Ice Melt: Match the Chemical to the Surface, Then the Temperature

How de-icers actually work

Ice always carries a microscopically thin film of liquid water on its surface. Dissolve a salt into that film and you lower its freezing point, so the ice in contact with it melts. The brine that forms spreads, works down through the ice, and eventually breaks the bond between the ice and the pavement — which is the point. You are not melting the ice away, you are breaking it loose so you can move it.

Two consequences follow immediately. First, de-icer needs liquid water to start, which is why it works badly on a dry, powdery surface at very low temperature. Second, more product is not faster: past the point where the film is saturated, extra pellets sit there doing nothing except running off into your soil.

The four chemistries

Sodium chloride (rock salt) is cheap and stops doing useful work somewhere around 15 to 20°F. On a cold night it is decoration. Magnesium chloride works to about -15°F and is the least irritating of the working chlorides — the sensible default around pets. Calcium chloride works to about -25 or -40°F depending on the product, dissolves exothermically so it starts fast, and is the hardest on concrete and plants. Chloride-free amide and acetate products remove the corrosion argument entirely and give up cold performance in exchange.

The surface table

This is the reference the site exists to publish. Nothing comparable is available for free, and it is the thing most people actually need before they buy a bag.

What ice melt does and does not do to concrete

The popular belief is that salt dissolves concrete. It does not. The damage mechanism is physical rather than chemical for the most part: de-icer keeps water liquid in the concrete's pores at temperatures where it would otherwise be frozen, so the surface goes through far more freeze-thaw cycles than it otherwise would, and each cycle expands water inside the pore structure. That is what causes scaling and spalling.

There is a genuine chemical component too — chloride ions migrating to embedded steel and corroding it, which is why bridge decks are a chloride problem. On a residential driveway with no reinforcement near the surface, the freeze-thaw mechanism is the one that will get you.

The practical rules that follow: keep de-icer off concrete poured within the last twelve months, seal decorative and stamped concrete, and do not over-apply. We work through it properly in does ice melt damage concrete.

Applying it correctly

• Spread before the storm where you can. A thin pre-treatment stops ice bonding to the pavement in the first place and uses far less product than breaking a bond after the fact.
• Shovel first, then treat. De-icer is for the thin layer bonded to the surface, not for melting six inches of snow. Using it that way is expensive and mostly ineffective.
• Check the temperature against the product. Applying rock salt at 10°F accomplishes nothing except cost and runoff.
• Use a spreader. Hand-broadcasting delivers several times the intended rate, which is where residue, slipperiness and plant damage come from.

Timing gets its own page, because getting it wrong is the most common and most expensive mistake in this category: when to apply ice melt.