Minimizing/Eliminating Alkali-Silica Reaction 

Mechanism
Concrete containing reactive aggregates can experience abnormal internal expansion and cracking due to alkali-silica reaction (ASR). As low alkali cements and good quality, non-reactive aggregates become more difficult to find, ASR is becoming more of a durability issue. ASR is a two-step reaction between dissolved alkalis (sodium and potassium) in concrete and reactive siliceous aggregates.

• Step 1: The chemical reaction between the reactive silica in the siliceous aggregate and the alkali present in concrete to produce an alkali-silica gel.
• Step 2: Expansion (swelling) of the alkali-silica gel when it comes in contact with moisture that creates internal pressures within the concrete. Visible symptoms of ASR include expansion-induced map cracking and the exudation of a white gel.

Prevention
Lithium compounds have been shown to be effective in mitigating ASR in concrete since the early 1950s. 

• ASRx 30LN  ASR inhibiting durability admixture is specifically formulated to inhibit and control deleterious expansion in concrete that may be susceptible to ASR because of the unavoidable use of reactive aggregates. The lithium ion in ASRx 30LN admixture interferes with the expansive mechanism of the alkali-silica gel by changing the reaction product.

Supplementary cementitious materials such as silica fume, fly ash and slag cement  may also be used in concrete to combat ASR.

• Rheomac SF 100 silica fume reduces concrete permeability and the amount of alkalis available to react with the silica in the reactive aggregates.

Articles

• 1994 ACI SP 148 "Practical Implications of Lithium-Based Chemicals and Admixtures in Controlling Alkali-Aggregate Reactions"
• Concrete 2000: Economic and Durable Construction Through Excellence, Vol. 1, pp. 1033-1042 "Chemical Admixtures for Inhibition of ASR in Concrete"