Why should aggregates used for concrete be free of certain contaminants?

Aggregates used for concrete must be free of certain contaminants because these contaminants can adversely affect the setting and overall performance of the concrete. Contaminants such as clay, silt, organic material, and salts can interfere with the bonding of the cement paste to the aggregates, which is crucial for the strength and durability of the finished concrete product.

When harmful substances are present, they can lead to weak points within the concrete matrix, leading to issues such as reduced load-bearing capacity, increased permeability, or even premature failure of the concrete. Ensuring that aggregates are clean allows for proper hydration of the cement and optimal adhesion, enhancing both the mechanical properties and longevity of the concrete structure.

In contrast, while aesthetics, weight, and moisture retention are important characteristics of concrete, they do not directly address the primary concern of setting and performance influenced by contaminants. Thus, maintaining aggregate cleanliness directly correlates to achieving a strong and durable concrete mix.