From cement production to pouring, building with concrete can be a messy business. Researchers are working on several fronts to make the process more sustainable. (Credits: left, Dori/Wikimedia Commons; right, US Navy.)
According to a paper in the August issue of ACerS’ International Journal of Applied Ceramic Technology (DOI: 10.1111/ijac.12083), projected annual demand for concrete by 2050 is approximately 16 billion metric tons.
“In general, only 13–15 percent (by total mass) of concrete consists of cement,” writes author Mohammad Pour-Ghaz of the Department of Civil, Construction, and Environmental Engineering at North Carolina State University (Raleigh). “In comparison with other materials, concrete is not essentially a high carbon footprint material. The significant use of concrete as a construction material, however, results in a large overall energy consumption and environmental loadings: Cement is responsible for 70–80 percent of the global industrial energy use by the nonmetallic production sectors, 5 percent of the global anthropogenic CO2 emissions, and 3.4 percent of the global CO2 emissions.”
Thus sustainability has become a key driver of research both in cement production and concrete formulation. Cement producers have been working hard to make their production processes as efficient and sustainable as possible, and have made significant strides, according to a recent World Cement article. Cement producers participating in the World Business Council for Sustainable Development’s Cement Sustainability Initiative have been able to decrease steadily CO2 emissions over the past couple of decades. The article attributes reduced emissions to more efficient kiln technology, increasing use of alternative fuels, reduced clinker content, and decreased electricity use per ton of product.
But Pour-Ghaz writes that efficient cement manufacturing technology is already relatively advanced, and further development may result in only incremental improvements. This has led cement manufacturers and other organizations to investigate other means of reducing emissions resulting from cement production.
One way to accomplish this is to use low-carbon raw materials. “Approximately 60 percent of the CO2 produced in the manufacturing of the cement is the result of carbonate calcination,” Pour-Ghaz writes. “Substitution of limestone with other low-carbon materials such as slag and fly ash can result in lower liberation of CO2.” Slag may also require less fuel for production, he adds.
With some researchers focused on reducing the amount of limestone used in portland cement production, it may seem paradoxical that scientists at the National Institute for Standards and Technology (Gaithersburg, Md.) are adding limestone powder to cement formulations. But that is exactly the case, according to this news release.
The research focuses on additions of limestone powder to concrete mixtures that contain significant quantities of fly ash, a byproduct of coal-burning power plants already in wide use to make concrete a more sustainable technology. The release says fly ash accounts for about 15 percent of the binder phase of ready-made concrete in the US. Working with colleagues at the Federal Highway Administration, NIST researchers hoped to enable incorporation of up to 40–50 percent fly ash content without the delayed setting times and reduced initial strength associated with such high fly ash content. The researchers found that a “judicious combination of fine limestone powder” can alleviate these potential problems.
“So-called high-volume fly ash ‘ternary’ mixtures (including some limestone) that replace between 40 percent and 60 percent of the cement portion not only set at rates comparable to those for typical concrete, but also were superior in terms of key properties,” the release says.
Increased use of cement containing high volumes of fly ash could simultaneously improve concrete sustainability and reduce construction costs, the release says. The researchers are currently field testing their limestone-enhanced mixtures to see what effect variable curing conditions have on the formulations.
Upping the sustainability ante by both reducing CO2 production during cement manufacture and actively sequestering the greenhouse gas during concrete curing is Solidia Technologies (Piscataway, N.J.).
The company is currently commercializing patented technology developed at Rutgers University, Piscataway, N.J. Solidia says its proprietary cement product and concrete production process can reduce CO2 emissions by up to 70 percent compared with conventional materials and processes. The subject of more than 50 U.S. and foreign patents and patent applications, the cement and CO2 sequestration technology were developed at Rutgers by professor of materials science and engineering and ACerS Fellow Richard Riman and Vahit Atakan, currently Solidia’s R&D director and a former doctoral student in Riman’s lab.
The process is said to require minimal changes to conventional cement and concrete production processes. According to its website, Solidia cement uses different ratios of limestone, clay, and sand than conventional portland cement and is produced at lower temperature, resulting in a 30 percent reduction in greenhouse gas and other emissions. To produce concrete, the cement is mixed with sand, aggregate, and water in conventional mixers. It can be shaped like conventional concrete; however, the product cures only when exposed to a CO2-containing atmosphere. The gas is injected into concrete during production, effectively sequestering it by setting it into the cured concrete.
Called Low-Temperature Solidification, the process is said to bind a wide variety of natural minerals and waste products to produce concrete with properties that exceed those of conventional concrete and natural stone. The technology was recently recognized with an R&D 100 Award.