Sources: Massachusetts Institute of Technology Concrete Sustainability Hub, Cambridge; CP staff

The process by which cement-based products sequester carbon dioxide is key to understanding the life cycle impacts of the world’s most widely subscribed building material. In a three-point question & answer sequence, MIT Concrete Sustainability Hub Deputy Director Hessam AzariJafari describes how CO₂ uptake is a key lever in the concrete industry realizing carbon neutrality:

What is carbon uptake in cement-based products and how can it influence their properties?

Carbon uptake, or carbonation, is a natural process of permanently sequestering CO₂ from the atmosphere by hardened products like concrete and mortar. Through this reaction, these products form different kinds of limes or calcium carbonates. It occurs slowly but significantly during two life cycle phases: use phase and end-of-life. In general, carbon uptake increases the compressive strength of cement-based products as it can densify the paste. At the same time, carbon uptake can impact the corrosion resistance of reinforced concrete. The corrosion process can be initiated if carbonation happens extensively (e.g., the whole of the concrete cover is carbonated) and intensively (e.g., a significant proportion of the hardened cement product is carbonated). 

What factors influence carbon uptake?

The intensity of carbon uptake depends on the climate; types and properties of cement-based products; composition of binders (cement type); and, geometry and exposure condition of the structure. In regard to climate, the humidity and temperature affect the carbon uptake rate. In very low or very high humidity conditions, the process is slowed; high temperatures accelerate it. Local atmosphere CO₂ concentration can likewise affect the carbon uptake rate.

In urban areas, carbon uptake is an order of magnitude faster than in suburban areas. The types and properties of cement-based products have a large influence on the rate of carbon uptake. Mortar (water, cement + fine aggregates) carbonates two to four times faster than concrete (water, cement + coarse and fine aggregates) because of its more porous structure. The carbon uptake rate of dry-cast concrete masonry units is higher than wet-cast products for the same reason. In structural concrete, the process is made slower as mechanical properties are improved and the density of the hardened products’ structure increases. 

Lastly, a structure’s surface area-to-volume ratio and exposure to air and water can have

ramifications for its rate of carbonation. When cement-based products are covered, carbonation may be slowed or stopped. Concrete that is exposed to fresh air while being sheltered from rain can have a larger carbon uptake compared to cement-based products that are painted or carpeted. Additionally, cement-based elements with large surface areas, like thin concrete structures or mortar layers, allow uptake to progress more extensively.

What is the role of carbon uptake in the carbon neutrality of concrete, and how should architects and engineers account for it when designing for specific applications?

Carbon uptake is a part of the life cycle of any cement-based products that should be accounted for in carbon footprint calculations. The MIT CSHub evaluation shows the U.S. pavement network can sequester 5.8 million metric tons of CO₂, of which 52 percent will be sequestered when the demolished concrete is stockpiled at its end of life. From one concrete structure to another, the percentage of emissions sequestered may vary. For instance, concrete bridges tend to have a lower percentage versus buildings constructed with concrete masonry. In any case, carbon uptake can influence the life cycle environmental performance of concrete.

The CSHub has developed a calculator for construction stakeholders to estimate the carbon uptake during a concrete structure’s use and end-of-life phases. Looking toward the future, carbon uptake’s role in the carbon neutralization of cement-based products could grow in importance. While caution should be taken in regards to uptake when reinforcing steel is embedded in concrete, there are opportunities for different stakeholders to augment carbon uptake in different cement-based products.

Architects can influence the shape of concrete elements to increase the surface area-to-volume

ratio. Concrete producers can adjust the binder type and quantity while delivering mixes that meet performance requirements. Finally, industrial ecologists and life-cycle assessment practitioners need to work on the tools and add-ons to make sure the impact of carbon is well captured when assessing the potential impacts of cement-based products in buildings and infrastructure. Currently, the cement and concrete industry is working with tech companies as well as local, state, and federal governments to lower and subsidize the cost of carbon capture sequestration and neutralization. Accelerating carbon uptake where reasonable could be an additional lever to neutralize the carbon emissions of the concrete value chain.