# What is Carbonation?
**Carbonation is a chemical process in which carbon dioxide (CO₂) from the air penetrates the concrete and reacts with calcium hydroxide — a product of cement hydration.** This lowers the pH of the concrete from around 12.5 to below 9, destroying the protective passivation layer around the steel reinforcement.
## How Does It Work?
The process moves from outside to inside:
1. CO₂ enters the concrete through its pores
2. In the presence of moisture, CO₂ reacts with calcium hydroxide (Ca(OH)₂)
3. Calcium carbonate (CaCO₃) — ordinary limestone — is formed
4. The pH drops in the carbonated zone
5. The carbonation front slowly advances deeper into the concrete
The rate depends on:
– The **density** of the concrete (porosity)
– The **relative humidity** (optimal between 50-70%)
– The **CO₂ concentration** in the environment
– The **concrete cover** over the reinforcement
## Consequences
Once the carbonation front reaches the reinforcement:
– The **passivation layer** around the steel breaks down
– In the presence of moisture and oxygen, **corrosion** begins
– Corroding steel expands, causing **spalling** of the concrete cover
– The structural capacity of the element is reduced
## Prevention and Repair
– Adequate **concrete cover** (at least 25-35 mm, depending on exposure class)
– Use of **dense concrete** with a low water-cement ratio
– Application of a **coating or impregnation** to the concrete surface
– For existing damage: **concrete repair** with alkaline repair mortar
## Related Terms
– Concrete
– Reinforcement
– Concrete deterioration
– Concrete cover
– Corrosion
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