form calcite.
Concrete absorbs carbon dioxide over time, so its carbon footprint may be smaller than once thought.
Source: National Science Foundation
A new study now shows 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 5% of the CO2 byproduct from cement production, when summed globally, they become significant,” Haselbach said. “Concrete is the most-used building material in the world.”
Researchers have known for decades concrete absorbs CO2 to form calcite (calcium carbonate, CaCO3) during its lifetime, and even longer if the concrete recycles into new construction. Also, 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.
Analysis of concrete samples appears to show other compounds, in addition to calcite, may be forming, Haselbach said. 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 work, where she is looking at evaluating the lifecycle carbon footprint of traditional and novel concrete applications, and looking for ways to improve them.
For related information, go to www.isa.org/environment.