Iron pyrites , coal , mica , shale , clay , alkali , organic impurities are some of the materials whose presence in the aggregates is viewed as harmful . These materials should not be present in such quantity which may affect the strength and durability of the concrete . They should also not be attacking the reinforcement embedded in concrete . Aggregates which are chemically reactive with alkalies of cement are harmful as they may cause cracking of concrete . Deleterious materials cause followig affects :
(i) They interfere with the hydration of cement
(ii) They affect bond between aggregates and cement paste .
(iii) They reduce the strength and durability of cement concrete .
(iv) They modify the setting action and contributes to efflorescene .
Maximum quantities of deleterious materials in the aggregates should not exceed the limits given in Table 15.4 as per IS : 383 -1963 .
As a Thumb rule , the sum of percentages of all the deleterious substances , should not exceed 5 percent
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