The alkalis of greatest importance are mainly sodium and potassium. They are present in the pore solution primarily as dissolved alkali hydroxides and are also supplied by cement and, in some cases, other constituents of the concrete.
AAR is a durability problem because the reaction products may absorb water and expand, generating internal stresses within the hardened concrete.
1. Types of Alkali–Aggregate Reactions
Alkali–aggregate reactions are generally divided into two major types:
- Alkali–silica reaction (ASR)
- Alkali–carbonate reaction (ACR)
| Type | Reactive Component | Main Reaction Product/Effect |
|---|---|---|
| Alkali–silica reaction (ASR) | Reactive forms of silica in aggregate | Alkali–silica gel and associated expansion when sufficient moisture is available |
| Alkali–carbonate reaction (ACR) | Certain susceptible dolomitic carbonate aggregates | Aggregate-related reactions and expansion; mechanism differs from ASR |
2. Alkali–Silica Reaction (ASR)
ASR is the more widely recognized form of AAR. It occurs when alkalis in the highly alkaline pore solution react with certain forms of reactive silica present in aggregate particles.
2.1 Reactive Forms of Silica
Not all silica-containing aggregates are reactive. ASR is associated particularly with poorly crystalline, strained or amorphous forms of silica.
Examples of potentially reactive materials include:
- Opal and opaline materials
- Chalcedony
- Volcanic glass
- Some forms of microcrystalline quartz
- Cryptocrystalline silica
- Some strained or highly defective quartz-containing rocks
3. Mechanism of Alkali–Silica Reaction
Step 1: Formation of an Alkaline Pore Solution
Hydration of Portland cement produces a highly alkaline pore solution. Sodium and potassium ions are present in this solution along with hydroxide ions.
Step 2: Attack on Reactive Silica
Hydroxyl ions attack susceptible silica structures. This breaks or weakens Si–O–Si bonds at reactive sites and promotes dissolution of silica.
Step 3: Formation of Alkali–Silicate Reaction Product
The dissolved silicate species interact with alkali ions such as Na+ and K+. This produces an alkali-containing silicate reaction product, commonly referred to as alkali–silica gel.
Step 4: Water Uptake and Expansion
The reaction product can take up water. Under conditions where the reaction product develops sufficient swelling pressure, it can generate tensile stresses in the surrounding cement paste and aggregate.
Once cracking develops, additional moisture can enter the concrete more easily, potentially allowing the reaction to continue.
4. Conditions Required for ASR
Three conditions are particularly important for significant ASR-related expansion:
- Reactive aggregate: The concrete must contain an aggregate capable of participating in the reaction.
- Sufficient alkali availability: The pore solution must contain sufficient alkali and hydroxyl ions.
- Sufficient moisture: Adequate internal moisture is generally necessary for significant expansion.
5. Alkali–Carbonate Reaction (ACR)
ACR is a less common form of alkali–aggregate reaction associated mainly with certain susceptible dolomitic carbonate rocks.
Its mechanism is different from the conventional ASR mechanism. It is associated with reactions involving the carbonate aggregate, alkalis and the surrounding cementitious system, and may produce expansion and cracking.
5.1 Mechanism of ACR
Certain dolomitic aggregates contain a characteristic microstructure involving dolomite crystals and a fine-grained carbonate matrix. In susceptible aggregates, interaction with the alkaline environment can cause alteration of the aggregate and associated volume changes.
A commonly discussed reaction is the dedolomitization reaction, in which dolomite is converted into calcite and magnesium-containing phases.
where M represents an alkali metal such as Na or K. For example, when M = Na:
The overall chemistry in real concrete is more complex than this simplified representation. Expansion may be associated with changes in the aggregate microstructure and moisture-related processes.
6. Symptoms of Alkali–Aggregate Reaction
The following signs may indicate AAR in concrete, although they are not by themselves sufficient to establish the diagnosis:
- Map or pattern cracking on concrete surfaces.
- Expansion and distortion of concrete elements.
- Cracks that may become wider with time.
- Local surface discoloration or deposits associated with cracking.
- Joint closure or movement caused by expansion.
- Loss of durability due to increased cracking and moisture penetration.
- In severe cases, reduction in serviceability and deterioration of the concrete.
7. Factors Affecting Alkali–Aggregate Reaction
7.1 Aggregate Reactivity
The mineralogical composition, particle size, texture and degree of crystallinity of the aggregate strongly influence its reactivity.
7.2 Alkali Content
Greater availability of sodium and potassium in the pore solution can increase the potential for alkali-related reactions, although total cement alkali content alone does not determine field performance.
7.3 Moisture
Moisture is an important factor in ASR because water participates in the reaction process and enables swelling of the reaction product.
7.4 Temperature
Higher temperature generally accelerates chemical reactions. Temperature therefore influences the rate of alkali–aggregate reactions, although actual expansion depends on the complete concrete system.
7.5 Concrete Permeability and Cracking
A permeable or cracked concrete can allow easier movement of moisture and dissolved species. This may influence the development and distribution of AAR-related deterioration.
8. Prevention and Control of AAR
The most effective approach is to prevent the combination of susceptible aggregate and environmental conditions that allow damaging expansion.
8.1 Use Non-Reactive or Proven Aggregates
Where possible, aggregates with demonstrated satisfactory performance should be selected. Aggregate testing and petrographic examination can help identify potentially reactive materials.
8.2 Use Suitable Supplementary Cementitious Materials
Materials such as fly ash, GGBS and silica fume can reduce ASR expansion in suitable mixtures. They modify the cementitious system and can reduce the availability or mobility of alkalis and hydroxide while refining the pore structure.
8.3 Control Alkali Availability
The use of suitable cementitious materials and appropriate mixture proportions can help control the alkali environment within concrete.
8.4 Control Moisture Exposure
Where practical, limiting moisture ingress can reduce the conditions required for significant ASR-related expansion. However, moisture control is often difficult for structures exposed to the environment.
8.5 Use Appropriate Concrete Mixture Design
A properly designed concrete with appropriate cementitious materials, aggregate selection, water-to-cementitious-material ratio and curing can reduce the risk of durability problems.
9. ASR vs ACR
| Feature | Alkali–Silica Reaction (ASR) | Alkali–Carbonate Reaction (ACR) |
|---|---|---|
| Reactive aggregate | Certain forms of reactive silica | Certain susceptible dolomitic carbonate rocks |
| Main chemical interaction | Alkaline pore solution with reactive silica | Alkaline environment with susceptible carbonate aggregate |
| Characteristic process | Dissolution of reactive silica and formation of alkali–silicate reaction products | Aggregate alteration, including dedolomitization in susceptible materials |
| Role of moisture | Very important for expansion | Important to the development of deterioration |
| Main deterioration | Expansion, cracking and loss of durability | Expansion and cracking associated with susceptible carbonate aggregate |
| Frequency | More widely encountered | Less common |
10. Key Points for Examination
- AAR is a chemical reaction between alkalis in concrete pore solution and susceptible aggregate minerals.
- The two major forms are ASR and ACR.
- ASR involves reactive forms of silica.
- ASR can produce an alkali–silicate reaction product that can absorb water and contribute to expansion.
- ACR is associated mainly with certain susceptible dolomitic carbonate aggregates.
- Moisture, alkali availability and aggregate reactivity are important factors in ASR.
- Reactive aggregate testing and suitable mixture design are important preventive measures.
- Supplementary cementitious materials can be effective in reducing ASR expansion in appropriately designed mixtures.
Concrete Technology: National Institute of Technology, Rourkela