5.2 : 骨料中的有害物质

Natural sand may contain organic impurities, such as decayed vegetative matter, which interfere with the hydration of cement.

A colorimetric test is conducted to detect the presence of organic matter, which can be judged by the color of the solution containing aggregate. The darker the solution, the higher the organic matter in it.

Fine impurities, such as clay, silt, and crusher dust, obtained from the crushing of stones, coat the aggregates and result in poor bonding with the cement paste.

Additionally, the presence of fine impurities increases the surface area to be wetted and thereby requires more water for mixing, placing, and finishing. This excess water increases drying shrinkage and the likelihood of cracking.

The wet sieve method is used to determine the presence of fine impurities in coarse aggregates. 

Excess salt in fine aggregates leads to corrosion of the steel reinforcements and absorbs moisture from the air, causing efflorescence in concrete.

The presence of low-density materials, like shale, and soft inclusions in aggregates like clay lumps, coal, or wood may break down under loads, leading to pitting and popouts in concrete.