The colour standard is designed for quick and accurate tests of organic matter in sand and aggregates for concrete, according to ASTM C-40. Organic impurities, usually in the form of tannic acid and its derivatives, are typically present in fine aggregates such as sand. These may interfere with the chemical reactions of hydration and may affect the strength of the cement, mortar or concrete where the aggregate is being used.
The results given by the colour comparison are designed to serve as a warning that unacceptable levels of organic impurities may be present.
In the test, a measured sample of fine aggregate is shaken with sodium hydroxide solution in a stoppered bottle and left to stand for 24 hours. The liquid above the test sample develops a colour, which is compared directly with coloured glass standards to indicate the level of organic impurities present.