Road salt

An Econ-brand salt spreader spreading salt on a road during the 2010 UK winter.

Road salt (Also known as de-icing salt, or snow salt) is a salt used mainly as an anti-slip agent in winter road conditions, but also to prevent dust and snow build-up on roads. Various kinds of salts are used as road salt, but calcium chloride or sodium chloride (rock salt) are among the most common. The more expensive salt magnesium chloride is generally considered safer, but isn't as widely used due to its price and effect on structural integrity. When used in its solid form, road salt is often pre-wet as this speeds up the melting process significantly.

Spreading

Road salt is generally spread using a winter service vehicle called a "salt spreader". A salt spreader is generally something added to larger truck, loader or tanker, if a salt brine is used instead of solid salt, rather than being its own specially designed vehicle. That way, it is very common for most winter service vehicles to have a salt spreader on them. The salt is stored in the large hopper on the rear of the vehicle, with a wire mesh over the top to prevent foreign objects from entering the spreading mechanism and hence becoming jammed. The salt is generally spread across the roadway by an impeller, attached by a hydraulic drive system to a small onboard engine. However, until the 1970s, it was often either spread manually using shovels by men riding on the back of the truck or smaller wheelbarrow-like contraptions, the latter still being used today for personal use. Some older spreading mechanisms still require it to be manually loaded into the impeller from the hopper.

Mechanics

Salt for use of melting ice and snow works through a phenomenon called freezing-point depression, the lowering of a substances freezing point after the addition of solutes. When road salt is added to roads, aside from providing better friction for vehicles on the road, it also dissolves in the water of the ice, resulting in a lower freezing point. As long as the temperature is above this freezing point, this in turn results in the ice melting. Due to this, ordinary rock salt is only effective down to minus 6-10 °C. At colder temperatures, it can have the opposite effect. Road salt is sometimes used even in colder conditions, if milder weather is expected. In very cold and dry weather, the road surface becomes rough and the need for de-icing is reduced. However, if it is both very cold and rainy, the roads can become extremely difficult and in some cases roads may need to be closed to traffic.

Types of salt

Storage tank for road salt in Puente de los Hocinos, Spain.

Sodium chloride

Sodium chloride is by far the most common kind of road salt. This is mainly due to its widespread use and low cost, and also because it already has a large industrial infrastructure, since it's already common-use for both industrial and consumer purposes. While it is common, and cheap, it does have its drawbacks. It does not have a very low effective temperature range; as mentioned previously it usually doesn't go lower than 6-10 °C, and under these temperatures it's often counter-productive. When used in large quantities, it can also often disrupt local ecosystems by heightening the salinity of bodies of water and the soil. Rock salt is also abrasive, and is known to eat away at concrete or asphalt if used decadently.

Calcium chloride

Calcium chloride is more uncommon compared to sodium chloride. While it is slightly more expensive than its sodium-based counterpart, it can cover a far larger area and melts ice almost three times quicker. It has recently started rising in popularity since it isn't as environmentally damaging as sodium chloride, and also due to its heightened effectiveness at clearing ice.

Magnesium chloride

Magnesium chloride is by far the most expensive of the common road salts, and is not in common use today. It has a very low environmental impact, and is quite effective at clearing ice. However, it has been discovered that magnesium chloride causes far more damage to concrete surfaces compared to the other salts, and has therefore been largely stopped being used as a de-icer. It is still widespread as a dust clearer in warmer weather, and is very effective at it.

See also

• Deicing