Wire brush on an angle grinder. Wear a respirator and safety glasses

In addition to what the other comment, there are roll on acid solvents that you put on and make it WAY easier to come off

The EMT to that receptacle has no offset. Would not hurt to strap the EMT closer-ish to the device location(add a strap). You could try a pressure washer on the walls, but you need a drain and lots of clean up.

I was planning on getting an angle grinder tomorrow!

Something like this? https://www.homedepot.com/p/Rust-Oleum-1-gal-Concrete-Etch-and-Cleaner-301242/206495770

Yep—I know it says to use it on floors but I’ve seen people use it on walls too. Also make sure whatever brush or roller you use is a material that it won’t melt (like cotton).

Even just a nylon paintbrush or rubber roller should work but ofc read the instructions

IMPORTANT EDIT: do NOT use an angle grinder and steel brush is you use this stuff lol. You will have to scrape it by hand after but it comes off like butter peel

An angle grinder is going to create a tremendous amount of dust that you definitely don’t want in your house or lungs. Grinding concrete will produce dust that can cause silicosis . If there is any black mold there, you don’t want that spread through the house. Is the paint lead based? Probably not, but it’s not a bad idea to get it tested. I would just scrape it as you are doing and spray it with a cleaner, and wipe it down. If you are going to paint it with DriLock, you don’t have to get every little piece off, just the loose stuff

Power washer??

If you keep it wet, you won’t get much dust. But wear a respirator just in case. Then use oil based primer to cover any stains - I like Zinsser 123. Oil primer is super stinky, so again, protect your lungs, but it will cover and stick better than water based.

Rent a giant hammerdrill with chisel bit. Same thing you would use to bring up tile and grout off the floor. Will make short work of it and will be worth every cent.

On second thought, you could also just texture over the whole wall. Maybe mix water primer thick with drywall mud, and roll it on. You probably want to remove the electric box first. Depends on how thick the existing gunk is, but just covering might be way easier.

Zinnser 123 (the original formula) is white pigmented shellac. It is not oil based. That said, it's my go-to primer in most situations.

I'm not sure if it's appropriate for OP's block wall. He should probably consult the manufacturer before using it. Or at least read the label on the can.

Round disc surface pressure washer attachments are amazing. And they use far less water than a wand.

Thanks for this reply. I didn't know that. Super helpful!!

Dynamite?

Scrape off any loose paint and seal with 2 coats of zinsser peel stop. Then your top coats when it's dry.

Don’t know the age of your house but I would test the paint for lead first.

Yea, so I have purchased three of these . It’s incredible how fast they clean and how little water they use compared to wands. I’d rate them one of my top three tools over all. Crazy efficient.