Carbonation of Concrete: Cement and Protection

Carbonation is the chemical ageing of the concrete's surface. Direct answer: airborne CO₂ slowly penetrates concrete and lowers its pH; when the front reaches the rebars, the steel loses its natural passivation and corrodes. The speed depends above all on porosity — W/C ratio, curing, compaction — and is simply measured by the phenolphthalein test.

The chemistry in one picture

Sound concrete is alkaline: its pH is around 13, and it is this basicity that maintains a protective layer on the steel — passivation. Carbon dioxide migrates through the pores, reacts with the cement hydrates (notably portlandite) to form carbonates: pH drops below 9. Bare steel, in the presence of moisture and oxygen, rusts. Rust swells, splits the cover, and the pathology self-accelerates.

What speeds up the progression

• Open porosity: high W/C, poor compaction, no curing — the trio that opens the doors.
• Medium relative humidity (50-70 %): carbonation is fastest in moist but unsaturated concrete — a permanently soaked or permanently dry concrete carbonates very slowly.
• Cracks and microcracks: local expressways for CO₂.
• Wetting-drying cycles: they pump the gas in with the absorbed water.

Measuring: the phenolphthalein test

On a freshly broken or cored face, the indicator is sprayed: sound zone turns pink, carbonated zone stays colourless. The front depth is compared to the cover: as long as the front is far from the steel, the structure sleeps; as it approaches, time to protect. Periodic measurements on the same structure give its actual speed — a few millimetres per decade for good concrete, far more for porous concrete.

Protecting yourself: defence in depth

• Compliant cover to the design — it is the CO₂ travel time that protects the steel.
• W/C ≤ 0.45 for exposed works: fewer pores, shorter paths.
• 7 days of curing: the concrete's skin is decided in its first week.
• Coatings and renders: finishing mortars and barrier paints slow CO₂ entry — render is a skin over the skin.
• Crack control: every crack is a shortcut.

Carbonation and chlorides: the two chemistries of corrosion

Carbonation destroys protection by neutralising the concrete; chlorides attack locally even in alkaline media — and in Dakhla, sea air brings its own. Both mechanisms can coexist on one coastal structure, and their defences largely overlap: density, cover, curing.

CPJ 45 and CPJ 55 (55 MPa,, recommended for demanding coastal works) give the structure its first line of defence. Free quote within 24 h, delivered in Dakhla City.