Looks like they boiled over and etched your concrete.
...buy some concrete etchant, miuriatic acid, at a box store.Power wash firstt, allow to dry thoroughly apply muratic with a natural bristle brush... (you'll find those in the concrete aisle.. or use a roofing tar brush )
..etch to match...
I get the feeling you already knew this and wanted to show cool pictures of what can happen..