Efflorescence grouting concrete flags

[HugoB]

I am renovating my front path, which was laid with coloured concrete slabs in the early 1970s and pointed in dark grey (or possibly originally black). I really like it and want to keep it, but having pressure-washed it, some of the pointing needs replacing.



On the advice of a local tile merchant, I used Bal Superflex Widejoint grey on a small part and the initial result was excellent, but within a few days efflorescence appeared and is getting quite bad. Not at all attractive. There is no dpc under the path and it appears to be laid on a weak mortar mix.

I've talked to Bal technical helpline and they recommend I talk to Fila about a sealer, which would apparently need reapplying regularly so the idea doesn't appeal too much. I am really not sure what to do for the best. Perhaps pointing with coloured mortar with a waterproofer added?

Here's a close-up of the problem (when I rub the white patches a white powder comes off):



Any help much appreciated.