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.
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