Sources: CalPortland Co., Glendora, Calif.
In a recent paper summarizing peer-reviewed scientific studies, the staff of key West Coast cement, aggregate and ready mixed producer CalPortland cites the exclusion of a critical factor in global, national, and regional greenhouse gas accounting methods: Concrete buildings, pavements and structures’ silent absorption of carbon dioxide from the atmosphere—to such an apparent degree that the built environment represents one of the world’s largest carbon storehouses. While considerable attention has been paid to quantifying the industrial phase emissions from cement production, authors observe, the natural reversal process of CO2 uptake during concrete’s complete material service life is just beginning to receive the consideration it deserves.
“It is time to further examine the value of concrete in the built environment as a significant carbon sink,” says CalPortland CEO Allen Hamblen. “To do so accurately, we must specifically look at the net effects of CO2 sequestration in concrete and evaluate all structures over their lifetime within a circular economy.”
Conversion of calcium carbonate in cement production, or calcination, is not chemically stable and therefore reversible, according to “Incorporating the Effect of Carbonation in Concrete Life Cycle Assessment.” CO2 in the atmosphere reacts with the hydrated cement in concrete and carbonates are regenerated, authors note. Hence, exposed concrete in the built environment absorbs carbon dioxide as compounds in the slab or structure matrix react with the gas to form CaCO3 or calcium carbonate. This reaction permanently removes CO2 from the atmosphere and binds it in a stable state within concrete—a process known as carbonation.
A Swedish Environmental Research Institute study examines data from several European countries to develop practical models to gauge the extent of CO2 uptake by concrete in the worldwide built environment. Researchers developed several approaches and statistical models of increasing complexity to come up with valid estimates; a Tier 1 model provides a simplified approach for use on a national basis relative to the annual emissions associated with cement production in the same year. It has two options for CO2 uptake calculation: A) uses the mean value of 20 percent for estimating uptake over the life of concrete structures; B) uses this mean value minus a standard deviation factor for estimating CO2 uptake, resulting in a 15 percent projection. The standard deviation adjustments are designed to account for various factors that could affect the rate of carbonation, among them: length of time of exposure to the atmosphere, humidity, concrete porosity, cement type, and water-cement ratios.
“Incorporating the Effect of Carbonation in Concrete LCA” authors are CalPortland’s William Larson, MBA, vice president, Marketing; Kirk McDonald, vice president, Technical Services; Tina McIntyre, general manager, Marketing and Government Affairs; and, Hartmut Riess, director of Process Engineering. Their report is posted here [https://www.calportland.com/incorporating-the-effect-of-carbonation-in-concrete-life-cycle-assessment/].