If your renovation is very recent, you might still be able to get the contractor back to clean up the mess. But if it has been a while, then you will have to do it yourself, which might be a little troublesome, for the tools needed.
Desmond Chew of Three-D Conceptwerke recommends using an acid wash to clean it off. Get the acid from the hardware store – just ask for the right type to clean off concrete. “Be careful and wear rubber gloves when you handle acid. Pour a little into a small pail diluted with some water (depending on how concentrated you want it), and use a sponge or brush to apply it on the concrete. “Avoid touching stainless steel surfaces with the acid as it will corrode,” he says.