Thick hard efflorescence on chimney stack

[Mattg4321]

Just had some scaffolding put up to deal knackered chimney flaunching that is causing water to soak into the chimney. Additionally part of the stack needs repointing, a few spalled bricks replaced and I’m going to apply stormdry to the brickwork. 
 

On the north side of the stack I’ve got some pretty heavy efflorescence in places. I’m not massively fussed about the appearance, although it would be nice to get rid of it. This side doesn’t really take the weather, but obviously hasn’t dried out as quick in the sun as the other sides, which has allowed the efflorescence to form. 
 

Can barely even scratch it with a screwdriver it’s so hard. Estimate 1-2mm thick or more in places. What should I do? Leave alone and stormdry cream over it? Try and remove it? Guess it would need a wire brush on a grinder or some extremely potent chemicals. 

[GEO-PAR]

I've got the same problem on some stonework - It's turned rock hard.  The only thing I've tried so far is the combination of gently chiselling with a nail and a rough brush to get the worst off. Then clearing the excess off with water.  It wasn't perfect though and will take me ages when I get around to doing the whole lot.  So if you come across any products that shift it faster, please let me know! 

[torre]

On sound brickwork we've had good results using brick acid. 

Soaking the wall first with water to minimise penetration into the bricks, brushing on the acid, leave for 30 seconds or so, give it a scrub with a scrubbing brush not wire brush and thoroughly rinse off.

You can try it diluted first and on a small area. Always wear eye protection and you should, if diluting, add acid to water not the other way around!

I'd be more wary of doing this on stonework, a stone mason who worked on our build suggested it's okay if the stone is saturated first and the acid not left on for long but I'd definitely want to try it on an off cut or something first, as the acid will react with the stone much faster than brickwork.