Having just returned from a Thanksgiving holiday where ice has been a major problem already this winter, a discussion on deicing materials seems appropriate. Ice on sidewalks and roadways makes for a hazardous combination to people and property. Removal of compacted snow and ice with shovels or snow blowers isn't always easy or effective. The use of a chemical deicer may be needed to facilitate removal.
In order to be effective, deicers must first attract or come into contact with sufficient moisture to form a liquid brine. The brine has a lower freezing point than water, which causes ice and snow to dissolve on contact. Deicing agents penetrate downward through the ice and snow layer until they reach pavement. Once on the pavement, the brine spreads outward to break the bond between the ice and snow and the pavement. After sufficient loosening, the ice and snow can be removed by shoveling or plowing.
Five chemicals are commonly used as deicers. While they can be used alone, they are often blended together or combined with other materials to improve performance, decrease cost, and decrease potential damage to plants, concrete, and ground water. Performance of a particular deicing agent is determined by factors such as temperature, traffic conditions, shape of deicer particles, duration of the deicer's melting action, and the time it takes the deicer to form a brine.
Calcium chloride (CaCl 2 ) is available in flake, pellet, or liquid form and often outperforms other deicing products especially at lower temperatures. It produces an exothermic reaction, giving off heat as it melts. Calcium chloride also has a greater capacity to attract and retain moisture directly from its surroundings, which enables it to dissolve faster and start the melting process.
Sodium chloride (NaCl) also known as rock salt was first used as a deicer in the 1940's. It is an effective deicer for areas that receive road traffic. It draws heat from the environment rather than releasing it and it loses most of its deicing effectiveness when temperatures are below 25 degrees F. Heat generated by the friction of moving traffic on busy roadways assists rock salt's effectiveness.
Potassium chloride (KCl) is a naturally-occurring material that is also used as a fertilizer (muriate of potash) and a food salt substitute. Because of its high salt index and the potential to burn foliage and inhibit rooting, its use is relatively limited.
Urea (NH 2 CO NH 2 ) is synthesized from ammonia and carbon dioxide. It is primarily used as a fertilizer. As a deicer, it has a lower burn potential than potassium chloride.
Calcium magnesium acetate (CMA) is a relatively new salt-free melting agent made from dolomitic limestone and acetic acid. It causes little damage to concrete or plants and is used as an alternative to salts in environmentally sensitive areas.
The shape of deicing particles affects the speed of their penetration through ice. Uniformly shaped spherical pellets of about 1/16 to 3/16 inch penetrate ice faster and more efficiently than other shapes. Irregularly shaped particles tend to melt randomly in all directions. Flake particles perform the worst because they melt as much horizontally as they do vertically. By the time flake particles reach the pavement, they are often too diluted to effectively contribute to undercutting and disbondment.
Corrosive effects of deicers on metals have been the subject of considerable research over the years. Deicers containing sodium chloride, calcium chloride, and potassium chloride can accelerate corrosion of iron and steel by holding moisture against metal surfaces. There is no significant difference among salts in their corrosive effect. Ordinarily, if the surface paint, chrome, or undercoating is not damaged or broken, corrosion will not occur.
Deicing salts can damage some concretes, causing them to scale or break up. Calcium chloride and calcium magnesium acetate cause the least damage to concrete and metal. To minimize injury, apply only minimal amounts of deicing materials and remove the slush and residue before it refreezes. Removal of the ice and residue will also reduce problems indoors where unsightly white powdery residues may damage floor coverings.
Deicing agents can also injure plants. Excess salts and over fertilization impede uptake of nutrients and affect water absorption. Symptoms of salt injury include desiccation, stunting, and dieback. Accumulation of salt in the soil over several years may result in progressive decline and eventual plant death. To reduce salt injury to plants, avoid piling "salted" snow or ice near them. Water salted areas heavily in the spring to leach or flush salt away. A physical barrier such as a burlap screen can be erected to intercept salt spray from passing vehicles on busy streets. When selecting plant material for use near heavily salted areas, choose plants known for their salt tolerance. These include Ponderosa and mugo pines, blue spruce, horse chestnut, white ash, honeylocust, Norway maple, red cedar, larch, sumac, willow, black locust, and red, white, and bur oaks. Avoid using balsam fir, douglas fir, white pine, hickory, redbud, hawthorn, walnut, American linden, linden, viburnum, dogwood, hackberry, American hornbeam, and black, red, and sugar maples. When selecting turfgrass, tall fescue and perennial ryegrass are more salt tolerant than red fescue and Kentucky bluegrass.
Deicers should be used only when necessary to help loosen snow and ice and make removal easier. Never over-apply deicing products and remove the slush before it has the opportunity to refreeze. Mix the deicers with sand to increase their effectiveness and reduce overall use. One pound of deicing salt mixed with 50 pounds of sand makes an effective mixture. When selecting deicing products, consider both cost and effectiveness. Sodium chloride is a relatively inexpensive salt compared to calcium chloride. However, in extreme cold, calcium chloride is the only product that works effectively. Always read and follow label directions when applying deicing materials.
This article originally appeared in the December 13, 1996 issue, pp. 177-178.