Recently, on one of our job sites, we were asked to clean the concrete slab (underside) because there were rust marks everywhere and they were not able to clean it with high pressure water.
Rust marks were everywhere, almost on 150 sqm, area. And presumably, they came from reinforcement bars or rod ties. Either way, poor concrete placement was the main cause, again.
We started with trying different things:
1- Acid washing: Hydrochloric acid washing could be a fast way to do this, but it didn't work. We even increased the dosage of acid in the solution, and sprayed on the concrete. This method could clean the rust marks at some places, but not all.
This method (acid washing) could have worked, if we increased the dosage of hydrochlorinc acid in the solution, even higher. But not only could it be a safety hazard, but also could affect long term life of the concrete. As concrete is an alkaline medium and introducing acid to it, could change this medium and affect the reinforcement bars.
2- Hot water high pressure cleaning:
Again, this method could help us go fast. We even set the temperature very high. But this didn't help us either.
3- Mechanical cleaning:
We even studied the feasibility of sand-blasting!
The rust marks were persisting and were going nowhere. Until we tried a "Belt Sander" with 40 grit sand paper rolls.
This was not an ideally fast solution, but was definitely the most effective one. The sander could help us remove almost all of the rust marks. And I say "almost", because we still had a bit of difficulty at the corners or on formwork marks.