Cement absorbs and stores millions of tons carbon dioxide from the air over the lifetime of urban buildings and infrastructure, scientists have calculated.
Cement is one of the key components of concrete, the most widely used manufactured material on earth, but it comes with a massive environmental cost. The production of cement is estimated to generate around 2.5 billion tonnes of CO2 every year – about 8% of the global total.
Nevertheless, the global carbon sink provided by concrete does help to counter the emissions released during cement production. When CO2 enters concrete through tiny pores, it reacts with the calcium-rich products in cement, and becomes locked into a stable mineral called calcium carbonate, or limestone.
Now, a study from the MIT Concrete Sustainability Hub has quantified this process and the amount of carbon uptake across the US in its entirety for the first time. The team found that the cement in US buildings and infrastructure sequesters over 6.5 million tonnes of CO2 annually which corresponds to roughly 13% of the process emissions in US cement manufacturing. In Mexico, the same building stock sequesters about five million tonnes a year.
The research finds that carbon uptake varies greatly depending on the specific size and shape of the structure, as well as the type of concrete used. For example, a concrete highway in Dallas sequesters CO2 differently from Mexico City apartments made from concrete masonry units (CMUs). Meanwhile, a foundation slab buried under the snow in Fairbanks, Alaska, “breathes in” CO2 at a different pace entirely.
“Carbon uptake is very sensitive to context,” said lead author Hessam AzariJafari. “Four major factors drive it: the type of cement used, the product we make with it – concrete, CMUs or mortar – the geometry of the structure, and the climate and conditions it’s exposed to. Even within the same structure, uptake can vary five-fold between different elements.”
As no two structures sequester CO2 in the same way, estimating uptake nationwide would normally require simulating an array of cement-based elements. Instead, the team developed hundreds of archetypes, typical designs that could stand in for different buildings and pieces of infrastructure.
With these archetypes in hand, the team modelled how each one sequesters CO2 in different environments and how common each is across every state in the US and Mexico. In this way, they could estimate not just how much CO2 structures sequester, but why those numbers differ.
It was also found that the ratio of mortar to concrete made a big difference, since porous mortars sequester CO2 an order of magnitude faster than denser concrete. In states where mortar use was higher, the fraction of CO2 uptake relative to process emissions was noticeably greater.
“We observed something unique about Mexico: despite using half the cement that the US does, the country has three-quarters of the uptake,” AzariJafari added. “This is because Mexico makes more use of mortars and lower-strength concrete, and bagged cement mixed on-site. These practices are why their uptake sequesters about a quarter of their cement manufacturing emissions.”
The team concluded that increasing the amount of surface area exposed to air accelerates uptake and can be achieved by foregoing painting or tiling, or choosing designs such as waffle slabs with a higher surface area-to-volume ratio.
Source: E&T
