The active ingredient in concrete is cement, and concrete’s ubiquitous usage makes cement the most widely used industrial commodity today. The world consumes about half a ton of cement per person per year. But alongside its positive benefits as a construction material, cement also puts a major strain on the environment. Cement production currently accounts for 3% of world energy consumption and roughly 8% of global greenhouse gas emissions. Reducing emissions from the cement sector is critical to meeting the United Nation’s goal of limiting global warming to 1.5°C, but the sector has proven inherently challenging to decarbonize. This challenge is prompting a major drive to develop technological solutions that can address these processing emissions, including significant upticks in both investments and startup activity in low-carbon cement.
How is Concrete Made Currently?
The manufacture of conventional concrete is fairly straightforward. First, the cement (usually Portland cement) is prepared. Next, the other ingredients — aggregates (such as sand or gravel), admixtures (chemical additives), any necessary fibers, and water — are mixed together with the cement to form concrete. The concrete is then shipped to the work site and placed, compacted, and cured.
Why is the Carbon Footprint of Conventional Concrete so Large?
The creation of cement is the most carbon-intensive portion of the concrete process. This comes down to two main activities: the calcination of limestone and the heating of cement kilns. To create Portland cement, limestone undergoes a calcination process, which releases large amounts of CO2 from the chemical reaction. This is the concrete industry’s most-pollutive activity, releasing up to 50 percent of the cement industry’s carbon emissions. Additionally, to create cement from raw materials into clinker (an intermediate product), large amounts of energy are required to heat, mix and cool the ingredients in giant kilns. It is estimated that, in traditional kilns, one ton of cement produces one ton of carbon dioxide.
What is Low Carbon Concrete?
Low carbon concrete is concrete produced with a lower carbon footprint than traditional concrete. Other than a reduced carbon footprint, low carbon concrete should behave identically to its high carbon counterpart. To create low carbon concrete, producers can implement a series of relatively low-impact changes to their production processes and mix designs.
How is Low Carbon Concrete Produced?
To create low carbon concrete, producers can implement a series of relatively low-impact changes to their production processes and mix designs. There are 3 primary ways by which concrete manufacturers can reduce carbon emissions related to their products, namely, 1). switching to lower emission fuel sources, 2). substituting other materials in the mix in lieu of cement, 3). and adopting carbon capture technologies during production. Depending on the type of alternative fuel used, dosage of the cement substitutes, or method of carbon capture utilized, the carbon footprint of concrete produced can be reduced by up to nearly 70%.
Switch to a Lower Emissions Fuel Source
This means moving from traditional fuels (e.g., coal) to lower-carbon fuels (e.g., renewable natural gas), waste fuels (e.g., non-recyclable plastics, non-recyclable tires, rail ties, etc.), and potentially even carbon-neutral fuels.
Partially Replace Cement Content with Mineral Compounds
A few substitutes to cement which can reduce the emissions in concrete are:
• Fly ash (by-product of coal-burning power stations)
• Ground granulated blast-furnace slag (GGBS) (by-product of the iron and steel industry)
• Silica fume (by-product from the manufacture of silicon)
• Limestone fines.
Install On-site Carbon Capture Technologies
Innovation in carbon capture, utilization, and storage (CCUS) technologies is an innovative development in the concrete industry. Carbon capture theoretically makes it possible to capture up to 100% of the carbon emissions from cement manufacturing. These captured emissions can be stored safely underground, injected back into concrete to strengthen it, or used to make other products like synthetic aggregates or fuels.
The concrete industry is in the early stages of the move towards a low-carbon future. How soon it gets there will go a long way towards determining how quickly the industry can chip away at that roughly 8% greenhouse gas emissions figure and make concrete production, an ancient technology that hasn’t really changed a whole lot over the centuries, into an industry friendly to the future of the planet.
For Further Reading:
The World Economic Forum has published an excellent overview of low-carbon concrete at: https://www.weforum.org/agenda/2023/01/low-carbon-concrete-is-the-future-now-emissions/.
“Alternative materials could shrink concrete’s giant carbon footprint” by Chemical and Engineering News is presented at their page: https://cen.acs.org/materials/inorganic-chemistry/Alternative-materials-shrink-concretes-giant/98/i45.
U.S. Concrete presents industry-based information on this subject in their article, “Low CO2 Concrete – Stronger, Cleaner, Greener Concrete”, found at: https://www.us-concrete.com/low-co2-concrete.