Razing a Common Misunderstanding: Early-Age Carbonation vs. Atmospheric Carbonation in Concrete

While the process of making concrete has been fairly consistent for hundreds of years—the Roman Pantheon is nearly 2,000 years old—it is not uncommon to see misunderstandings of concrete's chemistry, concrete manufacturing or the climate benefits of carbon mineralized concrete. In particular, atmospheric carbonation (sometimes referred to as "weathering" or “natural” carbonation), which is the absorption of CO₂ into hardened concrete over the course of decades, should not be confused or conflated with early-age carbonation, which is the injection and immediate mineralization of captured CO₂ in fresh concrete.

A Proven Solution With Immediate Benefits

CarbonCure Technologies is the world’s most well known and widely deployed early-age carbonation solution, advancing the decarbonization of the global concrete industry. Hundreds of concrete plants across more than two dozen countries use CarbonCure’s carbon utilization technologies to generate quantifiable and verifiable carbon savings through CO₂ mineralization in concrete and the resulting cement displacement.

CarbonCure verifies these savings following a robust methodology by the leading carbon credit standards body, Verra, known as VM0043 CO2 Utilization in Concrete Production. To date, CarbonCure and its concrete producer partners have generated more than 550,000 metric tons of CO₂ savings through this process. 

Early-Age Carbonation vs. Atmospheric Carbonation: What’s the Difference?

There is a misperception that CO₂ injection just speeds up or supplants the natural process of carbonation in concrete. In reality, these are two distinct processes with key differences, and early-age carbonation is driving measurable progress in concrete decarbonization.

To understand this distinction, it’s helpful to look at the chemistry of concrete. Concrete consists of cement, water and aggregates (gravel and sand). Cement consists mainly of clinker (superheated silicates—C₂S and C₃S) and lime (calcium oxide, CaO). As it hardens, cement undergoes a hydration phase, producing calcium hydroxide (Ca(OH)₂) and calcium silicate hydrate (C-S-H), the compound that binds all ingredients into hardened concrete.

The Chemistry of Carbonation in Concrete

Atmospheric carbonation in concrete occurs over decades as atmospheric CO₂ gradually penetrates hardened concrete, reacting with the calcium hydroxide (Ca(OH)₂) formed during the hydration phase. Because this process happens slowly and is influenced by external factors, its impact is inconsistent and difficult to quantify.

In contrast, early-age carbonation by CO₂ injection in fresh concrete is a rapid, controlled process occurring during mixing—largely before the hydration phase forms calcium hydroxide. As a result, there is far less calcium hydroxide available to bind with, and instead, the concentrated CO₂ reacts mainly with silicates (C₃S and C₂S), the key ingredients in cement clinker. Because calcium hydroxide is largely not impacted by early-age carbonation, the rate of atmospheric carbonation is also not impacted.    

The Unpredictability of Atmospheric Carbonation

While some may view atmospheric carbonation in concrete as a potential long-term solution to carbon emissions, this pathway is highly unpredictable and impractical for reliable carbon storage. Its efficacy is impacted by numerous factors, including the relative humidity and temperature where the concrete was placed, the lifecycle of the concrete, and the surface area of the concrete exposed to air, water or soil, including whether paint or sealant was applied to the concrete. 

In fact, the construction industry has a vested interest in limiting atmospheric carbonation rather than encouraging it. Given that rebar corrosion shortens the lifespan of concrete structures, many contractors apply anti-carbonation coatings on concrete, thereby eliminating the possibility that the concrete will serve as a carbon sink.

Finally, and importantly, rebar corrosion caused by atmospheric carbonation reduces the lifespan of concrete, forcing additional emissions from premature demolition, concrete waste management, manufacturing of virgin cement and the pouring of replacement concrete. In sum, atmospheric carbonation should not be viewed or relied upon as a solution to rebalancing the excess CO₂ in our atmosphere.

The Precision of Early-Age Carbonation 

In comparison, early-age carbonation via CO₂ injection into fresh concrete allows quantification of the carbon dioxide removed from the atmosphere and permanently stored in the concrete. Because CO₂ in early-age carbonation reacts with the silicates and not with calcium hydroxide, it does not significantly lower the long-term pH of concrete or increase corrosion risk [1]. And unlike atmospheric carbonation, which can take decades to accrue, early-age carbonation occurs seconds from the moment of CO₂ injection. 

Recent research has shown that, with CarbonCure CO₂ injection and other novel methods and technologies, building materials can store more than 16 billion metric tons of CO₂, roughly 50% of the annual emissions generated by human activity [2]. However, implementing these technologies requires dedicated action today from concrete producers, building owners, designers, construction companies and carbon credit buyers committed to accelerating this deployment. 

Accelerating Climate Action with Measurable Impact

We cannot wait decades for the perfect solution, nor can we afford to rely on unpredictable and corrosive methods of atmospheric carbon sequestration, to solve the climate crisis. And any false equivocation or, at best, flawed assumption that the latter will naturally, eventually deliver the same outcome by midcentury only encourages inaction this decade, at a time when urgent action is needed most.

Ready to act?

CarbonCure’s technologies are operating today and positioned to scale. If you’re looking for high-integrity carbon credits that deliver real impact, get in touch.

References

[1] S. Monkman, R. Cialdella, J. Pacheco, Performance, Durability, and Life Cycle Impacts of Concrete Produced with CO₂ as Admixture, ACI Materials Journal 120 (2023) 53–62. https://doi.org/10.14359/51734732.

[2] E. Van Roijen, S A. Miller, Steven J. Davis, Building materials could store more than 16 billion tonnes of CO2 annually, Science Vol. 387, No. 6730. https://www.science.org/doi/10.1126/science.adq8594.