You can try this Back Set by RoMix Chemical & Brush* 800-331-2243 , but I would give Romix a call they may have something better.
The instructions on this product say it works on concrete and grout.
Personally, I try and not pour concrete down drains.
so what was the final outcome?
Buy cheap, buy twice.
Have not gone yet, hopefully they will get back to me this week and I will let you know what happened