Clinker, produced by heating a limestone mixture to over 2,500 degrees, is the essential ingredient in cement, which is the primary ingredient in concrete, the world’s most human-made material. Traditional clinker production generates 90 percent of the emissions associated with cement. Below are a few examples of ways to address this challenge.
Reducing or replacing clinker with supplementary cementitious materials (SCMs)
Two of the most common SCMs:
Blast furnace slag, a byproduct of iron production, and fly ash, a byproduct of coal combustion, are two of the most common SCMs used to replace clinker. Both are expected to become less available as more steel facilities transition from blast furnaces to electric arc furnaces and the US phases out coal-fired power plants. Because most coal-fired power plants are in the eastern US, cement manufacturers in other regions already have limited access to fly ash.
Ground glass and natural pozzolans are two other common SCMs. While they may increase set time, they can also improve durability and strength. Blended cement and mixes that contain SCMs have become common in the US, accounting for 58 percent of cement shipments during the first eight months of 2024. Most of this was Portland Limestone Cement, which replaces a portion of the clinker with more limestone than traditional cement and reduces some emissions.
Reducing or replacing limestone with alternative materials
Cement manufacturers are also developing production methods to replace limestone with alternative materials, such as calcium silicate rocks. Calcium silicate rocks are abundantly available on every continent and contain no carbon. Production methods are also being developed to use industrial waste in place of limestone. Many companies developing these methods are actively commercializing these products to make them available at scale.
Limestone calcined clay cement (LC3)
Limestone Calcined Clay Cement (LC3) is another alternative being developed. LC3 replaces half of the clinker with widely available calcined clay and ground limestone, which releases less carbon dioxide than pure limestone. The clay is heated to a lower temperature than traditional cement, which reduces energy costs and means electricity can often be used instead of fossil fuels. As the grid is powered by more renewables, this will reduce emissions. In addition to reducing carbon emissions by approximately 40 percent relative to traditional cement, LC3 is highly durable and less permeable to water and salt, making infrastructure more durable.
As outlined in our Cement and Concrete 101, clinker is just one step in the production process, but it is one of the most impactful levers for reducing emissions. These examples demonstrate how the cement and concrete sector is advancing innovative, practical solutions.