Acoustic Emission and Low-Temperature Calorimetry Study of Freeze and Thaw Behavior in Cementitious Materials Exposed to Sodium Chloride Salt

Abstract

This paper describes a series of experiments that were performed to assess the freeze–thaw behavior of mortar specimens exposed to sodium chloride (NaCl) solutions. A low-temperature longitudinal guarded comparative calorimeter was used to perform cyclic freeze–thaw testing on mortar specimens saturated with NaCl solutions. Heat flow and acoustic emission activity were monitored during the freeze–thaw experiment to detect ice formation and cracking. Although the conventional water–NaCl phase diagram would suggest that no freezing or damage would occur in specimens saturated with 15% and 23.3% NaCl solution by mass within the applied freeze–thaw temperature range, damage was observed. For these specimens, an additional heat flow peak attributed to an unexpected phase change, accompanied by acoustic activity, was detected at a temperature higher than the expected freezing point. For better understanding of the source of this damage, a low-temperature differential scanning calorimeter was used to investigate the influence of NaCl on freeze–thaw behavior of water, two pore solutions, hydrated cement powder, and calcium hydroxide powder. The results showed that the pore solution alters the freeze–thaw behavior slightly; however, it does not exhibit the unexpected phase change at higher concentrations. The specimens made with hydrated cement powder showed the unexpected phase change in high concentrations of NaCl solution in a temperature range between 0°C and 8°C. Although the exact nature of this phase change is not definitively known, it appears to result in premature damage during freeze–thaw when high-concentration salt solutions are used, even if freezing of the solution is not occurring.

Early Detection of Joint Distress in Portland Cement Concrete Pavements

“…The chemical phase change is mainly due to a phase change (from liquid to solid) in the pore solution caused by these chemical products (Figure 5.1b). The chemical phase change during cooling was reported to be destructive and cause damage in cementitious materials (Farnam, Bentz, Hampton, et al, 2014;Farnam, Bentz, Sakulich, et al, 2014;Farnam, Dick, et al, 2014). During freezing, an increase in the temperature and an exothermic behavior (shown by an arrow in this figure) were observed due to latent heat release.…”

“…Figure 5.1b illustrates the phase diagram for a NaCl-H 2 O solution. (Li et al, 2012) and (b) comparison of NaCl-H 2 O phase diagram with chemical phase transition in NaCl-H 2 O-cement system (Farnam, Bentz, Hampton, et al, 2014). Figure 5.1b shows that in a cementitious systems exposed to NaCl, a chemical phase change can occur (as indicated by a dashed line), which was observed at a temperature range between -6uC and 8uC due to the presence of aluminate phases in cement.…”

“…For samples saturated with 15% and 23.3% NaCl, a chemical phase change was also observed in addition to the water/ice phase change. The reaction between cementitious matrix and solutions with higher concentrations (15% and 23.3% NaCl) may result in new chemical products (e.g., Friedel's salt, Kuzel's salt, calcium oxychloride, gypsum, or sodium sulfate) (Farnam, Bentz, Hampton, et al, 2014;Farnam, Bentz, Sakulich, et al, 2014;Farnam, Dick, et al, 2014). The chemical phase change is mainly due to a phase change (from liquid to solid) in the pore solution caused by these chemical products (Figure 5.1b).…”

“…Previous studies (Farnam, Bentz, Hampton, & Weiss, 2014;Farnam, Bentz, Sakulich, Flynn, & Weiss, 2014;Farnam, Dick, et al, 2014;Villani, Farnam, Washington, Jain, & Weiss, in press) identified that chemical reactions between the cementitious matrix and salt solution can result in a chemical phase change that can cause severe damage in cementitious materials. Figure 5.1b illustrates the phase diagram for a NaCl-H 2 O solution.…”

“…Figure 5.1b shows that in a cementitious systems exposed to NaCl, a chemical phase change can occur (as indicated by a dashed line), which was observed at a temperature range between -6uC and 8uC due to the presence of aluminate phases in cement. This chemical phase transition was detected using thermal measurement by observing the heat released during phase transition and the damage was assessed using acoustic emission technique (Farnam, Bentz, Hampton, et al, 2014;Farnam, Bentz, Sakulich, et al, 2014).…”