--- [log in to unmask] wrote:
> Original: feebly lime mortar made from oyster shell
> (lab report) performed
> very well until hit numerous  times by lightning;
> and the clay base of the
> hill shifted (over 98 yrs)

Sounds to me like it needs a good lightning conductor
and new footings / foundation.

But since that's probably already documented, and what
we are looking at is the mortar, here is my offering:

1. Granite units need something flexible in the outer
20-50mm of the joint, because the skin of the granite
will expand more than the core due to thermal movement
of the surface.  If there are any hard contact points
in the outer portion of the joint, the outcome is
likely to be shell-shaped spalls off the face of the
granite.

2.  Granite units need sound, solid, support so that
they do not move differentially due to settlement.

On this basis, I'd think about different mixes - one
for the bedding of the blocks, to provide good
support, and one for the pointing of the joints, to
provide movement and waterproofing.

I think the bedding mortar probably doesn't need to be
that flexible, so the Type S or similar might be
appropriate.  However, don't get carried away by the
loads.  The mass of the column will exert a maximum
dead load of around 0.7 MPa (if my arithmetic is
right) on the mortar joint at the base of the column.
Since Michael has said that the lateral loads are to
be taken out by stainless steel dowels, then I would
have thought that even a weak lime mortar should be
able to accommodate loads of that magnitude!
Challenge would be, however, whether the lime mortar
would have time to develop sufficient strength by the
time the column is complete, given modern construction
timetables!!

If it were new construction, I'd be tempted to think
about an elastomeric sealant for the pointing, but
service life is ratshit, so probably not a good
solution.

Lots of stone buildings in Sydney had a masons putty
(lime, whiting, linseed oil, and maybe a fibre for
shrinkage control) sausage laid along the outside face
of the joint distinct from the bedding mortar ...
trouble is that it is now hard and brittle, so not
really working the way it should.

Making a lime mortar 'stick' in the joints as pointing
could be problematic, given the relatively low
porosity and surface roughness of granite.

I think, on balance, I'd be trying to look for a lime
mortar that is capable of generating a compressive
strength of 1.5-2.0 MPa within 21 days.  This is where
my knowledge of these things ends ... I might be
looking for the impossible.

From hereon, perhaps I'm stumped.

Cheers
david

http://greetings.yahoo.com.au - Yahoo! Greetings
- Send your Valentines love online.

--
To terminate puerile preservation prattling among pals and the
uncoffee-ed, or to change your settings, go to:
<http://maelstrom.stjohns.edu/archives/bullamanka-pinheads.html>