Is the efflorescence visible as run-offs under butt joints or, rather, an even veil over the stones? You can also drip some vinegar on an affected stone (not on the joint!) And see if it bubbles.

Run-offs

They are white or yellowish run-offs, mainly from standing joints. Vinegar drops start to bubble.

Veil

You see a general white or grey haze on the bricks, which is difficult to come off. Vinegar drops on the stones do not start to bubble.

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