Chemical Mechanisms

In addition to the amplification of physical freeze-thaw damage, the potential for chemical attack associated with deicing salts was also noted by Taylor, Sutter, and Weiss (2012) and by Jones et al. (2013). Chemical deicer attack is a relatively new area of concern, at least partially spurred on by the increasing use of aqueous solutions of CaCl2 and MgCl2 as pavement deicers (Sutter et al. 2006; Jones et al. 2013; Weiss and Farnam 2015). An understanding of the mechanisms responsible for this form of attack is currently emerging and the true extent of risk to concrete pavement performance is being studied. Nevertheless, many consider it prudent to consider the potential for this type of chemical deicer attack when designing concrete pavements that will be subjected to these types of deicing chemicals.

It is known that reactions can occur between various deicers and a number of phases in HCP, including reactions with the tri-calcium aluminate phase to form Friedel’s salt or Kuzel’s salt. Yet the primary mechanism thought to be responsible for chemical deicer attack is a phase change that occurs in the presence of water, in which the calcium hydroxide present in typical HCP reacts with CaCl2 to form calcium oxychloride (Sutter et al. 2006; Sutter et al. 2008; Weiss and Farnam 2015; Monical et al. 2016). The source of CaCl2 can be directly from CaCl2-based deicers or as a product of chemical reactions that occur between MgCl2 found in MgCl2-based deicers and calcium hydroxide and calcium-silicate hydrate present in HCP (Sutter et al. 2008). The phase change to calcium oxychloride is highly expansive, with the resulting damage to the HCP likely due to crystallization pressures. Figure 6 shows the effect of this expansive reaction on cylinders of mortar soaked in solutions of CaCl2 and MgCl2 at 40 °F (4 °C) (Sutter et al. 2008). Scanning electron microscopy (see figure 7) shows that the distress occurred in specimens soaked in CaCl2 and MgCl2 solutions and is characterized by radial cracking from the outside of the cylinders inward, as if the surface were peeling away like layers of an onion. Further, affected areas of the HCP are altered, becoming more porous and nearly devoid of calcium hydroxide (Sutter et al. 2008).

Depending on the salt concentration in the solution, the phase change to calcium oxychloride occurs above the freezing point of pure water at temperatures between 32 °F and 122 °F (0 °C and 50 °C) (Weiss and Farnam 2015). At low salt concentrations, the chemical reaction may not occur at all and any distress that occurs will largely be due to physical freeze-thaw deterioration. As salt concentration increases, the chemical attack mechanism may become a more important, if not dominant, factor.

The importance of salt concentration in the solutions of CaCl2 and MgCl2 cannot be overstated. Deicers are applied at concentrations well in excess of what is needed for the calcium oxychloride phase change to occur. When these solutions are used in anti-icing operations, they remain at full concentration until becoming diluted as the precipitation event occurs, if it occurs at all. Further, anti-icing is often done when the pavement is dry prior to the start of the event, and thus the fully concentrated salt solutions can be drawn into the dry concrete through sorption. Sorption would be especially high if the solutions pool in non-draining joints. When used as deicers applied during or after a precipitation event, the melting of ice dilutes the solution and decreases the potential for an occurrence of a deleterious chemical reaction. But, as described previously, salts may concentrate over time if the solution is retained in the joint and the deicing cycle repeated, resulting in the potential for a damaging chemical reaction.