It is a process, and varies with where you live.

Muriatic Acid (follow package directions): https://www.tractorsupply.com/tsc/pr... d=2590323429

And a stiff wire brush are your best options. Muriatic acid has a significant virtue, the active ingredient (hydrochloric acid) is volatile if not reacted, and forms salts with many compounds such that it is (relatively) environmentally benign as compared to many other options. It also attacks only the CEMENT portion of the grout, leaving the sand and stone untouched.

Other stuff:

If the walkway is subject to exposure (snow/rain) and you are in an area where there are freeze/thaw cycles, you need to be very careful in how you treat the surfaces when you are done cleaning. You may choose to seal the surface, and if you do so, be sure that it is fully dried (days, at least), and that you treat the stone and grout uniformly and with the same material. The sealant wants to be a clear, non-volatile material when cured, and absolutely not anything like Thompsons - which is absolutely worthless in any case.

https://rainguardpro.com/product/micro-seal/ is one of many Silane-based materials. Thoroseal is another. But Silane/Siloxane is the gold standard - assuming you have the gold.

So you understand the "why" of it: Most masons these days are poorly trained in how to mix mortars and grouts. Generally they make the mortar far to rich, so that it cures (NOT dries) far too hard. When that happens, water penetration into the stone pavers is greater than what it is to the grout. At which point, during the freeze-thaw cycles, the harder material (grout) will cause damage to the pavers _AND_ if that were not enough, tend to spall in thin layers. So, sealing everything properly will eliminate that problem..

In a warm climate, note that the harder of the two materials will generally fail ahead of the softer material due to thermal expansion and contraction.. First it will separate by density, then the smaller sections will start to crack.

Best of luck with it!

Peter Wieck
Melrose Park, PA