---
title: "Carbonation of concrete"
subject: "Concrete Technology and Masonry Structure"
chapter: 6
key: "semester_5/concrete_technology_and_masonry_structure:carbonation-phenomenon"
source: https://www.bctengineer.com/q/semester_5/concrete_technology_and_masonry_structure/carbonation-phenomenon
site: BCT Engineering
---

# Carbonation of concrete

Concrete Technology and Masonry Structure · Chapter 6: Concrete durability

- Asked in: 2081 Bhadra, 2 marks
- Video: [Carbonation of Concrete - Durability of Concrete - Advanced Concrete Technology](https://www.youtube.com/watch?v=C4h651dL40g) — Ekeeda
- Answer checked by Claude Sonnet 5.5 on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/concrete_technology_and_masonry_structure/carbonation-phenomenon
- Whole chapter: https://www.bctengineer.com/md/semester_5/concrete_technology_and_masonry_structure/chapter-6

## Question

Describe the carbonation phenomenon in concrete.

## Short answer

Carbonation is the reaction of atmospheric CO₂ with Ca(OH)₂ in hydrated cement, forming CaCO₃ and water. It reduces pore solution pH from ~12.5 to below 9, depassivating steel and initiating corrosion. The depth advances as d=K√t, fastest at 50–70% RH, and is detected by phenolphthalein spray.

## Answer

**Carbonation** is the reaction of atmospheric carbon dioxide with the hydrated cement compounds, mainly calcium hydroxide, in the presence of moisture:

$$
\text{Ca(OH)}_2 + \text{CO}_2 \rightarrow \text{CaCO}_3 + \text{H}_2\text{O}
$$

- The reaction begins at the surface and moves inwards; the depth follows $d = K\sqrt{t}$ ($t$ in years).
- The pH of the pore solution falls from about 12.5 to 13.5 to about 8 to 9, which destroys the passive film on the steel and **starts corrosion** once the carbonation front reaches the bar.
- Rate is highest at relative humidity of 50 to 70% (dry concrete has no water; saturated concrete blocks CO$_2$ entry). It increases with high w/c, low cement content, poor curing and cracks, and high CO$_2$ concentration (cities).
- It is detected with **phenolphthalein** spray: the uncarbonated concrete (pH above 9.2) turns pink, carbonated concrete stays colourless.
- It causes some shrinkage (carbonation shrinkage), and slightly raises surface hardness and strength by filling pores.
- Prevent by low w/c, enough cement, proper curing, adequate cover (IS 456 Table 16), and protective coatings.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. What is the carbonation reaction and its effect?
   A. CO2 + C3A -> ettringite, causing expansion
   B. CO2 + Ca(OH)2 -> CaCO3 + H2O, which lowers the pH and leads to corrosion
   C. CO2 + Fe -> FeCO3 which protects the steel
   D. CO2 + CaCO3 -> Ca(OH)2, which raises pH
   Answer: B. Loss of alkalinity near the bar breaks the passive film. (Key point: Carbonation reaction)

2. How does carbonation depth vary with time?
   A. d increases linearly with t at constant speed
   B. d decreases with time
   C. d = K sqrt(t), advancing from the surface inwards
   D. d is independent of time
   Answer: C. The front slows as CO2 must diffuse through a thicker carbonated layer. (Key point: Carbonation depth)

3. At what condition is carbonation fastest?
   A. Completely dry concrete
   B. Fully saturated concrete
   C. Below 0 °C only
   D. Relative humidity of 50 to 70%
   Answer: D. Dry concrete lacks water for the reaction, saturated concrete blocks CO2 entry. (Key point: Humidity and carbonation)

## Related in Concrete Technology and Masonry Structure

- [Porosity, permeability, corrosion, carbonation](https://www.bctengineer.com/question.md?key=semester_5%2Fconcrete_technology_and_masonry_structure%3Aporosity-permeability-corrosion-carbonation) — Concrete Technology and Masonry Structure, chapter 6
- [Effects of carbonation and permeability on durability](https://www.bctengineer.com/question.md?key=semester_5%2Fconcrete_technology_and_masonry_structure%3Acarbonation-permeability-durability) — Concrete Technology and Masonry Structure, chapter 6
- [Creep phenomenon with time-dependent graph](https://www.bctengineer.com/question.md?key=semester_5%2Fconcrete_technology_and_masonry_structure%3Acreep-phenomenon-graph) — Concrete Technology and Masonry Structure, chapter 4, asked 2×