Carbonation of Concrete

Concrete Experts International has extensive, world-wide experience with concrete petrography. Carbonation is an integrated part of our petrographic analysis of concrete.

Carbonation of Concrete

Concrete Experts International has extensive, world-wide experience with concrete petrography. Carbonation is an integrated part of our petrographic analysis of concrete.

What is Carbonation?

Carbonation occurs in concrete because the calcium bearing phases present are attacked by carbon dioxide of the air and converted to calcium carbonate. Cement paste contains 25-50 wt% calcium hydroxide (Ca(OH)2), which mean that the pH of the fresh cement paste is at least 12.5. The pH of a fully carbonated paste is about 7.

The concrete will carbonate if CO2 from air or from water enters the concrete according to:

             Ca(OH)2 + CO2        ->       CaCO3 + H2O

When Ca(OH)2 is removed from the paste hydrated CSH will liberate CaO which will also carbonate. The rate of carbonation depends on porosity & moisture content of the concrete.

The carbonation process requires the presence of water because CO2 dissolves in water forming H2CO3. If the concrete is too dry (RH <40%) CO2 cannot dissolve and no carbonation occurs. If on the other hand it is too wet (RH >90%) CO2 cannot enter the concrete and the concrete will not carbonate. Optimal conditions for carbonation occur at a RH of 50% (range 40-90%).

Normal carbonation results in a decrease of the porosity making the carbonated paste stronger. Carbonation is therefore an advantage in non-reinforced concrete. However, it is a disadvantage in reinforced concrete, as pH of carbonated concrete drops to about 7; a value below the passivation threshold of steel.

How do you recognize carbonation?

Carbonation may be recognized in the field by the presence of a discolored zone in the surface of the concrete. The color may vary from light gray and difficult to recognize to strong orange and easy to recognize. Carbonation can be visualized by using Thymolphthalein.

In the optical microscope carbonation is recognized by the presence of calcite crystals and the absence of calcium hydroxide, ettringite and un-hydrated cement grains. Porosity is unchanged or lower in the carbonated zone.

Bi-carbonation – what is that?

Occasionally concrete may suffer from the so called bi-carbonation process. Bi-carbonation may occur in concrete with very high water to cement ratio due to formation of hydrogen carbonate ions at pH lower than 10. Contrary to normal carbonation, bi-carbonation results in an increase in porosity making the concrete soft and friable. Bi-carbonation may be recognized by the presence of large  “pop-corn” like calcite crystals and the highly porous paste.

Please do not hesitate to contact CXI if you have any questions regarding carbonation or any other deterioration mechanisms.

Terms and definitions

Using the same terms makes communication better! We have made this small dictionary which describes some of the terms used in concrete examination and description.

Terms and Definitions

The following small dictionary describes some of the terms used in concrete examination and description.

Agglomerate: 3 or more air voids in close contact

Aggregate: Component of the concrete consisting of a coarse aggregate fraction (more than 4 mm) and a fine aggregate fraction (less than 4 mm)

Alite: Tricalciumsilicate, C3S (3CaO-SiO2), a natural occurring mineral present in ordinary Portland cement.

Alkali silica reaction: Reaction between silica in the aggregates (e.g. chert, flint, micro-crystalline quartz), alkali (from cement, de-icing salt, sea water), calcium hydroxide and water. Alkali silica reaction causes aggregates to expand and cracks are formed radiating out into the ambient cement paste.

Alkali silica gel: A product of alkali silica reaction. Gel is usually clear, transparent and non-crystalline (amorphous), although in some cases it may crystallize. It expands during water absorption. Gel is usually found lining or filling cracks and air voids.

Aluminate phase: Tricalciumaluminate, C3A (3CaO-Al2O3)

Anhydrate: CaSO4

Belite: Dicalciumsilicate, C2S (2CaO-SiO2), natural occurring mineral present in ordinary Portland cement.

Bleeding: Water separation in the concrete. "Bleeding" is the phenomenon where water migrates towards the surface of the concrete and collects on the surface or underneath coarse aggregates. Seen as narrow voids along the interface between aggregate and cement paste. Calcium hydroxide may be deposited in the voids.

Calcium carbonate: CaCO3 , calcite, found as deposits in voids and cracks and on the surface.

