Rock salt, also known as (a.k.a.) sodium chloride, road salt or halite, is the least expensive of the ice melters. It is effective down to about
plus 20° F. When it’s colder than that, salt just lays there. It can damage concrete, grass, plants and wood decks. If concrete is less than one year old, rock salt is not recommended.
Magnesium Chloride works down to minus 13° . This is another fertilizer product, so it avoids some of the negative associated with rock salt.
Calcium Chloride is the gold standard of ice melters. It works down to 25° below zero and is known to be quite safe around plants, decks and concrete. This is the one generally recommended for ice build-ups on roofs and in gutters.
How they work
Many people think the object of using ice melters is to MELT ALL THE ICE.
Well, they’re wrong. Ideally, the ice melter will melt through the ice, down to the concrete or other solid surface, where it breaks the bond between the pavement and the ice so that the ice can be shoveled away.
Ice melters work poorly on gravel walks and drives, because there is no pavement surface where they can work. They tend to go through the ice and into the gravel without having much effect. | 
