Sidewalk slabs re-grouted and messed up

Hi, the joints of the paving slabs in the entrance area were partially washed out and have been renewed. Fugli Pflaserfugen mortar was used and the work was done untidily. Partially, when sweeping in the mortar, it was apparently wiped over the concrete stones with a wet broom. Now, after a few days and also after rain, the paving slabs have gray-white discolorations, partly large-scale. Rinsing with water made no difference. What can be done, which cleaner should be used? Thanks for your help.

Hello questioner. Some problems, probably including the one you mentioned, cannot simply be answered with 100% certainty. My suggestion, based on the described background, would be to remove the discolorations, which are probably cement and lime residues, with an acidic cleaning agent. Now you could go out and buy corresponding cleaning additives after some searching. I recommend conducting a so-called "field test," simply quickly and inexpensively. No joke: please take regular COLA for this, apply it to the (dry) entire surface on one plate first in the side area, apply some mechanical action in circular motions to the treated plate surface with soft bristles (e.g., a hand brush), and let it soak for 3 or 4 hours. Afterwards, rinse with clear water. The tip regarding COLA is serious! Cola consists largely of phosphoric acid, which, although associated with a pH value of 1, is only weakly acidic as a triprotic acid. If the cleaned plate’s surface still shows some imperfections but is noticeably improved, repeat the procedure, possibly extending the application time. Good luck: KlaRa

Hello and thank you for the reply. Acidic cleaning agent.....mh, I have read that concrete slabs are not acid-resistant. Or is that not true?

Yes, that's true. But we are not working here with concentrated hydrochloric acid (just kidding), but with phosphoric acid. It is also a component of many "cement residue removers." Nothing happens, there is no chemical reaction between the acid and the alkali of the cement (or rather, it is lime).

o.k. I will try it with cola. If that doesn't work, there are such cleaners from Lithofin. Which one would be suitable?

If it doesn’t work with a phosphoric acid (see COLA), then it certainly won’t work with a usually diluted phosphoric acid in cement residue removers.