Cradle-to-gate CO2e emissions vs. in situ CO2 sequestration of structural concrete elements

Life cycle assessment (LCA) is an emerging methodological tool used in building design and construction to quantify the environmental impacts of materials, components, and whole buildings. While calculation of embodied cradle-to-gate greenhouse gas (GHG) emissions is commonplace for ordinary portland cement (OPC) structural concrete elements in building-related LCAs, the carbon dioxide (CO2) sequestered by in situ carbonation of exposed OPC concrete is often neglected because the quantity of sequesterable CO2 is assumed trivial compared to the initial GHG emissions (kg CO2e) associated with their manufacture. Using a screening cradle-to-gate LCA and a previously developed and validated CO2 sequestration model for OPC concrete, this paper quantifies and compares estimates of the initial CO2e emissions to the CO2 sequestration potential of several OPC concrete elements at both finite (25 years) and infinite time intervals. The results demonstrate that, depending on cement type, compressive strength, structural geometry, and time, approximately 19%-a non-trivial sum-of initial CO2e emissions could be recoverable via CO2 sequestration for the concrete elements considered herein. Notably, however, concrete elements that sequester the most CO2 do not always result in the lowest net CO2e emissions.