3.1. What is Carbonation?

Carbonation is a chemical process in which carbon dioxide (CO₂) from the atmosphere reacts with calcium hydroxide (Ca(OH)₂) in hydrated cement paste to form calcium carbonate (CaCO₃).

Reaction: Ca(OH)₂ + CO₂ → CaCO₃ + H₂O

Carbonation occurs when CO₂, which is a common gas found in the air, interacts with calcium hydroxide present in concrete. When these two substances come into contact, they undergo a chemical reaction. This reaction transforms the calcium hydroxide into calcium carbonate, which is often a stable and solid compound.

This process can be visualized as a building block system. The CO₂ is like a puzzle piece that fits into the structure of the concrete, changing it from something vulnerable (calcium hydroxide) to a more stable form (calcium carbonate).

Think of carbonation like the way a soda goes flat. When an unopened bottle is sealed, the carbon dioxide bubbles are kept in. But once opened, the gas starts to escape and interacts with everything around it. Similarly, when concrete is exposed to air over time, CO₂ seeps into the material and affects its composition.

The carbonation reaction progresses inward from the concrete surface, forming a "carbonation front" which can be detected by phenolphthalein indicator. The uncarbonated zone turns pink; carbonated areas remain colorless.

As carbonation occurs, it does not happen all at once but rather starts at the surface of the concrete and moves inward. The boundary between the area that has reacted with CO₂ and the area that has not is called the carbonation front. A chemical indicator like phenolphthalein can be used to visually identify this boundary: if the area turns pink, it means that the concrete still has calcium hydroxide, and hence, hasn't carbonated yet, while colorless areas indicate carbonation.

Imagine painting a wall. Initially, when you apply the first layer, the paint only changes the surface color. As you apply more paint, it seeps deeper, changing the colors beneath it, just like carbonation progresses through the concrete.

Several factors influence the rate and depth of carbonation:

• Concrete permeability: More porous concrete allows faster CO₂ ingress.
• Relative humidity: Optimum carbonation occurs at 50–70% RH.
• CO₂ concentration: Higher levels accelerate carbonation.
• Curing and cover depth: Poor curing and insufficient cover depth accelerate carbonation depth.

There are multiple factors that affect how quickly and deeply carbonation occurs in concrete:

1. Concrete permeability: If the concrete is very porous, CO₂ can pass through it more easily, accelerating carbonation.
2. Relative humidity: Carbonation happens best in an environment that is neither too dry nor too wet, with 50-70% humidity being ideal.
3. CO₂ concentration: Where there are higher concentrations of CO₂, carbonation happens faster.
4. Curing and cover depth: How well the concrete was cured and how thick the cover of the concrete is also play important roles; inadequate curing allows carbonation to penetrate deeper.

Think of carbonation like a plant growing. If you provide the right conditions—like the right amount of water (humidity), sunlight (CO₂), and soil quality (permeability)—it will grow quickly. Conversely, if the soil is poor or the environment is unfavorable, growth will be stunted. Similarly, concrete 'grows' more susceptible to carbonation based on these factors.

The effects of carbonation include:

• Reduction in alkalinity (pH drops from ~12.5 to <9).
• Loss of passive protection layer on reinforcing steel.
• Initiation of steel corrosion.
• Shrinkage due to CaCO₃ formation → microcracking.

Carbonation has several detrimental effects on concrete structures:

1. Reduction in alkalinity: The pH level of the concrete drops significantly, which reduces the ability of reinforcing steel to remain protected.
2. Loss of passive protection: As the alkaline environment dissipates, the protective layer that prevents the steel from corroding is lost.
3. Initiation of steel corrosion: Without protection, the steel begins to rust, which can lead to structural failure over time.
4. Shrinkage: The formation of calcium carbonate can lead to internal stresses, causing microcracks in the concrete.

Imagine your skin getting sunburnt; with a protective layer (like suntan lotion), your skin stays safe. If you don’t use it, the sun damages your skin cells (akin to steel corrosion). Furthermore, just as sunburn leads to peeling (microcracking), carbonation leads to the internal stresses that damage the concrete.

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