Acid Attack on Concrete

Concrete Experts International has extensive, world-wide experience with deteriorated concrete suffering from acid attack caused by acid smoke, rain and exhausting gasses. Diagnosing acid attack is an integrated part of our petrographic analysis of concrete.


Delayed Ettringite Formation

External Sulfate Attack

Carbonation of Concrete

Alkali Silica Reactions in Concrete

Freeze - Thaw

Acid Attack on Concrete

Concrete Experts International has extensive, world-wide experience with deteriorated concrete suffering from acid attack caused by acid smoke, rain and exhausting gasses. Diagnosing acid attack is an integrated part of our petrographic analysis of concrete.

What is Acid Attack?

Concrete is susceptible to acid attack because of its alkaline nature. The components of the cement paste break down during contact with acids.

Most pronounced is the dissolution of calcium hydroxide which occurs according to the following reaction:

2 HX + Ca(OH)2 -> CaX2 + 2 H2O 
(X is the negative ion of the acid)

The decomposition of the concrete depends on the porosity of the cement paste, on the concentration of the acid, the solubility of the acid calcium salts (CaX2) and on the fluid transport through the concrete. Insoluble calcium salts may precipitate in the voids and can slow down the attack. Acids such as nitric acid, hydrochloric acid and acetic acid are very aggressive as their calcium salts are readily soluble and removed from the attack front. Other acids such as phosphoric acid and humic acid are less harmful as their calcium salt, due to their low solubility, inhibit the attack by blocking the pathways within the concrete such as interconnected cracks, voids and porosity. Sulphuric acid is very damaging to concrete as it combines an acid attack and a sulfate attack.

Microscopic appearance

An acid attack is diagnosed primarily by two main features: 

  • Absence of calcium hydroxide in the cement paste
  • Surface dissolution of cement paste exposing aggregates

Please do not hesitate to contact CXI if you have some problems regarding acid attack 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.

Carbonation of Concrete

Carbonation occurs in concrete because the calcium bearing phases present are attacked by carbon dioxide of the air and converted to calcium carbonate.

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.

Contact Us

Concrete Experts International ApS

Gøngehusvej 242
DK-2950 Vedbæk

+45 2835 0660

This website uses cookies. By continuing to use this site, you accept our use of cookies.