Remove burn marks from concrete by using washing soda and water, or a homemade alkaline cleaner containing washing soda, borax, dish soap and water. Strong chemicals such as acid-based cleaners may etch or otherwise damage the concrete.
Simple Spot Treatment
Wet the burn mark with a little water.
Mix a little water into some washing soda in a shallow bowl to make a paste. Make more than enough paste to cover the burn mark.
Pour the washing soda paste over the burned area. Scrub the spot with a scrub brush while wearing rubber gloves.
Rinse the area after several minutes using a garden hose or bucket of water.
If the stain remains, apply washing soda directly to the wet burn mark, allowing it to sit for 10 minutes or longer. Scrub the area. If the stain remains, apply the paste again and allow it to sit until it dries completely. It may take several attempts to completely remove the stain.
Homemade Alkaline Cleaner
If the concrete suffers from a number of burn spots, mix up your own that is also good at removing dirt and oily or greasy spots. An alkaline cleaner has powerful detergent properties that can remove stains while also neutralizing acidic substances that cause stains.
Add one gallon of hot tap water to a bucket. Mix in 1/2 cup each of borax and washing soda, followed by a tablespoon of liquid dish soap. Scrub the affected concrete with the solution using a stiff-bristled scrub brush or push broom. Hose or rinse the concrete afterwards.