Why estimation of organic impurities in aggregates is important?

The presence of organic compounds in natural sand in sufficient quantities to be harmful, and hence is intended to show whether further tests are necessary or desirable.

Harmless organic materials may cause colouration and certain naturally occurring organic compounds do not cause colouration.

Estimation of Organic Impurities in Aggregates as per IS: 2386 Part-2 (1963)

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The test gives an approximate value of organic impurities in aggregates.

Example of Organic Impurities in Aggregates:-

Organic substances in fine aggregate or sand: If sand is extracted from the river bed; it may contain humus, organic soil, decayed vegetation, decayed animal material, etc. These organic materials interfere with the cement setting and ultimately affect the strength of concrete. It shows that the presentence of organic materials excess of a limit in aggregates may be harmful.

IS Code for Estimation of Organic Impurities in Aggregates:-

IS: 2386 Part-2 (1963); Estimation of deleterious materials and organic impurities in aggregate.

Apparatus and Solutions Required for Estimation of Organic Impurities in Aggregates:-

• A 350-ml graduated clear glass bottle
• Stopper
• 3 percent solution of sodium hydroxide
• 2 percent solution of tannic acid
• 10 percent alcohol

Procedure for Estimation of Organic Impurities in Aggregates:-

Part-A: Testing the aggregate specimen (sand).

1. The sand (fine aggregate) shall be tested as delivered and without drying.
2. A 350-ml graduated clear glass medicine bottle shall be filled to the 75-ml mark with 3 percent solution of sodium hydroxide in water.
3. The sand shall be added gradually until the volume measured by the sand layer is 125 ml.
4. The volume shall then be made up to 200 ml by adding more solution.
5. The bottle shall be stoppered and shaken vigorously and then allowed to stand for 24 hours.

Part-B: Preparation of standard solution.

1. Add 2·5 ml of 2 percent solution of tannic acid in 10 percent alcohol, to 97∙5 ml of a 3 percent sodium hydroxide solution.
2. Place in a 350-ml bottle, stopper, shake vigorously and allow standing for 24 hours before comparison with the solution above the sand.
3. Alternatively, an instrument or coloured acetate sheets for making the comparison can be obtained, but it is desirable that these should be verified on receipt by comparison with the standard solution.

Result and Report:-

Other tests shall be made if the colour of the liquid from Part-A is darker than the standard solution freshly prepared from Part-B.

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