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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.
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).
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.