"NY" wrote in message
o.uk...
On 30/05/2021 17:48, NY wrote:
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 forgot to say: the mortar is brand new and the bag was only opened a few
days ago and has been kept in a dry garage.

It's made with grey Portland cement, not white lime.

Another thing I forgot to say. Where I've dug up the loose mortar from when
the patio was first laid (about 10 years, according to neighbours), it's
been a thin crust, in some places only about 5 mm thick, with a large void
below it down to the level of the base on which the slabs have been laid. Is
this normal? I'd have thought the whole depth of the crack between slabs
would have been filled with mortar.