Deleterious substances in aggregates can be detrimental to the quality and durability of concrete. These substances include organic impurities like loam, which interfere with cement hydration and are usually present in the sand. These prevent a good bond between aggregate and cement paste. Organic impurities can be detected using the colorimetric test, where the darkness of a solution after agitation indicates the level of organic content.

Another type of impurity is clay and fine material that may coat aggregate particles and weaken the bond with cement paste. The presence of clay and silt content in the aggregate is often assessed using sedimentation or wet-sieve methods. Salts from marine environments are also considered deleterious substances, particularly if present above 6% by mass, leading to the corrosion of reinforcements embedded within the concrete. Washing aggregates with fresh water is recommended to reduce salt content.

Lastly, aggregates containing particles such as shale, coal, or mica that are non-durable or absorb water can lead to concrete degradation. American Society for Testing and Materials (ASTM) C 33-03 sets permissible levels for these unsound particles in aggregate. Proper testing and adherence to standards are crucial to ensure the quality of aggregates used in construction, thus safeguarding the integrity and longevity of concrete structures.