Just the FAQs: Early Carbonation vs. Atmospheric Carbonation

At CarbonCure Technologies (CCT), we’re part of a broader approach - sharing information with our peers, learning from each other, and inspiring innovation to make the built environment more efficient and more sustainable. Carbonation is a topic that’s come up frequently in discussions around making concrete greener, but before we can optimize it, we need to explain it. 

Concrete producers are very familiar with atmospheric carbonation. It’s the commonly known chemical reaction that takes place when environmental carbon dioxide (CO2) gas penetrates the concrete material and reacts with calcium hydroxide (CH), one of the two main hydration products of the cement reaction. This reaction forms calcium carbonate and takes long periods of time (e.g., years) as the CO2 must diffuse into the mature concrete (see Figure 1).

The depletion of CH causes the reduction of the concrete pH to levels that endanger the integrity of the material, making it more prone to corrosion damage. In other words, it directly affects the concrete durability, thus giving the term ‘concrete carbonation’ a negative connotation.

If carbonation has a negative connotation, wouldn’t that make early carbonation even worse?

Well, not exactly. Early carbonation involves utilizing CO2 in the production of concrete wherein the carbonation reaction occurs within minutes. In this case, the CO2 reacts with the calcium ions provided by the cement to form nano-sized particles of calcium carbonate. This reaction occurs alongside the early cement hydration reaction in fresh concrete [1].

The reaction is extremely accelerated - from years to minutes.

Does ‘early carbonation’ increase risk of corrosion? Would I need to increase the cover of my concrete structures?

The depletion of CH will cause the concrete pH to drop below 13, and it can even reach as low as 8 for fully carbonated concrete. Concrete with ferrous reinforcement requires a high pH (larger than 11.5) to ensure the stability of the protective passive layer on the surface of the reinforcement [2]. A drop in the pH level can cause the passive layer to deteriorate, thereby making the reinforcement susceptible to harmful corrosion.

Early age carbonation reactions can involve calcium that, on balance, would otherwise have hydrated to form CH and contribute to high pH. However, the early age carbonation does not hinder the long-term development of the concrete microstructure as the concrete matures. Therefore, CH will develop during later hydration and the pH development continues as normal once the carbonation application ends. Experiments at CCT show that the decline in pH is minimal and only within the first few minutes (it does rebound back to normal levels after 30 minutes or so), suggesting no risk of depassivation of ferrous reinforcement (see Figure 2). 

Figure 2. pH mature concrete values as CO2 dosage increases in early carbonation CCT process

How do we know how much CO2 is mineralizing?

A decade of extensive testing has indicated a very high rate of carbon mineralization using our ready mix technology. And recent lab work by our astute research team confirms that, with an average uptake of 90-percent. These experiments were carried out where the total carbon content of CO2-injected samples were measured and compared to a control sample to determine changes in total carbon after the injection of CO2 [3]. 

What does all of this mean?

In short, when injected into concrete, CO2 converts to calcium carbonate, an inert material that has a negligible effect on the pH of the concrete and has no potential for corroding rebar. Academic concrete durability experts, such as Dr. Doug Hooton (University of Toronto) and Dr. Michael Thomas (University of New Brunswick), have conducted studies [4, 5] quantifying the pH impact of mineralized CO2 in ready mix concrete and concluded that there is no meaningful impact on pH.

As demonstrated by these third party studies, early carbonation is safe and reliable, backed up by a decade of analysis conducted by our research team as well as proven results across millions of truckloads of carbon mineralized concrete and thousands of applications worldwide.

Explore some notable reference projects using concrete made with CarbonCure. And if you’ve got questions, or you want to learn more, please contact us.

References

1. Monkman, S., Kenward, P. A., Dipple, G., MacDonald, M., & Raudsepp, M. (2018). Activation of cement hydration with carbon dioxide. Journal of Sustainable Cement-Based Materials7(3), 160-181.

2. Dyer, T. (2014). Concrete durability. Crc Press.

3. Monkman, S. (2018). Sustainable ready mixed concrete production using waste CO2: A case study. Am. Concr. Ins330, 163-174.

4. Thomas, M. (2019). Impact of CO2 Utilization in Fresh Concrete on Corrosion of Steel Reinforcement.

5. Hooton, D., MacDonald, M., Monkman, S., Sandberg, P. (2016). Properties and durability of concrete produced using CO2 as an accelerating admixture. Cement and Concrete Composites, 74, 218-224.


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