In article ,
p.k. wrote:

The key to the method is to use *kiln dried ready mix* that flows into the
cracks and crevices packing just as well (possibly even better than) a semi
dry mix well trowelled down. A normal builders sand and cement mixed by hand
will not work and will behave as you describe. ( NB I am not talking about
block paving kiln dried but the bags of dry ready mix) I does work!

Totally unnecessary - but I agree about the normal mixtures not working.
Unless you collect from the kiln, the sand will have reached ambient
humidity by the time you buy it - those bags are NOT airtight. You
can trivially make it yourself, in the drier parts of the country and
in summer almost everywhere, by simply drying appropriate sand.

Builder's sand will produce a normal mortar. If you sieve sharp sand
though a kitchen sieve, you will get a fine sand that makes a very
porous mix and does not typically set hard. I use that, both because
it lets water through and because it is easy to remove if you need
access or to relay. You must sieve the sand when it is really dry,
and it is the right texture for using for mere sand filling of slab
or block joints.

You can also get there from builder's sand by washing it - but you end
up rejecting most of it.


Regards,
Nick Maclaren.