Why Are There Dark Spots on My Concrete? Here’s What Might Be Happening

Dark spots on concrete can be puzzling, especially when you’re using chloride-free, colorless deicers like Safe Paw or Safe Thaw. So where do those stains come from if not the product? The answer often lies deep within the concrete itself.

One of the most common, natural causes of dark spotting is iron oxidation. Concrete contains trace amounts of iron—even if it wasn’t deliberately added. Over time, exposure to water and air can cause this iron to rust, forming iron oxide. The resulting rust-like stains can appear as dark blotches or discolored patches, especially in damp, shaded, or high-traffic areas.