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In article ,
(N. Thornton) writes:
"Toby" wrote in message ...
N. Thornton wrote:
1:1:6 is very common, but has been widely criticised by the experts,
since the lime content causes thr cement to not checmically set
properly, and the cement clogs the pores preventing the lime going off
properly as well. The result is regular failures. But its still
commonly used. I expect it'll take another 10 years for the knowledge
to permeate.

So what would you suggest, or an expert recommend?

I'm no expert on this, I've just read a few papers on it, so I'd stick
my head in the sand and suggest asking an expert Seriously, when I
read it I was boggled with the complexity of what I thought was such a
simple thing.

It sure isn't simple, and neither is it the case that it's
completely understood. I used to have a neighbour who taught
brickwork, and his comment was that we're discovering what
we don't know about cement based mortars faster than we're
working how to use them properly. We know (or at least used
to know) much more about lime based mortars, since we've been
using them at least 200 times longer.

However, I recall reading somewhere a few years ago (possibly
in this newsgroup) that the BRE had experimented with various
cement/lime mixtures, and the 1:1 was quite good. Any less lime
and the mortar loses all the movement and self-repair qualities
of lime, so you'd be back to the problems you get repointing a
lime mortar house with cement. I think there were problems with
mixtures containing more lime too (until you get to pure lime
of course), but I can't recall what that was.

I did discover one thing though, which is inclusion of 1% plastic
fibres increases longeveity by controlling cracks and increasing
strength under tension. The Victorians knew this principle, and it was
standard practice to include horsehair in their mixes because it
extended mortar life significantly.

I've not come across it in mortar, but it's certainly used in
scratch coat lime plaster. This stops it cracking as the lime
mortar walls move around. The dot-n-dab plasterboard fixing
is loaded with plastic fibres too to give it tensile strength.
The tutor on my plastering course demonstrated how effective
that is by hanging his weight on a piece of board fixed up with
the stuff, and it showed no sign of giving way.

I wish I could give us all a better answer. I still dont know what to
repoint the house with.

Silicone bath sealant? ;-)

--
Andrew Gabriel