Carbonation refers to a chemical reaction that can occur between carbon dioxide (CO₂) and cement. In the past, we assumed that carbonation is damaging to concrete products. However, we now know that not all carbonation has the same effects. Recent research has shown that carbonation can occur in concrete in two very different ways:
- Weathering carbonation: Mature concrete reacting to carbon dioxide in the atmosphere over an extended period.
- Early age carbonation (CarbonCure): Fresh concrete reacting to carbon dioxide introduced to the concrete mix during production.
While the effects of weathering carbonation can harm mature concrete, early age carbonation involves different chemical reactions affecting fresh concrete that result in strength and durability benefits, thereby reducing cement requirements. These effects also impact the carbon footprint of concrete products.
Carbonation Chemical Reaction
This article summarizes both types of carbonation and explains their differences.
Weathering carbonation occurs when CO₂ from the atmosphere slowly enters concrete over time, reacting with some of the reaction products of cement hydration and water in the pores (i.e., small pockets in the structure of hardened concrete).
In ideal conditions for weathering carbonation, CO₂ from the atmosphere will react with the water that remains in the porous structure of mature concrete to form carbonic acid. Subsequent reactions caused by the presence of carbonic acid can then lead to a reduction in the pH of concrete, which can cause the concrete to deteriorate and can make steel reinforcement (e.g., rebar) susceptible to harmful corrosion. The presence of salts containing chloride ions (e.g., road salt) will increase the rate of corrosion.
Proper design and consideration for durability mitigate these effects. Over time, however, this weathering carbonation will damage all concrete and can seriously affect the resilience of any concrete structure.
Early Age Carbonation
The early age carbonation process (CarbonCure) that is used to make concrete has a very different effect. Early age carbonation happens when carbonation reactions occur alongside the early hydration of the cement through deliberate exposure to carbon dioxide. Unlike weathering carbonation, which takes place over time, early age carbonation occurs rapidly and can contribute to a denser, stronger concrete — even while using less cement. Early age carbonation only occurs when concrete producers intentionally choose to make concrete products with CO₂.
In early age carbonation, CO₂ that is introduced to the concrete mixing process rapidly converts to calcium carbonate (CaCO3), or limestone, in the presence of cement particles. The creation of calcium carbonate during early age carbonation improves the compressive strength of concrete without the negative effects of weathering carbonation.
Early Age Carbonation with CarbonCure
Early age carbonation is the type of carbonation that occurs in concrete made with CarbonCure’s technology. CarbonCure injects a precise dosage of CO₂ into concrete during mixing, where it becomes chemically converted into a mineral.
Once injected into the wet concrete mix, the CO₂ reacts with calcium ions from cement to form calcium carbonate, which becomes embedded in the concrete and makes the concrete stronger. Concrete producers see an average compressive strength gain of 10% at 28 days when CO₂ is injected into concrete via CarbonCure.
This gain gives producers the ability to optimize their mix designs and reduce cement content while maintaining the concrete’s strength.
Aside from increasing its strength, CO₂ does not impact the concrete properties. Fresh properties including set-time, slump, workability, pumpability, air content, temperature, and finishing are not affected; neither are hardened properties, including pH, freeze-thaw, density, color, texture, and durability.
Environmental Implications of Carbonation
All carbonation reactions can have an environmental benefit, namely the absorption of CO₂ by the concrete itself. Recent research by concrete producer, CalPortland, suggests that 20% of the CO₂ generated through the production of concrete is reabsorbed over its life cycle. These findings cast doubt on our previous conceptions about the environmental impact of concrete.
Fortunately, the ability of concrete to reabsorb atmospheric CO₂ is not affected by using early age carbonation to make concrete. In fact, by using CO₂ in the concrete production process, concrete producers can actively reduce the carbon footprint of concrete products while still delivering the same quality concrete and seeing the same long-term CO₂ absorption that normally occurs.
For a deeper dive into this topic, download the CarbonCure white paper that discusses the chemistry of the two carbonation processes in greater detail.