Concrete or Grouting between flagstone in walkway ? Advice needed

Hello :
I have a flagstone walkway (natural Vermont slate actually, that I scavenged from a local quarries slag heap). It is 5 feet wide and 20 feet long.
See http://www.shawn-and-lisa.com/PhotoAlbumOurHouse.htm for 2 pictures of it (the pics are in the first row, 2nd and 3rd pic across).
Anyways, the walkway has been in place now for 3 years. We first thought that planting the irregular gaps between the individual pieces of slate with moss and "steppable" plants creeping thyme etc would look really nice. At first it did. But now it's more grass and weeds than pretty, low growing plants and it's a pain to constantly weed it, so I'm thinking of mortaring or grouting or whatever between the pieces.
Having never done this before, I don't have a clue where to start. I've heard of dry grout mix made for this purpose that you sweep into the cracks (after digging them out a little of course) and then you wet it with a spray and it hardens. But, I'm not sure if mine is the right application for that. I live in Vermont, so it's exposed to severe winters.
Any advice on products/techniques would be greatly appreciated !
Thanks - Shawn
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wrote:

DON'T do it! Use sand. Frost will lift and break the grout and it will be awful.
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Sand is what I have now. The thing I don't like about it, is that weeds grow in the sand, ants dig it all up into anthills, and it's just generally unappealing and ugly now after I spent so long building it 3 years ago.
See http://www.shawn-and-lisa.com/PhotoAlbumOurHouse.htm for 3 pictures.
I thought maybe cement them in might look nice but I was wondering just exactly what cement/mortar/grout product could be used for this application.
Shawn

cracks
spray
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Grout will crack without a base with winter heave. The best way is dry set base and all, it gives you allot of work time for leveling and spacing. Wet set is a rushed job. Or blast out the seams with a power washer, 2500lb or more and see if you like it for a few years. Grout only will last a few years but when it fails its hamer time to get rid of the grout. Gravel will wash onto the stones, can be a pain, but if lowered down can be fair.
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