Calcium hydroxide: Ca(OH)2, Portlandite, hydration product of C3S and C2S. Appears throughout the non-carbonated cement paste and occasionally as larger crystals in voids and cracks

Carbonation: Transformation of the calcium containing constituents of the concrete by reaction with the carbon dioxide in air. Calcium hydroxide is transformed to calcium carbonate and consequently the paste becomes chemically neutral.

Cement: The visible cement minerals are: C3S, C2S and C4AF. C3A is not visible.

Cement paste: Compound of hydration products from the cement-water reaction and unhydrated cement. Micro silica (MS) and fly ash (FA) is usually treated as part of the paste.

Cracks: Are divided in 3 groups in relation their width: coarse cracks are cracks wider than 0.1 mm, fine cracks varies from 0.01 - 0.1 mm and micro cracks are less than 0.01 mm. Cracks usually run in the cement paste; however, occasionally the aggregates are traversed. Cracks form either in the early (plastic stage) or at a later stage (fully hardened stage) in the life of the concrete .

Delayed Ettringite Formation (DEF): Hardened concrete that has been subjected to high-temperature heat curing or high internal hydration heat temperature may suffer from expansion and cracking during subsequent exposure to moisture. DEF is diagnosed by the presence of gaps around aggregate particles.

Entrained air: Usually defined as the small and medium sized spherical air voids with maximum size 0.5 - 1 mm

Entrapped air: Comprises irregular and angular air voids of all sizes

Ettringite: 3CaO.Al2O3.3CaSO4.32H2O. Needle shaped crystals of calcium sulfo-aluminate hydrate produced by constituents in the cement paste, including gypsum. It is also produced by sulfate attack on the concrete.

Ferrite phase: Tetracalciumaluminateferrite, C4AF (4CaO-Al2O3-Fe2O3)

Filled voids: Air voids with complete or partial filling of e.g. alkali silica gel, ettringite, calcium hydroxide, calcium carbonate or gypsum.

Friedel Salt (mono-chloride): 3CaO.Al2O3.CaCl2.10H2O

Gaps: Cracks observed all the way around aggregate particles in the interface zone between aggregate and cement paste. Gaps can be empty, or be partly or completely filled with ettringite. The width of the gaps is positively correlated with the size of the aggregate. Gaps are evidence of some sort of paste expansion occurring from delayed ettringite formation (DEF), freeze/thaw and sulfate attack.

Gypsum: CaSO4.2H2O, calcium sulfate which is found as a common constituent of cement. It is also produced by acid attack (sulfuric acid) on concrete.

Hydration: Reaction between cement and water, producing cement gel (cement paste, cement glue) serving as an adhesive in the concrete.

Mono-sulfate: 3CaO.Al2O3.CaSO4.12H2O

Plastic shrinkage: Shrinkage caused by strong drying of newly cast concrete. The result is often plastic shrinkage cracks in the concrete surface.

Plastic settlement: Settlement of fresh concrete, often causing plastic settlement cracks.

Re-crystallization: Dissolution of crystals and re-deposition either of the same mineral in a different form or of new minerals, mostly stabilized due to the presence of water.

Segregation: "Segregation" is the phenomenon where the concrete has no internal adhesion, and the mortar (therefore separates from the coarse aggregates.

Thaumasite: CaSiO3.CaSO4.CaCO3. A mineral not naturally present in concrete. Thaumasite may occur in concrete suffering from sulfate attack. The presence of thaumasite depends according to literature on temperature and presence of carbon dioxide.

w/c ratio: The ratio of water to cement by weight of the cement paste. If the cement paste contains fly ash (FA) or micro silica (MS) the addition of these materials can be taken into account using an activity factor and an equivalent w/c ratio can be calculated.

Other Deterioration Mechanisms

Alkali Silica Reactions in Concrete

Alkali silica reaction is a heterogeneous chemical reaction which takes place in aggregate particles between the alkaline pore solution of the cement paste and silica in the aggregate particles.

Delayed Ettringite Formation in Concrete

Generally DEF is seen as a form of internal sulfate attack.

External Sulfate Attack

External sulfate attack is a chemical breakdown mechanism where sulfate ions from an external source attack components of the cement paste.

Acid Attack on Concrete

Concrete is susceptible to acid attack because of its alkaline nature.

Freeze - Thaw Deterioration of Concrete

Deterioration of concrete from freeze thaw actions may occur when the concrete is critically saturated, which is when approximately 91% of its pores are filled with water.

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