Characterization of the ASR rim: Application to the Potsdam sandstone

Chemical properties of the reaction rim associated with alkali-silica reaction (ASR) were investigated using microprobe and scanning electron microscope (SEM). The studied aggregate is the Potsdam sandstone, a Cambrian siliceous sandstone well known for its reactivity. This particular rock is composed of well-crystallized quartz grains surrounded by a poorly crystallized siliceous cement that is considered to be the reactive constituent. Research was conducted on laboratory concrete specimens having reached various expansion levels and on some samples taken from an ASR-affected dam. Results indicate that the dark rim surrounding reactive particles is mainly composed of silica. This suggests that the reaction rim is formed by the precipitation of dissolved silica. Some alkalis and calcium were detected inside the thin intergranular joints in concentration ranging from 1% to 10%. These ions come from the cement paste and play a major role in dissolving original reactive silica.