Final answer:
The expansive reaction in question is known as sulfate attack, which occurs in concrete when sulfate ions react with calcium hydroxide in the hardened cement paste, often leading to damage through swelling and cracking. It exemplifies a precipitation reaction, where the new products can cause the paste to expand. The rate and severity of these decompositions are significantly influenced by the concentration of reactive pollutants such as sulfur dioxide in the environment.
Step-by-step explanation:
The expansive reaction that occurs when the cement matrix of concrete or mortar comes in contact with sulfates dissolving in groundwater or in soil is known as sulfate attack. This chemical reaction can cause significant damage to concrete structures. The sulfate ions react with compounds in the hardened cement paste, such as calcium hydroxide, to form products, like gypsum, which can cause the paste to swell and lead to cracking, disruption, or loss of bond between the cement paste and the aggregates within the concrete.
Such reactions are a form of a precipitation reaction, where the resultant products are often insoluble and form as precipitates. Sulfate attack is an important consideration in the construction industry, particularly when laying foundations in sulfate-rich soils or using water from sources high in sulfate content for making concrete.
Reactions with Sulfurous Acid
Similarly, an environmental occurrence involving a precipitation reaction is observed when calcium carbonate (CaCO3) deteriorates as a result of its reaction with sulfur dioxide to form sulfurous acid, demonstrating how reaction rates vary with reactant concentration. The deterioration is more severe in areas with higher concentrations of sulfur dioxide due to air pollution.