I just happen to have some of that in the garage! Never thought to use it. Thank you much.Phosphoric Acid Cleaner is a fast act-ing, easy to use acid-base cleaner for pre-cast concrete, concrete, masonry,grout, tile and natural stone surfaces. Phosphoric Acid Cleaner will removecement grout haze and mortar residue, joint cement, lime & hard waterdeposits, efflorescence, rust and other dirt and grime. Phosphoric AcidCleaner is suitable for interior and exterior use. Due to the nature of phos-phoric acid, Phosphoric Acid Cleaner is a milder alternative to other acid-base cleaners for concrete, tile, grout and natural stone surfaces
https://miraclesealants.com/pds/Phosphoric Acid Cleaner_PDS_08.12.16.pdf
We used it on repairing refractory, helps the new stuff adhere to old.