For less common types of alkali-driven degradation of concrete see also Alkali-aggregate reaction

Alkali-aggregate reaction

Alkali–aggregate reaction is a term mainly referring to a reaction which occurs over time in concrete between the highly alkaline cement paste and non-crystalline silicon dioxide, which is found in many common aggregates...

 (disambiguation page).

The alkali–silica reaction (ASR) is a reaction which occurs over time in concrete

Concrete

Concrete is a composite construction material, composed of cement and other cementitious materials such as fly ash and slag cement, aggregate , water and chemical admixtures.The word concrete comes from the Latin word...

 between the highly alkaline cement

Portland cement

Portland cement is the most common type of cement in general use around the world because it is a basic ingredient of concrete, mortar, stucco and most non-specialty grout...

 paste and reactive non-crystalline (amorphous) silica, which is found in many common aggregate

Construction Aggregate

Construction aggregate, or simply "aggregate", is a broad category of coarse particulate material used in construction, including sand, gravel, crushed stone, slag, recycled concrete and geosynthetic aggregates. Aggregates are the most mined material in the world...

s.

The ASR reaction is the same as the Pozzolanic reaction

Pozzolanic reaction

The Pozzolanic reaction is the chemical reaction that occurs in hydraulic cement, a mixture of slaked lime with amorphous siliceous materials , forming non-water-soluble calcium silicate hydrates...

 which is a simple acid-base reaction between calcium hydroxide

Calcium hydroxide

Calcium hydroxide, traditionally called slaked lime, is an inorganic compound with the chemical formula Ca2. It is a colourless crystal or white powder and is obtained when calcium oxide is mixed, or "slaked" with water. It has many names including hydrated lime, builders lime, slack lime, cal, or...

, also known as Portlandite

Portlandite

Portlandite is a rare oxide mineral, the naturally occurring form of calcium hydroxide . It is the calcium analogue of brucite .-Occurrence:...

, or (Ca(OH)₂), and silicic acid

Silicic acid

Silicic acid is a general name for a family of chemical compounds of the element silicon, hydrogen, and oxygen, with the general formula [SiOx4-2x]n...

 (H₄SiO₄, or Si(OH)₄). For the sake of simplicity, this reaction can be schematically represented as following:

Ca(OH)₂ + H₄SiO₄ → Ca²⁺ + H₂SiO₄²⁻ + 2 H₂O → CaH₂SiO₄ · 2 H₂O

This reaction causes the expansion of the altered aggregate by the formation of a swelling gel

Gel

A gel is a solid, jelly-like material that can have properties ranging from soft and weak to hard and tough. Gels are defined as a substantially dilute cross-linked system, which exhibits no flow when in the steady-state...

 of Calcium Silicate Hydrate

Calcium silicate hydrate

Calcium Silicate Hydrate is the main product of the hydration of Portland cement and is primarily responsible for the strength in cement based materials.-Preparation:...

 (CSH). This gel increases in volume with water and exerts an expansive pressure inside the material, causing spalling and loss of strength of the concrete, finally leading to its failure.

So, ASR can cause serious expansion and cracking in concrete, resulting in critical structural problems that can even force the demolition of a particular structure.

The mechanism of ASR causing the deterioration of concrete can be described in four steps as follows:

1. The alkaline solution attacks the siliceous aggregate to convert it to viscous alkali silicate gel.
2. Consumption of alkali by the reaction induces the dissolution of Ca²⁺ ions into the cement pore water. Calcium ions then react with the gel to convert it to hard calcium silicate hydrate
   Calcium silicate hydrate
   Calcium Silicate Hydrate is the main product of the hydration of Portland cement and is primarily responsible for the strength in cement based materials.-Preparation:...
   
   .
3. The penetrated alkaline solution converts the remaining siliceous minerals into bulky alkali silicate gel. The resultant expansive pressure is stored in the aggregate.
4. The accumulated pressure cracks the aggregate and the surrounding cement paste when the pressure exceeds the tolerance of the aggregate.

Mitigation

ASR can be mitigated in concrete by three complementary approaches:

1. Limit the alkali metal content of the cement. Many standards impose limits on the "Equivalent Na₂O" content of cement.
2. Limit the reactive silica content of the aggregate. Certain volcanic rocks are particularly susceptible to ASR because they contain volcanic glass (obsidian
   Obsidian
   Obsidian is a naturally occurring volcanic glass formed as an extrusive igneous rock.It is produced when felsic lava extruded from a volcano cools rapidly with minimum crystal growth...
   
