Deicers and Chemical Damage to Concrete

Posted on by Bert Kopp

The short version of the article reads as follows: Do not apply chemical deicers to concrete!

As winter approaches, you gaze at the now limp plastic sack which held last year’s supply of deicer. You know that you need to pick a deicer unless you plan on spending the winter in a cast, but with so many options which one do you pick? There are a great many claims about which deicers are harmful to concrete, and how you can keep your walkways safe without damaging them.

In this series of articles I will explore some of the major causes of deicer damage to concrete and how you can select a deicer to best fit your needs and protects your concrete products. But before I get into the nitty-gritty, here are some general tips to keep your concrete looking brand new.

  1. Wait 6 months to two years after installing new concrete to apply any chemical deicers. Concrete gains most of its strength in the first 6 months. No matter what you put on your steps waiting at least 6 months will substantially reduce the risk of damage to your concrete.
  2. Apply a sealer before using any deicer. Sealers will protect concrete from both chemical and physical damage and are very inexpensive. For best results see a professional on concrete sealing and wait at least 28 days after concrete is poured to seal it.
  3. Never use ammonium nitrate.
    The undisputed worst deicer for concrete. It does chemical damage and will even corrodes cured concrete.
  4. Avoid Magnesium Chloride. This is the runner up for the most destructive deicer to concrete. Magnesium Chloride causes both chemical and physical damage. Though this product is often advertised as being safe for concrete, these claims are entirely inaccurate.
  5. Use sand or kitty litter, if possible. Though these products do not melt ice, they will provide traction and cause virtually no damage to concrete surfaces of any age.



Chemical Damage to Concrete

Despite what is often advertised, most deicers do chemical damage to concrete. All salts, including calcium chloride, sodium chloride, ammonium nitrate, and even magnesium chloride do chemical damage[1,2,3]. Many companies which manufacture deicers will disagree with this claim, and I do not intend to cast stones at the deicer industry without just cause. Fortunately a number of scientific studies have been conducted to tackle this subject. I will review here the mechanics of chemical damage to concrete and how they relate to deicer corrosion.

Concrete Structure

To understand how various chemicals damage concrete, we must first understand how concrete derives its strength. Concrete is made from a combination of stone, sand and cement. Cement is the component of the mix which gives the concrete strength and shape, and is composed of calcium silicates such as (CaO)2(SiO2). When cement is mixed with water these calcium silicates combine to form larger molecules which for a hard gel with water and calcium hydroxide. [3]This process occurs according to the following equation:

2(CaO)2(SiO2) + 5 H20 → (CaO)3(SiO2)24(H2O) + Ca(OH)2

Figure 1

The result is a firm material which holds the sand and aggregate in concrete together. Figure 1 shows the  calcium silicate hydrate crystalline structure [3]. The expansive bond structure which connects all of these molecules together is responsible for the strength of concrete. At the center of each of the molecules is a calcium atom (green in figure 1). Salts and other deicers can damage concrete by removing this atom from these calcium silicate molecules. Without this central molecule, the stability of the crystalline structure breaks down and the concrete loses its strength.

Experimental Support

A study was conducted by the university of Kansas which dealt specifically with the chemical damage that deicers cause to concrete. In the experiment, a number of concrete ingots were cast and exposed to deicer solutions of various concentrations. The strength of the concrete was measured by its change in elasticity after immersion in the salt solutions.

Figure 2

Figure 2 shows the results of this experiment[5]. You can see the 15% drop in elasticity from the solutions of both magnesium chloride and calcium chloride in only the first 10 weeks of exposure. This is because both magnesium chloride and calcium chloride contain ions which can bond to the calcium in the calcium silicate hydrate structure, and remove it [1, 2, 4]. You can see a similar effect with CMA or calcium magnesium acetate, wherein the ions that form from CMA leach calcium from calcium silicate hydrate, although in CMA this process takes much longer.

This is because different elements have varying abilities to ionize and react with calcium and other chemicals. Halides, such as chlorine, tend to ionize easily in salts and be very reactive with other compounds [3]. Thus different salts cause varying amounts of damage to concrete.

Figure 3


Deionized water- For Comparison

  • CaCl2 Low Concentration

Figure 3 shows photos of the concrete samples taken from the University of Kansas Experiment. The photos corroborate the results of the elasticity testing in that magnesium chloride and calcium chloride had the greatest appearance of physical damage, followed by CMA. The experiment also showed that sodium chloride (table salt) is actually far less harmful to concrete than other deicers. Finally, the difference in the damage from the high and low concentration solutions shows that there may be a great difference in the amount of damage done to concrete depending on the amount of deicer used.

In Conclusion:

To hit the main points that I want to stress in this article:

  • Calcium chloride, magnesium chloride and all salts do damage to concrete.
  • Magnesium chloride and ammonium nitrate are especially bad – do not use these!
  • If you have to use salt, use a small amount. It will be better than the alternative.



Good luck dealing with the snow this year. Have a safe and happy winter from Steps Plus!

 

References:

  1. Carde, C., R. François, and J.-M. Torrenti. Leaching of Both Calcium Hydroxide and CS-H from Cement Paste: Modeling the Mechanical Behavior. Cement and Concrete Research, Vol. 26, No. 8, 1996, pp. 1257-1268.
  2. CODY, R. D. Experimental Deterioration of Highway Concrete by Chloride Deicing Salts: Environmental & Engineering Geoscience, Vol. 2, No. 4, 1996, pp. 575 – 588
  3. H. F. W. Taylor, Cement Chemistry, 2nd Ed., Academic Press, London (1997).
  4. Darwin, D., J. Browning, L. Gong, and S. R. Hughes. Effects of Deicers on Concrete
    Deterioration. ACI Materials Journal, Vol. 105, No. 6, 2008, pp. 622-627.