Magnesium and calcium chloride

The grit and salt bins section also includes this alternative to traditional road salt.

Magnesium and calcium chloride are alternative de-icing agents that remain effective at lower temperatures than traditional road salt, while being less aggressive towards road surfaces and roadside greenery.

They are increasingly chosen by site managers who need to limit the negative environmental impact of de-icing agents.

Despite their higher purchase price, magnesium and calcium chloride can be more cost-effective per square metre of treated surface, as they require smaller doses and remain effective even in severe frost.

Magnesium chloride and calcium chloride differ slightly in how they work. Calcium chloride releases heat as it dissolves, so it melts ice more quickly at very low temperatures, whereas magnesium chloride acts somewhat more gently and is preferred where the aim is to reduce irritation to animals' paws and to limit the impact on the root systems of trees growing along pavements.

Both agents are less aggressive towards reinforcing steel and concrete than traditional road salt, making them the preferred choice for de-icing multi-storey car parks, ramps to underground garages, and bridges and viaducts, where corrosion of reinforcement poses a serious structural risk over the many years of a structure's service life.

The dosage of both chlorides should be adjusted to the forecast temperature and the type of surface. Using too much does not speed up melting in proportion to the cost; it merely increases the burden on the surrounding greenery. Manufacturers specify recommended doses per square metre depending on the thickness of the ice layer, and it is worth reviewing them before the winter season.

Chlorides must be stored in a dry, covered place, as both compounds are highly hygroscopic and absorb moisture from the air, which leads to caking and a partial loss of their performance. They should therefore be kept in sealed salt bins rather than in open heaps outdoors.

The use of chlorides can also be economically justified in view of penalties for damage to urban greenery. A growing number of local cleanliness regulations specify which de-icing agents are permitted in the immediate vicinity of plantings, and using substances that are gentler on root systems helps avoid disputes with the parks department during the end-of-winter inspection.

It should be borne in mind that the effectiveness of both chlorides, although higher than that of salt at low temperatures, is not unlimited. Below a certain temperature threshold, which depends on the concentration of the solution, no chemical agent can replace mechanical ice removal, so a stock of grit sand to improve surface traction remains a useful addition.