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Aggregates for concrete, if containing deleterious substances, may affect strength, workability and long term performance of concrete in the long term. These substances or materials are normally referred to as deleterious materials. They are undesirable due to their weakness, softness and fineness or any other physical or chemical characteristics harmful to properties of concrete.
These substances can be grouped under three categories:
1. Impurities - these refer to organic matter which may interfere with hydration of cement. Common examples are decayed vegetables in the form of humus or organic loam which are more likely found in fine aggregates though these can be easily washed away.
2. Fine materials - such as silt and crusher dust. These may be present in the form of surface coatings. This may interfere with bonding between aggregates and cement paste. This is important as a strong bonding is critical to the strength and durability of concrete. As with impurities, these also can be washed away.
3. Unsound particles - these can be either particles failing to maintain their integrity or particles leading to disruptive expansion on freezing or exposure to water. Examples of these are low density shale, clay lumps, wood and coal. These materials lead to pitting and scaling. If found in excess quantities, the strength of concrete may be affected.