Our house has a patio of stone slabs (slightly irregular size, shape and
thickness - not rectangular concrete slabs). They seem to have been laid
on bare earth, as far as I can see from looking into the gaps, without
levering up a slab to look.

Some of the mortar "grouting" between the slabs has cracked and
disintegrated, so I'm trying to re-mortar the broken bits. I've removed
the broken bits and brush away as much of the powder residue that
remains. I'm using ready-mixed mortar from B&Q, with water added to form
a stiff paste which is thick enough to adhere to a palette knife so I
can direct it into the gap (typically 5-20 mm) and then tamp it down
with my fingers (wearing rubber gloves in case the cement causes skin
irritation).

I then use the hosepipe to spray a fine mist over the sections every few
hours (given that it's hot and sunny here) so it has enough water to
cause the cement to set.

But some sections never set: even after a couple of days the mortar can
be rubbed away in a powder of sand and cement. Other sections have set
rock solid. The colour of the dry mortar varies from slate grey where
it's set to almost white (maybe with golden sand granules on top) where
it remains powdery after several days.

I've tried various alternative techniques to the stiff-paste consistency:

- adding a bit more water to the mix so it's a bit more runny, and
letting it fall off the palette knife into the gap, building up the
level gradually and then tamping it down to make it roughly level with
the slabs

- filling the section with dry ready-mix and then spraying a mist of
water over to start the setting process: repeating the misting every few
hours

I've checked that all the dry powder is thoroughly wetted, to avoid dry
sections.

No technique seems to give better or worse results. Sometimes sections
set, sometimes they dry to a firm dust that can be rubbed or washed away.

I don't know what the proportions of sand and cement are - just the
standard mortar mix.

I'm making up small batches of a few hundred grammes of dry mix, mixed
in a plastic bowl, so as not to make so much that it has started to set
before I've finished the painstaking job of tamping the mortar into the
gaps.


Should I try soaking the ground through the cracks that I'm about to
fill, to prevent the water in the mortar leaching into the ground before
it has had chance to set the mortar?


I don't need it to be a perfect job, as long as there are no longer gaps
that weeds can grow through.