   ) and should not be used as aggregate. The use of calcium carbonate aggregates is sometimes envisaged as an ultimate solution to avoid any problem. However, while it may be considered as a necessary condition, it is not a sufficient one. In principle, limestone (CaCO₃) is not expected to contain high level of silica, but it actually depends on its purity. Indeed, some siliceous limestones (a.o., Kieselkalk
   Kieselkalk
   Kieselkalk is also known as the Helvetic Siliceous Limestone. It has sedimented during the Lower Cretaceous epoch. It can contain up to 40 % of very fine , evenly distributed authigenic quartz crystals...
   
   found in Switzerland) may be cemented by amorphous or poorly crystalline silica and can be very sensitive to the ASR reaction as observed with some siliceous limestones exploited in quarries in the area of Tournai
   Tournai
   Tournai is a Walloon city and municipality of Belgium located 85 kilometres southwest of Brussels, on the river Scheldt, in the province of Hainaut....
   
    in Belgium. So, the use of limestone as aggregate is not a guarantee against ASR in itself. The silica content of the limestone and its reactivity must remain below a threshold value that has to be carefully experimentally assessed by the aggregate producer.
3. Add very fine siliceous materials to neutralize the excessive alkalinity
   Alkalinity
   Alkalinity or AT measures the ability of a solution to neutralize acids to the equivalence point of carbonate or bicarbonate. The alkalinity is equal to the stoichiometric sum of the bases in solution...
   
    of cement with silicic acid
   Silicic acid
   Silicic acid is a general name for a family of chemical compounds of the element silicon, hydrogen, and oxygen, with the general formula [SiOx4-2x]n...
   
    by voluntary provoking a controlled pozzolanic reaction
   Pozzolanic reaction
   The Pozzolanic reaction is the chemical reaction that occurs in hydraulic cement, a mixture of slaked lime with amorphous siliceous materials , forming non-water-soluble calcium silicate hydrates...
   
    at the early stage of the cement setting. Convenient pozzolanic materials to add to the mix may be, e.g., pozzolan
   Pozzolan
   A pozzolan is a material which, when combined with calcium hydroxide, exhibits cementitious properties. Pozzolans are commonly used as an addition to Portland cement concrete mixtures to increase the long-term strength and other material properties of Portland cement concrete, and in some cases...
   
   , silica fume
   Silica fume
   Silica fume, also known as microsilica, is a fine-grain, thin, and very high surface area silica.It is sometimes confused with fumed silica and colloidal silica...
   
   , fly ash
   Fly ash
   Fly ash is one of the residues generated in combustion, and comprises the fine particles that rise with the flue gases. Ash which does not rise is termed bottom ash. In an industrial context, fly ash usually refers to ash produced during combustion of coal...
   
   s, or metakaolin
   Metakaolin
   Metakaolin is a dehydroxylated form of the clay mineral kaolinite.Rocks that are rich in kaolinite are known as china clay or kaolin, traditionally used in the manufacture of porcelain. The particle size of metakaolin is smaller than cement particles, but not as fine as silica fume.-Forming...
   
   . These react preferentially with the cement alkalis without formation of an expansive pressure, because siliceous minerals in fine particles convert to alkali silicate and then to calcium silicate without formation of semipermeable reaction rims.

In other words, as it is sometimes possible to fight the fire by the fire, it is also feasible to combat the ASR reaction by itself. A prompt reaction initiated at the early stage of concrete hardening on very fine silica particles will help to suppress a slow and delayed reaction with large siliceous aggregates on the long term. Following the same principle, the fabrication of low-pH cement also implies the addition of finely divided pozzolanic materials rich in silicic acid to the concrete mix to decrease its alkalinity.

ASR test

The concrete microbar test was proposed by Grattan-Bellew et al. (2003) as a universal accelerated test for alkali-aggregate reaction.

See also

• Alkali–aggregate reaction
• Alkali–carbonate reaction
• Calcium silicate hydrate
  Calcium silicate hydrate
  Calcium Silicate Hydrate is the main product of the hydration of Portland cement and is primarily responsible for the strength in cement based materials.-Preparation:...
  
  
• Carbonatation
  Carbonatation
  Carbonatation is a chemical reaction in which calcium hydroxide reacts with carbon dioxide and forms insoluble calcium carbonate:The process of forming a carbonate is sometimes referred to as "carbonation", although this term usually refers to the process of dissolving carbon dioxide in...
  
  
• Pozzolanic reaction
  Pozzolanic reaction
  The Pozzolanic reaction is the chemical reaction that occurs in hydraulic cement, a mixture of slaked lime with amorphous siliceous materials , forming non-water-soluble calcium silicate hydrates...