Road salt is used in many municipalities to remove ice from streets in order to make them safer for driving. This type of salt is usually ground into small bits and sprayed from the back of trucks; it assists both in gritting roads (providing more traction) and in lowering the melting temperature of the ice it comes in contact with, making it easier to break up and remove with plows.
Chemical Composition
Most rock salt used for road work is made up primarily of sodium chloride (NaCl) with some additional impurities to make it hydrophilic. The primary impurities are made of soluble sulfates, which is one of the reasons why this is considered an inedible salt; the sulfates make the runoff taste bad. There is an additional anti-caking agent (which varies somewhat by manufacturer) to keep the rock salt easily spreadable after prolonged periods of storage. All rock salt deicers work by absorbing water and making a brine, which reduces the melting point of the ice underneath it; this generally requires low-moisture rock salt.
Mechanical Properties
Rock salt is ground to roughly a quarter inch (6 mm) particle size as part of its manufacturing process; this allows it to provide extra traction on a road. The particle size is somewhat random, with a fair number of the components at roughly an eighth of an inch in diameter (3 mm) although this varies from manufacturer to manufacturer. Rock salt, in storage, has a density of 1.2--roughly 1,200 kilos per cubic meter, or just about 80 pounds per cubic foot.
Environmental and Corrosion Concerns
Rock salt as a de-icing agent raises some additional concerns; the runoff can impact local agriculture by "salting the field" and reducing crop yields while the use of de-icing agents increases the number of freeze-thaw cycles that a road bed undergoes. Every thaw and freeze cycle causes ice crystals to expand in the road material, increasing maintenance costs. Furthermore, rock salt tends to bond with and stay with sand and other gritting agents and therefore greatly accelerates the rate of corrosion on vehicles, especially in their undercarriage and muffler systems.
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