Set in stone? A perspective on the concrete sustainability challenge

Sustainability challenges and opportunities With more than one-half of the world’s growing population (now ∼ 7 billion) living in cities, a sustainable physical infrastructure is central to improving and maintaining a high quality of life. Concrete is an important component of this infrastructure, with a current annual per capita consumption of about 2.8 tonnes (t) ( Figure 1 ). Concrete powers a worldwide US$35 billion industry, employing more than two million workers in the United States alone. This high demand is driven by a number of remarkable properties of this material ( Table I ), with which a structural composite with complex geometry and high strength can be created on demand by mixing water with cement powder and stone. This “liquid stone” processing capacity enables rapid construction and repair of geometrically complex pavements, bridges, buildings, and waterways. However, such high usage carries a price associated with Earth’s fi nite resources and limited tolerance for industrial byproducts. Sustainable use of concrete requires that its function and costs (in economic, environmental, energy, and social terms) be evaluated within the context of its end use and that new and highly optimized materials be developed ( Table I ). Life-cycle assessment shows quantitatively that the greatest environmental burden from structures such as buildings occurs during the use stage, 5 which presents new priorities for the exploration of property and processing optimization. As illustrated in Figure 2 , concrete is manufactured locally and directly from readily available limestone and clay, which are mixed without refi nement and then heated in a large, rotating kiln (up to 200 m in length) to temperatures of about 1500°C. Carbon dioxide is generated from both the combustion to provide heat and the decomposition of limestone (CaCO 3 ). A large kiln can produce up to 10,000 t of so-called clinker nodules per day, which are then ground into a fi ne gray powder, with addition of gypsum. The resulting so-called portland cement is transformed into familiar concrete by the addition of sand and gravel and, most importantly, water. Whereas sand and gravel are largely inert fi llers, a chemical transformation occurs that turns the cement–water mixture from a liquid suspension into a solid binding matrix called cement paste. One indication of the Set in stone? A perspective on the concrete sustainability challenge