It is possible that the bag had been in store for awhile and had a
small hole or other damage which has allowed the cement fines to
start to set but without turning the whole lot hard. It's possible
instore staff had noticed some harness and bashed it to break up any
fine lumps. This is never going to be any use for a decent mortar.

Also, when mixed and a proper admix used it should give a nice
'fatty' mortar. If it doesn't, don't use it. If the soil is liable
to 'heave' in cold weather, a lime-based mortar might be better but
this needs weather protection while it achieves full strngth.

Also possible that the whole batch was poorly mixed by the store
supplier and has less cement than it should.

For grouting slabs, it must be protected from frost for at least
7 days.

On 31/05/2021 16:23, Brian Gaff (Sofa) wrote:
No he needs to bite the bullet and look under one and see what was done
properly. If its on soil, depending on the composition, round here its got
a lot of clay, the soil swells and contracts, as I found at a feeble attempt
at crazy paving some years ago!
That is enough to crack any cement between stones.
As for drying but not setting, is it the narrowest slits that dry out? I
would imagine that would be the case. Cement is not very strong unless under
compression and the less there is the less the compression and the more it
turns to just powder again.
Brian