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Effect of clay in fine aggregate
Grading, shape and impurities a fine aggregate are key properties which need to be controlled for use in concrete. Clay contamination in sand is a natural impurity but excessive clay can affect water demand, strength and consistence. A particle size grading (PSD) will only provide the total fines content passing the 0.063µm sieve when carried out by using a wet method. It is also recognised that some clay lumps do not disperse during the wet sieving method.
The maximum passing 0.063µm in BS EN 12620 is 3%. Crushed rock fines have a higher permitted value but are unlikely to contain clay impurities.
The Methylene blue test (BS EN 933-9) provides an assessment of the quantity of clay in a sample of rock or aggregate. No limits are given in UK standards as it is not considered sufficiently precise for the purpose of determining harmful fines content, see BS EN 12620 Annex D. The French National Standard has a limit of 1%.
Acknowledgement:
The Concrete Society
