If you want to keep your slate or other type of flagstone walkway in good condition, one of the things you need to do is avoid using rock salt to keep it from getting icy during the cold months. There are a few other things you can use besides rock salt that will keep you safe and not harm your slate walkway.
How Salt Damages Slate
Rock salt damages slate and other types of flagstone by causing it to chip away gradually. The salt causes ice to melt, and the melting and freezing process shrinks and expands the slate, causing cracks in the surface. These cracks become deeper, weaken the slate and cause parts of it to chip away from the surface and away from the edges. Salt also causes discoloration in the form of white patches, or patches that are a few shades lighter than the rest of the stones.
Calcium magnesium acetate (CMA) is used as an alternative to sodium chloride to deice roads, making them safer to travel on in the winter. CMA has some drawbacks, however -- it loses its effectiveness in temperatures below 23 degrees and reacts more slowly than salt, especially during a snow or ice storm. On the upside, CMA lasts longer than salt and doesn't cause the damage that salt causes on road or walkway surfaces.
Instead of using a deicer, consider sprinkling a fine material over your slate walkway that will give the surface some traction or "grab" in the cold months. Sand is often mixed with things such as CMA to give the icy surfaces more texture so people are less likely to slip on the sidewalks or lose control on the roads. Cat litter is also cheap and sticks to the ground even in icy conditions, so you won't have to worry about falling on your walkway.
How to Apply Deicers or Traction
Mix some coarse builder's sand with a bag of CMA, both of which can be found in home-improvement stores. Scoop up the mixture with a small plastic cup or hand shovel for a small job, or use a large garden shovel to spread the mixture over a longer walkway. Walk along the side of the icy slate walkway and shake the shovel or cup back and forth as you move along to distribute it evenly. The sand provides traction while the CMA goes to work to break up and melt the ice.