What Is Carbonation in Concrete?

What Is Carbonation in Concrete?

What Is Carbonation in Concrete?

Carbonation is the process in which carbon dioxide in the air reacts with the calcium hydroxide in concrete to create calcium carbonate, which can lower the pH of the concrete to around 9. This can cause the protective oxide layer around reinforcing steel to break down, potentially leading to corrosion.

The rate of carbonation depends on the moisture content of the concrete; if it is very dry or saturated, the reaction will be slower.

Carbonation is most likely to occur in concrete that has enough moisture to facilitate the reaction but not so much that it acts as a barrier. In high-quality concrete, it may take many years for carbonation to reach the level of reinforcing steel.

How Does Carbonation Work?

Carbonation is a chemical process that occurs when CO2 in the atmosphere reacts with the calcium oxide in concrete, forming calcium carbonate. This process is essentially the reverse of the calcination of lime that occurs during the production of cement, which is a major contributor to the embodied CO2 in concrete.

Carbonation occurs slowly and continuously, starting at the surface of the concrete and moving inward. Over the lifetime of concrete, carbonation can reabsorb about a third of the CO2 emitted during cement production, reducing the overall CO2 footprint of both the cement and the concrete.

However, if the carbonation front reaches steel reinforcement, it can cause corrosion, so the mix design of structural concrete is designed to limit the rate of carbonation.

Carbonation is also more rapid in lower-strength concrete without steel reinforcement, such as blocks, and can increase the strength of these materials while also potentially extending their lifespan.

During the end-of-life phase, when concrete is crushed for reuse as aggregate, the crushing process increases the surface area, allowing for more rapid absorption of CO2.

Additionally, the newly crushed aggregate can undergo carbonation due to leaching from exposure to rain and during its secondary use in various applications.

Reversible Calcination – Carbonation

The calcination reactions in concrete are reversible, meaning that the carbon dioxide that was driven off during the cement-making process can be taken up again through a process called carbonation.

However, the speed at which this process occurs depends on various factors, including the availability of carbon dioxide and the ability of the CO2 molecules to penetrate the concrete.

The carbonation process can take place over a long period of time and may be accelerated if the concrete is crushed, increasing its surface area to volume ratio. Ultimately, the rate of carbonation in concrete is influenced by both the time and the availability of carbon dioxide.

Concrete carbonation is typically assessed using an alkalinity indicator, such as Phenolphthalein, to determine the extent of the carbonation and its proximity to the reinforcing steel. If the carbonation is more than 10mm away from the steel, it is generally not considered a risk for corrosion.

However, recent research suggests that the passive protection of the steel reinforcement in concrete may be lost at a higher pH than previously thought, around pH 11 rather than pH 9-10. This creates a challenge in accurately detecting the extent of carbonation, as the Phenolphthalein indicator is only effective up to a pH of around 9.

The research found instances of corrosion caused by carbonation occurring 5-10mm ahead of the pH 9 area detected by Phenolphthalein, indicating that carbonation-induced corrosion may begin at a pH of approximately 11.

Factors Affecting the Rate of Carbonation of Concrete

There are several factors that can impact the rate at which concrete undergoes carbonation. These factors include the grade of the concrete, the water-cement ratio, the permeability of the concrete, the cover provided to reinforcements, the ambient relative humidity, the concentration of carbon dioxide in the environment, the presence of surface protection, the age of the structure, the orientation of the structure, the use of admixtures, the porosity of the concrete, and the curing period.

Modifying these factors can potentially decrease the rate of carbonation of the rebar within the concrete.

Impacts Of Carbonation

It is important to consider the impact of carbonation on greenhouse gas emissions when calculating the environmental impact of cement and concrete. Currently, guidelines for calculating these emissions, as established by the Intergovernmental Panel on Climate Change (IPCC), do not take into account the carbonation of concrete. This means that the emissions calculations may not be entirely accurate.

It is estimated that the use of concrete contributes to 5-8% of global carbon dioxide emissions, with 50-60% of these emissions coming from the raw materials used in its production.

These emissions have the potential to be reabsorbed through the carbonation of concrete during its use and at the end of its lifespan.

Improving calculations to accurately reflect the role of carbonation in reducing emissions is an important part of climate change strategies. This includes developing methods and models for calculating the uptake of carbon dioxide through carbonation in various cement-containing products.

Concrete Carbonation And  Steel Reinforcements

Concrete carbonation is the process by which carbon dioxide from the air reacts with the calcium hydroxide in concrete, forming calcium carbonate. This reaction can cause the embedded steel reinforcements in concrete to corrode, leading to expansion and cracking that weakens the structure.

Carbonation occurs as soon as concrete is exposed to the atmosphere and can progress at a rate of 1mm to 5mm per year, depending on the porosity and permeability of the concrete.

Carbonation is the most common cause of reinforcement corrosion in above-ground structures, but it can be prevented or stopped through the use of protective anti-carbonation coatings.

During the carbonation process, the alkalinity of the concrete decreases from a pH of 12-13 to around pH 9, hardening the concrete and increasing its compressive strength.

However, this reduction in pH also breaks down the protective passivation layer around the reinforcing steel, making it vulnerable to corrosion. When the steel reinforcements rust, the resulting expansion of the iron oxide (rust) can cause the concrete to crack and spall.

Rust has a volume that is up to six times larger than the original steel, which can lead to significant expansion and damage to the concrete structure.

How Is Concrete Carbonation Treated Or Prevented?

Carbonation in concrete can be prevented or stopped through the use of protective anti-carbonation coating systems. These coatings are available in a range of forms, including epoxy coatings, acrylic sealers, and silane sealers.

It is important to choose a coating system that provides protection against the penetration of carbon dioxide, oxygen, and water, while also allowing damp substrates to breathe without blistering.

To ensure the best protection for new and existing concrete structures, it is advisable to seek the assistance of a technical team with expertise in anti-carbonation coatings. The Flowlock range of anti-carbonation coating systems is one option that can provide effective protection against carbonation in concrete.

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