Concrete, the most widely used human-made material, shapes much of our built world. The manufacture of one of its key components, cement, creates a substantial yet underappreciated amount of human-produced carbon dioxide: up to 8 percent of the global total, according to London-based think tank Chatham House. It has been said that if cement production were a country, it would be the third-largest emitter after China and the U.S. Currently four billion tons of cement are produced every year, but because of increasing urbanization, that figure is expected to rise to five billion tons in the next 30 years, Chatham House reports. The emissions from cement production result from the fossil fuels used to generate heat for cement formation, as well as from the chemical process in a kiln that transforms limestone into clinker, which is then ground and combined with other materials to make cement.
Although the construction industry is typically resistant to change for a variety of reasons—safety and reliability among them—the pressure to decrease its contributions to climate change may well accelerate disruption. In 2018 the Global Cement and Concrete Association, which represents about 30 percent of worldwide production, announced the industry's first Sustainability Guidelines, a set of key measurements such as emissions and water usage intended to track performance improvements and make them transparent.
Meanwhile a variety of lower-carbon approaches are being pursued, with some already in practice. Start-up Solidia in Piscataway, N.J., is employing a chemical process licensed from Rutgers University that has cut 30 percent of the carbon dioxide usually released in making cement. The recipe uses more clay, less limestone and less heat than typical processes. CarbonCure in Dartmouth, Nova Scotia, stores carbon dioxide captured from other industrial processes in concrete through mineralization rather than releasing it into the atmosphere as a by-product. Montreal-based CarbiCrete ditches the cement in concrete altogether, replacing it with a by-product of steelmaking called steel slag. And Norcem, a major producer of cement in Norway, is aiming to turn one of its factories into the world's first zero-emissions cement-making plant. The facility already uses alternative fuels from wastes and intends to add carbon capture and storage technologies to remove emissions entirely by 2030.
Additionally, researchers have been incorporating bacteria into concrete formulations to absorb carbon dioxide from the air and to improve its properties. Start-ups pursuing “living” building materials include BioMason in Raleigh, N.C., which “grows” cementlike bricks using bacteria and particles called aggregate. And in an innovation funded by DARPA and published in February in the journal Matter, researchers at the University of Colorado Boulder employed photosynthetic microbes called cyanobacteria to build a lower-carbon concrete. They inoculated a sand-hydrogel scaffold with bacteria to create bricks with an ability to self-heal cracks.
These bricks could not replace cement and concrete in all of today's applications. They could, however, someday take the place of light-duty load-bearing materials, such as those used for pavers, facades and temporary structures.