A concrete structure made of a concrete which had opal mineral in the aggregate
ID: 1765761 • Letter: A
Question
A concrete structure made of a concrete which had opal mineral in the aggregate showed deleterious expansion after 20 years in service. Describe the chemical reaction the concrete is undergoing. A concrete structure made of a concrete which had opal mineral in the aggregate showed deleterious expansion after 20 years in service. Describe the chemical reaction the concrete is undergoing. A concrete structure made of a concrete which had opal mineral in the aggregate showed deleterious expansion after 20 years in service. Describe the chemical reaction the concrete is undergoing. Describe the chemical reaction the concrete is undergoing.Explanation / Answer
Opal is a hydrated amorphous form of silica
The reaction can be visualized as a two-step process
Alkali + reactive silica == alkali-silica gel
Concrete consists of aggregates—stone or gravel and sand, in a matrix of cement paste. The cement paste contains interconnected microscopic pores through which water or ions in solution can migrate. The pore water in concrete is an alkaline solution; the measure of alkalinity is pH.† The alkali-silica reaction forms a gel that swells as it draws water from the surrounding cement paste. Reaction products from ASR have a great affinity for moisture. In absorbing water, these gels can induce pressure, expansion, and cracking of the aggregate and surrounding paste. The presence of gel does not necessarily indicate destructive ASR. Some gels expand very little or not at all. If a gel is low swelling, it will not create problems. High swelling gel may cause pressures exceeding the tensile strength of concrete, which results in cracking of the concrete. Rate of migration of pore fluids to the reaction site and temperature also influence swelling pressures.
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