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Chemical reactions in concrete

As you learned in the concrete section, a chemical reaction occurs when concrete is formed from water, cement, and aggregate. There is another chemical reaction that may occur later in the life of the concrete, called alkali-aggregate reaction (#). In this case, the cement reacts with a chemical in the aggregate to produce a gel or some other substance. There are three types of alkali-aggregate reactions:

  • alkali-silica reaction, which produces a silica gel
  • alkali-silicate/silica reaction, very slow
  • alkali-carbonate reaction
Fontana dam experienced alkali-silica reaction which caused large cracks to form.

How does the reaction cause cracking? The reaction creates the silica gel or other product. Then the concrete has to make room for the silica gel, so the concrete has to expand, or "grow". This expansion, like expansion due to heat, causes tension in the concrete. As you saw in the concrete section, concrete is not strong in tension and will crack. Fontana dam is still "growing" because of the reactions!

Why doesn't this happen to all concrete structures then? Because it takes just the right amount of cement, chemicals in the aggregate, and moisture to make the reaction happen.

the cracked Fontana dam, North Carolina

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