Road salt, used each winter to protect roadways from ice and preserve safe driving conditions, can slowly degrade the concrete on roads. Engineers have known for some time that calcium chloride (CaCl2) salt, commonly used as deicer, reacts with the calcium hydroxide [Ca(OH)2] in concrete to form a chemical byproduct that causes roadways to crumble.
To address this concern, a civil engineer from Drexel University (Philadelphia, Pennsylvania), is working on a new formula for concrete by using cast-off products from furnaces. According to the researcher, these products can hold up better against chemical erosion.
Yaghoob Farnam, an assistant professor in Drexel’s engineering school, created a method for using fly ash, slag and silica fume—leftovers from coal furnaces and the smelting process—in a new concrete mix that is more durable because it doesn’t react with road salt.
“Many departments of transportation have reduced the amount of calcium chloride they use to melt ice and snow, even though it is very efficient at doing so—because it has also been found to be very destructive,” Farnam says. “This research proves that by using alternate cementitious materials to make concrete, they can avoid the destructive chemical reaction and continue to use calcium chloride.”
The goal of Farnam’s work is to produce a concrete mix that contains less calcium hydroxide—the ingredient that reacts with road salt to form calcium oxychloride [Ca(ClO)2]. This chemical tends to expand when it is formed, and when that reaction happens in the pores of cement, it can cause degradation and cracking. Farnam’s research led him to the conclusion that supplementary cement materials could be substituted into the mix in place of calcium hydroxide.
“There is a great push to use these power industry byproducts because they take up space and some of them can be harmful to the environment,” Farnam says. “We believed that portions of the byproducts such as fly ash, slag, and silica fume could be used to make concrete that is both durable and cheaper, because it uses recycled materials.”
To test his theory, Farman’s lab created cement samples using varying amounts of fly ash, silica fume, and slag and compared them to samples of ordinary Portland cement—the most common type used in roads. His findings confirmed his hypothesis—namely, that the samples containing more cement substitute materials did not produce as much calcium oxychloride.
An examination of the ordinary Portland cement samples, via acoustic emissions, x-rays, and microscopy, revealed damage after just eight days of exposure due to the formation of calcium oxychloride. Meanwhile, samples with a proper amount of fly ash, silica fume, and slag did not show damage during the testing period.
The study also revealed that higher concentrations of calcium chloride produce more calcium oxychloride when reacting with concrete. So, theoretically, using lower concentrations of calcium chloride on roads could help extend their life, but it would also make it less effective as a deicing agent.
“An additional concern is that calcium oxychloride can form even if the concrete is not undergoing a freeze-thaw cycle,” Farnam says. “It is a chemical reaction that can happen at room temperature, so it can take place when the roads are pre-salted, even if ice doesn’t form. And as the salts remain on the surface after a snowstorm, the reaction will continue to degrade the road, so it is vitally important to minimize this reaction in order to preserve the infrastructure.”
Farnam says he plans to continue searching for ways to improve the materials used in infrastructure. He and his lab research team are currently pursuing a method to create a protective layer on the surface of concrete by using bacteria that can prevent calcium oxychloride formation.