“Concrete plays a critical role in achieving societal goals for sustainable development,” he said. “Calls for increased housing to address affordable housing shortages and more resilient buildings and infrastructure to mitigate the impacts of natural disasters will…lead to increased construction using concrete.”
Due to its importance in these construction applications, concrete has become the world’s most popular building material. In fact, production volumes of cement, the ingredient that gives concrete its strength, surpass that of steel by a factor of three. “This significant consumption is…important to address when setting industrial emission targets,” says Gregory.
Much of concrete’s emissions derive from the production of portland cement, which binds together sand and gravel to make a strong and durable final product. To make cement, “limestone and other raw materials are mined and [treated] before entering [a] kiln which requires significant amounts of energy,” said Gregory. “The limestone is [then] transformed into clinker in the kiln in a process…that emits carbon dioxide.” In the final stage of the process, the clinker is then mixed with other binding materials to form cement.
Though the production of cement is energy-intensive, concrete is primarily composed of aggregate materials, like sand and gravel, with low environmental footprints. This means that, overall, concrete’s emissions are much lower than other materials. “On a per unit weight basis, concrete and cement have low embodied carbon dioxide and energy footprints, [which are] the emissions and energy associated with production,” said Gregory.
Concrete also has life cycle environmental benefits that can offset its initial emissions. “MIT case studies have shown that embodied environmental impacts of buildings associated with material production and building construction are at most 10% of the total life cycle greenhouse gas emissions. Energy use represents the vast majority of environmental impacts in most buildings,” said Gregory.
Since concrete buildings tend to be energy-efficient, the life cycle greenhouse gas emissions of such buildings are comparable or lower than buildings constructed using other materials. Concrete pavements can also have comparable or lower lifetime emissions than other paving materials because they reduce the emissions of the vehicles that drive on them.
Regardless, since concrete is so heavily used, reducing its emissions is still critical. In his testimony, Gregory discussed several strategies to minimize these emissions.
One of the steps cement manufacturers have taken is to increase the energy efficiency of their cement plants. According to Duke University, the industry improved energy intensity during cement production by 13 percent over a decade-long period. “The energy performance of the industry’s least efficient plants changed most dramatically, total source energy savings were 60.5 trillion Btu annually, and environmental savings were 1.5 million metric tons of energy-related carbon emissions,” said Gregory.
The use of alternative fuels, like tires, biomass, or post-consumer paper, represents another opportunity for cement manufacturers to reduce emissions. Gregory noted that legal barriers have required American manufacturers to use around only 15% alternative fuels. In Germany, the use of alternative fuels is far higher—up to 60% of the total fuel used.
Manufacturers can also reduce the emissions of portland cement by blending it with other binders like portland limestone cement (PLC). “[PLC] is made by blending limestone with clinker. The limestone replaces clinker in the cement and therefore, has lower carbon dioxide emissions…The costs of PLC are similar to [portland cement], as is its performance,” said Gregory.
Even though it offers a sustainable advantage, PLC makes up only 1% of all cement produced in the US. “This is primarily due to an unwillingness of concrete specifiers, such as engineers, to choose PLC over [portland cement], which has a longer history of use,” Gregory explained.
Innovative technologies like carbon capture offer opportunities to lower the emissions cement as well. Carbon capture works by injecting CO2 into concrete during the mixing process. This effectively traps the CO2 in the concrete, indefinitely sequestering it inside the material.
Existing policies, however, can make it difficult for manufacturers to invest in the installation of energy efficiency and carbon capture technologies. “Congress should…encourage, rather than discourage, investments in energy efficiency and carbon capture, use and storage technologies,” Gregory advised.
Gregory’s testimony provided policymakers with the fundamentals of concrete’s environmental impact and presented a roadmap for future action. “The concrete and cement industries will need help from Congress,” he said. “Congress can start by reducing the barriers manufacturers face to taking early action.”