Fwd: Re: Re: mortar for limestone blocks

[John Twilley <jtwilley@zzzzzzzzzzz>]

Marcus,

For low-tech work like most here will be concerned with, the best guides
to
serviceability with the local stone types and local weather conditions
are
likely to be some of the older masons who've had to live with their
mistakes.
the 7/1/1 ratio (by volume ?) doesn't seem excessively sandy for some
applications.  If you are making walls out of rubble where most of the
loads
are compressive you probably want the mortar in between not to be too
hard in
order to avoid "point loading", to act as a cushion in the event of
settling
and not to act like a fulcrum over which some stones would break.  All
stones
expand slightly when water-saturated and shrink slightly when dry but
some that
contain clays can do this more than others.  That is another reason to
want
something that is not too rigid.

If you are depending on the mortar to bond more strongly in a case where
there
might be changing loads from different directions (furniture) you
probably want
something with more cement.  But there's no point in making a cement
that bonds
more strongly than the sand is bound in the sandstone.  One month after
cure,
if breaking the joint between two pieces results in material pulling out
of the
stone instead of the mortar failing, there's no point in using more
cement
because the stone is the weaker link.

One way to increase the bond strength without increasing the overall
amount of
cement in the mix and creating an excessively hard mortar is to paint a
slurry
of cement alone onto the surface of the stone just before it is going to
be
set.

Another point to remember is that none of these materials will cure
properly
unless they are kept damp.  (You weren't going to set dry stones were
you?)  If
the pores in the stone draw the water out of the mix it won't cure
properly.
That's easier to avoid in a wall where the water content of the stones
will act
as a source of moisture even when the air is drying the mortar at the
surface.
But in a free-standing installation of just a few stones it can be
harder to
manage.  Masonry mortars with lime in them require free air circulation
as well
as damp because the lime component cures by picking up carbon dioxide
 from the
air over a much longer period.  But this process stops in dry
conditions.

Last, for outdoor work in climates where there are winter freezes, the
issue of
freeze-thaw damage can be a factor.  For commercial work there are
additives
that help to keep the air bubbles in the right size range to limit this
freeze-thaw susceptibility but for small projects, that's hard to deal
with.
Depending on the style of what you are making, you might consider a two
stage
operation where pointing mortar is used in the joints that would be more
weather resistant and be replaceable, while a softer mix is used for the
actual
setting of the stones.

John