Let me just start by saying I’ve applied about 40 tons of grout over the years as one of my side hustles along with customer tile work in contracting. Much of it commercial.
To bleach clean concrete that has been used as grout you would use Muriatic Acid available in any pool supply shop for about $30/gallon. It is unlikely they use concrete for the actual grout even if they were oblique enough to apply a tile application directly to a concrete floor. But…this is not uncommon in industrial / commercial applications where they call it “industry standard” when they mean “It’s good enough and will last past the call back date”. Run a screw driver or grout pick over an out of the way bit of cracked or ruined grout to see if it is grout (it will scratch easily) or concrete (will appear hard and unscratched). Don’t use a lot of elbow grease, you are not prying out the grout, you are just trying to lightly scratch through whatever miasma has built up over the years to see the original material. Treat it like you are keying you least favorite neighbor’s car to try and scratch the actual grout line. The tile will not scratch. Grout will scratch and look powdery and concrete will chip or only scratch lightly even with lots of effort (which, again, not needed). If it’s concrete us muriatic acid. If it’s grout the use a professional product called 477 (It’s been about 25 years since my last job so I don’t know if they still make it and sell it. If so…USE GOOD VENTILATION!!!! It works but it can really give a guy a headache! Probably have really good ventilation with ANY chemical you use! If it were me and a floor looked like that I would pull out all the grout (grout picks are designed for this) about a 20 minute job, then use the pick to pull lightly on all tiles to see if any are ready to pop or already loose, then I would vacuum every last bit of dust and old grout up and spritz the grout line with a spray of water and regrout the whole bathroom with a fresh clean grout that can be properly sealed. If you are this ambitious us SANDED GROUT! It’s a little harder and messier to work with but it is far better for commercial use. SEAL your new grout properly! (TWICE).. Sanded grout expands and contracts more closely to concrete than unsanded grout. And for god sakes don’t use silicon or some silly stuff in a tube that promises lifetime adhesion…you will be pulling it out in a month when it starts mildewing and stinking and staining. Buy yourself a $30 grout bag and make a clean job of it or do it like us old farts and trowel it and clean up after. Concrete and tile/grout expand and contract at different rates so you are going to have small cracks and spaces along the sides of the tiles that will darken over time. If you go to a box box home DIY store you can buy an adequate sealer for ‘stone’ that will fill in the tiny cracks and keep the tiles from ‘popping’ over time…and they WILL start popping off over time and you will try to glue them back down or some such craziness because…”It’s just a couple tiles”. Once you use the acid to clean (following the safe handling instructions) and it looks great, go back in and seal everything. Wait for it to dry to the touch, tie small garbage bags around your feet like medical footies to keep from fouling the uncurled sealer and go back in with a dry Terri cloth towel and wipe the sealer off the TOP of the tile (but don’t push down into the grout!). Keep your towel folded flat so it skims only the tops of the tiles and removes the sealer. That stuff will build up and look awful if left on the tiles. But it fills in the small gaps and after it’s cured will allow you to go back in and scrub the grout every few months to keep it looking clean. Nothing is forever and you will need to do this every three or four years if you want that floor to last. No one likes ugly, but why spend money if it still functions as it should?