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Reactive Silica Supply

The primary source for reactive silica necessary for ASR to occur comes from the aggregates used to make the concrete.  There is a long list of rocks and minerals that exhibit ASR tendencies due to their reactive silica phase. The table below lists some of them.

Reactive Minerals
Volcanic glass
Cryptocrysalline (or microcrystalline) quartz
Strained quartz

Figure A2c.T1. Reactive Rocks and Minerals. 

Limestone is predominantly composed of the mineral calcite.  The chemical composition of calcite is calcium carbonate (CaCO3).  Calcite is chemically inert in concrete and pure limestone is not reactive. However, limestone as a rock type may contain other minor minerals in addition to calcite. For example, Spratt limestone contains about 9% silica (SiO2) some of which is present as a highly disordered opaline material. The presence of the opaline material renders Spratt rock a highly alkali-silica reactive aggregate (not alkali-carbonate reactive).

Not all minerals that contain silica react to a significant degree in concrete. In fact, most siliceous aggregates do NOT cause deleterious reaction. What makes silica reactive is the nature of its molecules. The mechanism of ASR requires OH- molecules to "attack" the silica phase of an aggregate. This is more likely to occur successfully if the silica is in an amorphous phase. There is more opportunity for the attacking OH- ion to penetrate an amorphous phase and break free the silica ion that can then combine with the alkalis, Na+ and K+ that are also able to move more freely through the disordered structure.  A more rigid crystalline structure does not provide the same opportunity for OH-, Na+, or K+ ions to maneuver between and attack. Therefore, aggregates that have silica phases of more rigid crystalline structure do not tend to exhibit ASR symptoms.

Figure A2c.F1. Illustration. Silica Crystals. This illustration is a schematic showing difference in crystal structure of quartz, on the left, and opal, on the right. The quartz crystal (on the left) shows a well-organized molecule structure of oxygen and silicone molecules. Two hydroxide and sodium molecules are shown on top of the structure. The opal structure, on the right, shows a more disordered structure of silicone, oxygen, hydroxide, and sodium molecules. At the bottom of the illustration are explanatory shapes indicating silicon, oxygen, hydroxide, and sodium or potassium.

Figure A2c.F1. Silica Crystals. 

The  pore solution pH influences the solubility of silica.  The higher the pH (the more OH-), the more likely silica will dissolve.

Figure A2c.F2. Illustration. The Effect of pH on Silica Solubility. This illustration graphically shows the effect of increasing pH values on silica's ability to dissolve.
Figure A2c.F2. The Effect of pH on Silica Solubility.

Updated: 02/20/2015

United States Department of Transportation - Federal Highway Administration