Fastest way there is to go with strong acids, like ABC. If a customer had Super Ox on hand they could do that as well. If the residues are not excessive (like maybe in a shower) use of a lesser powdered acid solution (like Scale Away) would be in order. The downsides to use of strong acid solutions is that they do give off fumes (especially in humid conditions) which are very annoying and potentially dangerous. Obviously they are also dangerous to get on your skin and eyes. But they are the fastest way to go. NOTE: Always wear appropriate Personal Protective Equipment (PPE) when working with strong acids and read the label and SDS before use.
Concrete is a mix of chemicals that falls on the alkaline side of the pH scale and therefore is better removed with products on the acid side of the pH scale. These would include No-Ox and Super Ox. An alternative product for lighter duty removal of concrete would be CJ Industrial Cleaner. It has the advantage of being highly buffered and will have less potential for surface damage than products using “free acid” technology, like the No-Ox and Super- Ox.