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Understanding the Salt Phase Diagram

Project #: 4W8916
Start Date: 01/07/2021
End Date: 01/01/2022
Status: Completed

A deliverable for this project was an instructional video “Understanding the Salt Phase Diagram” targeted at winter maintenance operators and professionals. Found here


The research team will develop training materials (a fact sheet and a video) to help provide winter maintenance practitioners with a better understanding of the phase diagram for sodium chloride and how to apply it to yield the best results in roadway deicing. To provide visual aids for the training materials, Contractor will demonstrate the behavior of salt solutions in a laboratory setting, capturing key portions of the demonstration through video and still images.

The sodium chloride phase diagram is a visual representation of the behavior of NaCl solutions across a range of temperatures and salt concentrations. Winter maintenance practitioners are primarily concerned with the portion of the phase diagram where ice is formed. The NaCl phase diagram shows a familiar process: salt depressing the freezing point of ice. However, there has been some confusion regarding the portion of the diagram to the right of the eutectic point. In brine solutions with salt concentrations beyond the eutectic point, salt precipitate and ice begin to form in the solution as the temperature drops. Practitioners are unclear as to whether spraying a supersaturated brine can cause roads to become more slippery under certain circumstances.

In a laboratory setting, first using beakers and then through friction testing of pavement samples sprayed with a salt brine solution, this project will demonstrate the behavior of salt solutions of various concentrations across a range of winter temperatures. The results will be documented in detail in a final report and will be used to create two user-friendly training aids: a one-page fact sheet and a short training video. These training aids will be clear and concise, geared toward helping winter maintenance managers and supervisors understand key aspects of the NaCl phase diagram and apply that knowledge to deicing decisions.

This project was developed by Clear Roads, an ongoing pooled fund research project aimed at rigorous testing of winter maintenance materials, equipment and methods for use by highway maintenance crews.


The goal of this project is to develop materials that will help winter maintenance practitioners to make informed decisions on the use of road salts, specifically sodium chloride (aka rock salt, solar salt, salt brine, etc.), at varying temperatures and concentration.