Why Treated Rock Salt is Better for your Sidewalks than Calcium Chloride
Much of the discussion about the effects of deicing products on concrete centers around the corrosive effects deicing products have on embedded steel used to reinforce concrete. This is for good reason, the deterioration of embedded steel in concrete can be directly connected to catastrophic failures in bridges, parking garages, or other structural concrete exposed to deicing products. But if you are a facility manager whose primary concern is nonstructural concrete such as sidewalks, concrete trailer parking areas, steps, and entryways, treated rock salt is a better choice for your concrete surfaces.
First, let’s dispel the myth that calcium chloride is safe for concrete. The next time a contractor tells you that calcium chloride is better for your concrete sidewalks than salt, ask why? He/she will probably say that rock salt eats concrete. At that point, ask how? In most cases, that will be the end of the conversation.
The truth is unless we are talking about the corrosive chemical reactions between chlorides and embedded steel in concrete, the primary cause of damage to concrete by deicing products is that caused by increased freeze-thaw cycles and not chemical damage. That’s not to say that there is no chemical damage caused to concrete by the use of chloride-containing deicing products, but first, let’s look at how physical damage takes place.
Concrete is often referred to as a hard sponge because of its sponge-like appearance under magnification and its propensity to absorb moisture. When we apply deicers to snow and ice the resulting water is absorbed by the concrete. If the deicer products become diluted or the temperature continues to fall, that water or moisture in the concrete will freeze again. As this meltwater refreezes, it expands within the concrete microstructure and can separate paste from aggregate or expand pores. As this happens over and over again the damage increases exponentially. Spalling and pitting will start to occur. As the top surface of the concrete weakens, the water is able to be drawn in deeper within the concrete and the damage can intensify. Deicing products create artificial freeze-thaw cycles that the concrete would not normally have to endure. Due to the increasing frequency of slip and fall lawsuits, contractors and property managers have had to use increasing amounts of deicing products on concrete sidewalks, steps, and entryways. The result has been increased damage to these concrete surfaces.
Why treated salt and not calcium chloride?
To answer this question, we look at the chemical damage caused by deicing materials. Calcium Silica Hydrate(C-S-H) and Calcium Hydroxide(C-H) are the two principal products of cement hydration (concrete curing process). Exposure to any chloride can cause concrete to leach C-H. The loss of C-H can lead to an increase in concrete porosity as well as a decrease in concrete PH level. The PH level is important because at high PH levels, there is little chance of reinforcing steel corrosion. C-S-H is the primary source of concrete strength and can account for more than 50% of the paste which holds the aggregate together. The interaction of sodium chloride ions with C-S-H can cause C-S-H to lose some of its calcium ions. Calcium Chloride and Magnesium chloride can similarly cause C-S-H to lose calcium ions and therefore some of its strength, but they have additional reactions as well.
Calcium Chloride ions react with leftover cement hydration products to form complex salts. These salts form crystals that grow within the concrete and create tensile stresses in the concrete. Additionally, calcium chloride damages concrete through the formation of oxychloride. Oxychloride is formed between calcium chloride and water. This new compound creates destructive hydraulic pressures.
Magnesium ions, supplied by magnesium chloride react with the C-S-H to produce M-S-H. The resulting M-S-H is not cementitious and therefore provides no strength to the concrete. Over time this can lead to a significant loss of concrete strength. Magnesium ions will also react with the CH to form brucite. Brucite crystals, within the concrete pores, create internal pressures and can lead to increased porosity.
Of the three most common deicers, treated salt, calcium chloride, and magnesium chloride, I believe treated salt is the best choice for concrete sidewalks. My reasons are listed below;
• Treated salt, while it may not work as quickly, will last longer and may reduce the amount of freeze/thaw cycles.
• Treated salt does not have the same chemical reactions with the concrete as calcium chloride or magnesium chloride.
• Treated salt is half the cost of calcium chloride. Treated with a small amount of liquid calcium or magnesium chloride and an agricultural enhancer it can work to much colder temperatures than straight rock salt.
• Treated rock salt contains corrosion inhibitors and is colored to enhance visibility to aid the applicator in putting the proper amount down.
Of course the use of any deicer product is going to some extent be detrimental to the concrete surface. The question is which deicing product is going to be the least harmful and how can we best manage its use? The last question we should ask is; what is the value in paying twice as much for calcium chloride when treated rock salt will work just as effectively and is less harmful to the concrete?