In the last post we have discussed about ASTM specification for organic impurities in fine aggregates of concrete. There we ignored source and impact of such impurities in aggregates. Now we will learn these facts, notice that we are always discussing about fine aggregates. This is due to washability of coarse aggregates which is not almost available in case of fine aggregates.
At first, we will learn about sources of organic content in fine aggregate. Most common fine aggregates is sand which is of two types
We are concerned about natural sand as manufactured sand is a relatively controlled product. We have provided some information about organic sources in glass powder sand in previous post.
The organic particles in fine aggregates are occurred due to decomposition of vegetable residue in sand (or other fine aggregates). In some regions tannic acid is found left by root of pine trees. This acid deposited on fine particles cannot be washed. Other organic acids that can impair concrete properties are:
a. Acetic acid
b. Citric acid
c. Formic acid
d. Humic acid
e. Lactic acid
Such organic matter found in aggregate as organic loam or humus.
Organic content is always suspected as harmful. Some type of organic content even as small amount in fine aggregate can retard or even prevent hardening of concrete and can produce severe impact on concrete reducing its strength.