As technology deployment to eliminate carbon pollution in the power generation and transportation sectors are taking hold in greater ways, solutions for addressing industrial emissions takes on greater urgency.
If the cement manufacturing sector were a country, it would be the third largest emitter behind China and the United States, responsible for 8 percent of total global carbon dioxide emissions.
Cement is both the key ingredient and main source of emissions in concrete, the most widely used building material in the world—the United States alone produced 92 million tons of cement in 2022. It is notoriously challenging to decarbonize because so much of the emissions come from the chemical composition of the raw materials that are fired in kilns (i.e., process emissions), not just from the energy that runs the kiln.
However, modifying existing methods could significantly reduce emissions in the near term—if states and standard-setting groups more easily allowed substitutions in concrete recipes. A new report led by ClearPath and supported by C2ES and the Clean Air Task Force details how these rules hold back decarbonization without necessarily improving the performance of the world’s most essential building material and ways to unlock abatement opportunities.
Cement (the binding component of concrete) is responsible for nearly 80 percent of concrete’s emissions, while composing only 10 percent of its mass. Cement is made by mining and grinding materials, primarily limestone, with other additives; then, the mixture is heated in a kiln at high temperatures. The process emissions associated with cement are largely from the carbon dioxide-releasing chemical reaction that occurs when limestone is heated; a step which results in more than two-thirds of the emissions in cement manufacturing.
One way to reduce concrete emissions is to alter the process or raw materials associated with cement production. Another way, which was the focus of the report, is to simply use less cement by introducing greater flexibility to substitute materials that have cement-like binding qualities but without the associated emissions. Supplementary cementitious materials (SCMs) are material byproducts of various industries, including fly ash, blast furnace slag, and silica. They can serve as a partial replacement for cement in concrete, while maintaining product quality and decreasing emissions.
Because governments are the largest buyers of concrete in their respective states, they hold significant purchasing power to shape the market and encourage widespread adoption of these lower-carbon cement substitutions through the specifications they choose to set for materials used in publicly-funded projects. Currently, many states have prescriptive standards that often narrow the composition of acceptable concrete by setting rules on the mixture rather than how well the concrete performs in key tests—often through maximum water-cement ratios, minimum cement per volume of concrete, and maximum amounts of SCMs that are allowable—all of which tend to push cement content beyond what is needed to optimize performance.
A performance-based approach would determine the acceptability of a material through measures in various strength and quality tests without regard to specific rules on the mixture, while maintaining safety standards and reducing lifetime costs. It also allows for significant emissions reductions due to flexibility on the concrete mix. The use of SCMs to partially displace the amount of cement can reduce concrete emissions by up to 70 percent relative to concrete made only with cement as the binding agent.
The report found that every state across the United States has at least one type of prescriptive standard, restricting material use and restricting the flexibility of manufacturers to reduce emissions of their concrete or cement products. State departments of transportation (DOTs) are highly influential on local markets. Since governments are the largest buyers and often set the strictest specifications, they act as the common code when contractors work in various localities on different projects. The government’s procurement power as they build huge infrastructure projects, combined with their impact on private contractors, determines the types of materials developed in those areas. Shifting to performance-based specifications at the state level could therefore catalyze the adoption of much lower emission concrete.
The report also shows how entrenched the problem of prescriptive specifications can be. They are often based on standards designed by national and international associations such as the American Concrete Institute (ACI) or the American Society for Testing and Materials International (ASTM) and adopted by state DOTs as-is, without exceptions or additions. If the standards set by organizations like ACI or ASTM could be updated to be more performance-based, with fewer restrictions on the composition of concrete mixes, it too would create opportunities to significantly reduce emissions.
A dozen states have already successfully implemented some form of performance metrics into their construction standards. Expanding the uptake of performance specifications will require support across the cement and concrete industry and levels of government. Research on increased SCM substitutions, optimal low-carbon concrete mixtures, and new tests that ensure the performance of alternative cement would support increased inclusion of performance specifications.
Support through demonstration projects, where federal construction projects can showcase newly validated materials, will be crucial to increase the acceptance of new materials in a risk-averse industry. Lastly, technical support from the U.S. Department of Transportation would aim to educate the agency’s staff, material producers, and contractors; these decision-makers are key to influencing greater adoption of performance specifications and often need additional training or resources to comply with performance specifications and testing. These relatively small recommendations could spur big changes in the way cement and concrete are produced, creating an opportunity for significant emissions reductions from.