De-Icing.

In winter, ensuring road safety is crucial, and salt plays a fundamental role. Road salt is an effective and economical solution for combating ice and snow, helping to prevent accidents and protect drivers.

Why is salt used for de-icing?

Salt (NaCl), also known as sodium chloride, lowers the freezing point of water, meaning it takes much colder temperatures for water to freeze. This occurs because, when dissolved, salt releases sodium and chloride ions, which make it more difficult for water to freeze even at very low temperatures. When salt is applied to ice, it dissolves in the thin layer of water that is always present on the surface of the ice. This saltwater mixture freezes at much lower temperatures than pure water, down to around -9ºC, which facilitates the melting of ice and prevents further formation.

Salt is considered effective down to -9ºC because, although saltwater can remain liquid at temperatures as low as -21,1ºC depending on the salt concentration, the process of melting the existing ice slows down significantly at lower temperatures. For this reason, when temperatures drop below -9ºC, more effective alternatives are used, such as calcium chloride (CaCl2), magnesium chloride (MgCl2), or acetated-based salts like potassium acetate (KAc), among other deicers, which perform better in extreme conditions.

Different types of de-icing salt

There are three main types of salt that can be used for de-icing:

• Rock salt: Mined from underground salt deposits.
• Solar salt: Created by naturally evaporating seawater through the action of sun and wind.
• Vacuum salt: Produced through a carefully controlled vacuum evaporation process of seawater.

Traditionally, rock salt is the most widely used because it strikes the best balance between cost, purity, and de-icing efficiency. Its optimal grain size and low moisture absorption make it particularly well-suited for this purpose.

 How do humidity and grain size affect de-icing efficiency?

Humidity: Salt with high moisture content tends to cluster, making it harder to apply and reducing its effectiveness. To address this issue, anti-caking agents are added to keep the salt free-flowing and easy to spread.

Grain size: Fine-grained salt dissolves more rapidly, delivering immediate results but lasting for a shorter period. However, if the grains are excessively fine, they can be blown away by the wind, diminishing their effectiveness. On the other hand, larger grains dissolve at a slower rate, providing a more gradual but prolonged impact. The selection of grain size should be tailored to the specific situation and the type of de-icing needed.

References:

[1].- Autelitano, F.; Rinaldi, M.; Giuliani, F. Winter highway maintenance strategies: Are all the sodium chloride salts the same? Constr. Build. Mater. 2019, 226, 945–952. DOI: 10.1016/j.conbuildmat.2019.07.292. 

[2].- Klein-Paste, A.; Wåhlin, J. Wet pavement anti-icing — A physical mechanism. Cold Reg. Sci. Technol. 2013, 96, 1–7. DOI: 10.1016/j.coldregions.2013.09.002. 

[3].- Ullah Sajid, H.; Naik, D. L.; Kiran, R. Improving the ice-melting capacity of traditional deicers. Constr. Build. Mater. 2020, 121527. DOI: 10.1016/j.conbuildmat.2020.121527.