How can I remove the efflorescence from Basalt

Question

We have basalt tiles on our outside steps. These were laid on the concrete. Over the years the white deposit, (I am assuming efflorescence from the concrete underneath) is increasing. I finally want to remove it.

Hard brushing and water blaster has not removed it.

We used to have Granite kitchen benchtops (countertops), so I know that the Granite is not resistant to many acids, such as lemon or fruit juice. So I don't want to use CLR (Calcium Lime Rust) remover.

I am wondering if I should try Sulphuric acid or Phosphoric acid. I cant tell if either will react with the Basalt. I don't have any left over tiles to experiment on.

Suggestions ?

Note this is not the same as stains on Granite posts on this site.

Answer 1

The efflorescence will be a relatively soft/soluble mineral or mix thereof. Calcium is likely.

So I would, in fact, try a mild acid to start with, combined with physical scrubbing. Those really shouldn't harm real granite significantly, especially if rinsed off after use. Vinegar is my initial go-to in that category, since I always have it on hand and it's cheap.

If you need something more aggressive, you can test it in an unobtrusive corner to confirm that the granite won't mind. But also, remember that this stone is going to be sitting out in the weather for decades, and the area you've looking at isn't even going to be stepped on; a bit of wear won't be noticed by anyone but you.

The professional solution might actually be a small sandblasting kit or something similar. (Bead blasting? Powder blasting?). If folks concur on that idea, such things can be rented ...