Concrete is the second-most-used substance in the world after water. It's also the most widely used building material. Concrete is great for its strength and resilience. It's also simple to produce and inexpensive. For centuries, concrete has been the foundation of buildings, roads, bridges, and more. Globally, about 30bn tons of concrete are used every year. And the total market value of concrete is expected to exceed $970bn by 2030.

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The problem of concrete production: one of the least sustainable materials

As essential as it is for modern construction, concrete also presents significant environmental issues. Manufacturing concrete requires large amounts of energy and produces high carbon dioxide emissions. Cement, a key component of concrete, is responsible for 8% of global CO₂ emissions. According to CarbonBrief.org, that's greater than any other country's share besides China and the U.S.

Concrete is also responsible for 10% of the world's industrial water usage. In addition, the production process has major impacts on air quality due to dust and other particulate matter.

Concrete production consists of multiple steps, from quarrying, grinding, and clinker production to mixing and transportation. Throughout the production, more than half of the emissions are released by the heating process of the limestone, a process called calcination. When limestone, predominantly made of calcium carbonate, is heated, it breaks down into calcium oxide and CO₂. The emissions associated with calcination are, therefore, inherent to the chemistry of cement itself and thus are considered “hard to abate.”

Besides the calcination process, around 40% of the emissions associated with cement production are generated by burning fossil fuels to heat the kiln. In the kiln, comparable to a very high-temperature rotating oven, pre-heated elements are converted into clinker via a sintering process. The chemical reactions require very high temperatures, generally over 1400°C. Accordingly, fossil fuels like coal dust, brown coal, oil, and low-grade fuels are typically used to fuel the burner that supplies such temperatures. As part of this combustion process, CO₂ gets released together with several other gases.

In conventional cement plants, process emissions (which are pure streams of CO₂) are mixed with flue gases resulting from the combustion of fossil fuels, which have a significantly lower concentration of CO₂. This results in a flue gas that is “impure” with a low percentage of carbon dioxide, making it much harder to separate and capture.

Finally, about 10% of the emissions come from the electricity used to power additional plant machinery, quarrying, grinding, transportation, and cement mixing.

The solutions: opportunities for sustainable concrete

As the concrete industry will continue to grow in the foreseeable future, its environmental problems need to be solved urgently. While the current production process for concrete is unsustainable for the environment, sustainable solutions to "green concrete" are emerging.

CCS and CCU

One of the solutions to help reduce the amount of pollution from concrete production is CCS and CCU. CCS stands for Carbon Capture and Storage, a technology that aims to capture carbon dioxide before it enters the atmosphere. It stores CO₂ underground, where it eventually mineralizes. Similarly, CCU, or carbon capture and utilization, converts CO₂ into resources like industrial feedstocks and materials.

• CarbonCure integrates its technology into concrete plants. The company injects captured CO₂ in fresh concrete during mixing. After the injection process, the CO₂ reacts with the mix and eventually turns into a mineral that will be permanently embedded within it. Besides carbon reduction benefits, the CO₂ mineralization augments the solidity of concrete, providing both financial and environmental advantages.
• Carbon Clean claims that its modular system provides a fast, scalable, and standardized solution that can be adapted to any site at an affordable cost. Its CycloneCC technology combines the company's advanced, proprietary buffer salt solvent (APBS-CDRMax®) and process technology, rotating packed beds (RPBs). Each CycloneCC unit is delivered ready to install and can start operating in less than two months.

Oxyfuel combustion and indirect calcination

One of the factors that make point-source carbon capture expensive in a cement plant is the heterogeneous nature of flue gases. It’s hard to isolate and capture CO₂ from it. Accordingly, some processes are being developed to produce pure CO₂ emissions, which are significantly easier to convert into useful products.

• Furno Materials is developing a material technology that leverages oxyfuel combustion to produce a pure stream of CO₂. Oxyfuel combustion involves burning fuels with oxygen instead of air (predominantly inert nitrogen), yielding emissions with a much higher concentration of CO₂.
• Calix Global, on the other hand, isolates process emissions from combustion emissions with an indirect calcination unit. This indirect calciner heats limestone indirectly, as opposed to conventional cement plants that use combustion gases to heat limestone.

Clinker substitution or reduction

Clinker substitution occurs when clinker, the primary material used in cement manufacturing, is substituted with other cementitious materials and pozzolans. It's often replaced with coal fly ash or blast furnace slag. These materials are byproducts of other industrial and energy processes and can displace the need to create clinker, thus reducing the overall carbon footprint of cement. Although this is already common practice in the industry, a few companies are innovating on supplementary cementitious materials.

• Carbon Upcycling's technology can permanently store CO₂ in industrial byproducts and natural, sustainable concrete materials. These come from coal plants, steel mills, glass manufacturing, and mine sites. The company's technology reduces clinker in cement and cement in concrete, providing a solution to cement and green concrete producers.

Novel cement

Novel cement is a type of cement made from new, sustainable materials. It can be used in concrete production instead of traditional materials like clinker. Different types of novel clinkers and cement are being developed with new production processes. For example, researchers at Northwestern University have developed a material technology to incorporate nanomaterials into traditional cement. This helps improve water and fracture resistance and greatly reduces emissions.

• Brimstone's technology makes carbon-negative portland cement that can be used for sustainable concrete. Traditionally, cement is made from limestone, which releases much CO₂ during production. Brimstone produces cement from calcium silicate rock, a carbon-free, sustainable material much more abundant than limestone. This material technology can cut out process emissions from the start. Brimstone's process produces cement clinker and supplementary cementitious materials (SCM). In addition, the rock it uses contains magnesium, which is used to remove atmospheric CO₂ permanently.
• Biomason claims to use natural microorganisms to grow structural Biocement®. Its Biocement® grows in ambient temperatures, building with carbon to create controlled, structural cement for products or applied services. Biolith, the company's first commercially available Biocement® product, comprises approximately 85% natural aggregate and 15% Biocement material. Its precast product is utilized in projects across the U.S. and Europe.

Kiln substitution

Another way to reduce emissions is by replacing the energy source with renewable electricity. Some startups are working on adding alternative paths to substitute kilns with electrochemical processes. This can eliminate the emissions from both burning fossil fuels and process limestones.

• Sublime Systems claims to make low-carbon cement without using the kiln. With an electrochemical production process, the company can extract calcium using low-cost electricity rather than heat. Its product can replace traditional cement without compromises. Sublime Systems is on track to produce 1mn tons per year of low-carbon cement by 2028.
• Similarly, Chement focuses on driving the key reactions during cement production using electrochemistry instead of combustion. With a room-temperature electrochemical process, Chement can eliminate the use of highly-polluting kilns. This process relies on renewable electricity and the same materials to conduct a chemical reaction with fewer energy costs and CO₂ emissions.

What else is being done to make concrete production sustainable?

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The adoption of innovations and technologies can often greatly increase with the support of government policy and incentives. The government is also discussing ways to produce more green concrete. For instance, a "carbon tax" system can push the industry to upgrade its processes to include carbon capture and other sustainable materials technologies. Meanwhile, research and development policies can encourage acknowledging and adopting new sustainable concrete production techniques.

Decarbonizing concrete production is still an emerging area. But the need to address the industry's environmental issues will only grow continuously. Through collaboration among startups, corporations, and governments, we can help the concrete industry move forward in a more sustainable direction.