I assume that there is a reasonable gap between the slabs? - about 10
mm. (If the slabs are butt jointed like you find on pavements with
concrete slabs then you can't do the following)
They need to be repointed with mortar, scrape out the old stuff and
repoint. I would do this with a wet mortar mix.. You can do it with a
dry mix brushed into the joints, but on already laid slabs with dry
joints I don't think it works as well.
Have a look at:
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