Road Salt

Hundreds of millions of tons of salt are produced every year, all across the world, but with China and the US as the biggest, saltiest players. Approximately 293 million tons of the white stuff were made in 2019 by one of three processes. The first and by far the oldest method is solar evaporation from sea water in open-air salt pans, which dates back to at least the Bronze Age in the UK. The second process involves mining from underground seams of rock salt (halite) that were left behind by the evaporation of prehistoric oceans. The third method involves pumping water into those deposits to dissolve out salt and then evaporate the brine.

Only 6% of that enormous global glut of salt is used in our food, and more than half of the salt produced is used by cold countries to prevent roads from icing over in winter. The mixture of rock salt and grit used on our roads combines with water on the surface of the tarmac to produce a salty solution that has a lower melting point than pure water, meaning that it no longer freezes at zero degrees. This salty solution gradually dissipates, so the ice doesn’t stay melted for long if temperatures stay below zero. This is where the other ingredient of road salt comes in: grit is added to the salt to make the road surface grippier, preventing the development of dangerously smooth, slick ice.

This salty, gritty sprinkling keeps traffic moving in a blizzard and is thought to reduce ice-induced accidents by nearly 90%, but comes with its own hazards. Concrete roads, bridges and tunnels can suffer extreme degradation in countries where road salt is routinely used in the winter. This is because the salt gets into the concrete and starts to corrode the steel reinforcement, the resulting rust expands creating cracks and allowing salt to infiltrate further. Road salt run-off has known environmental impacts: accumulating in waterways and killing off rainbow trout, roadside and aquatic plants, and salamanders and frogs. It is also thought to attract birds, deer, elk and moose to roads to satisfy their salt cravings, ironically increasing the risk of car vs. animal collisions.

Check out our longer blog post that looks in fine-grained detail at simple sodium chloride.