Alkali-silica reaction in cement: causes and mitigation strategies

The alkali-silica reaction (ASR) is a significant durability issue in concrete structures, resulting in internal cracking, which compromises structural integrity. ASR is caused by a chemical reaction between the alkali hydroxides (NaOH and KOH) present in cement and reactive silica in the aggregates used. This reaction forms an expansive gel that absorbs moisture, leading to internal stresses and cracking of the concrete.

The main factors contributing to ASR include:

• Alkali content: high levels of alkalis, usually sodium and potassium oxides present in the cement, increase the pH of the pore solution in the concrete. This high alkalinity facilitates the dissolution of silica from aggregates.

• Reactive aggregates: certain aggregates, such as those containing poorly crystalline or amorphous silica, are highly reactive in alkaline conditions. Examples of such aggregates include opal and chalcedony, which are much more reactive than crystalline forms like quartz.

• Moisture availability: water is essential for ASR, as the alkali-silica gel absorbs water and swells, leading to cracking. ASR is particularly problematic in structures exposed to humid environments or direct water ingress.

Effects of ASR
The expansion of the ASR gel exerts internal stresses within the concrete matrix, leading to cracking. This process significantly weakens the concrete, making it more porous and allowing further ingress of water and harmful agents, such as chlorides, which can accelerate the corrosion of steel reinforcements. Over time, this deterioration compromises the load-bearing capacity of the structure.

Mitigation Strategies
Mitigating ASR involves addressing one or more of the contributing factors through various strategies:

• Low-alkali cement: reducing the alkali content in cement, typically below 0.60% Na₂O equivalent, can limit the potential for ASR. This is one of the most direct methods for preventing ASR in new concrete.

• Use of Supplementary Cementitious Materials (SCMs): SCMs, such as fly ash, ground granulated blast-furnace slag (GGBFS), and silica fume, can reduce the alkali concentration in the pore solution and lower the pH, making it less conducive to silica dissolution. SCMs rich in aluminum, like metakaolin and fly ash, are particularly effective as they slow down the dissolution of reactive.

• Use of non-reactive aggregates: the most straightforward method of preventing ASR is the use of aggregates that are less reactive. Aggregates should be tested for reactivity before being used in concrete mixtures.

• Lithium-based admixtures: lithium salts, such as lithium nitrate, can mitigate ASR by stabilizing the silica gel and preventing it from expanding. These admixtures can be used in both the new concrete and the existing structures that are already affected by ASR.

• Moisture control: reducing the availability of water to the concrete can limit the expansion of the ASR gel. This can be achieved by using sealants, waterproof membranes, and ensuring good drainage systems around structures to prevent water ingress.

Conclusion
ASR poses a significant risk to the long-term durability of concrete structures, especially those exposed to moist environments. By understanding the causes of ASR and implementing effective mitigation strategies—such as using low-alkali cement, SCMs, non-reactive aggregates, and lithium-based admixtures—the detrimental effects of ASR can be controlled. Continued research and advancements in testing and modelling will further aid in preventing ASR-related damage in concrete structures.