On Saturday, 29 July 2017 19:00:07 UTC+1, Roger Hayter wrote:
Dave Plowman (News) wrote:
In article ,
newshound wrote:

Sounds like a plausible idea. The other thing you might think about is
ways to improve the keying. For example you could drill some holes in
the face to be repaired, insert plastic wallplugs, and then put in some
screws protruding a certain amount from the surface. I think I would use
stainless steel screws.

That was in my mind too. One thought was to cut the coping part off with
an angle grinder and cut down one of those readily available cast ones to
the correct length. And fit it to the original stone in some way as well
as with the mortar, like screws.

Sadly, there's nothing in the instructions about what metal is best with
lime. SS would be easy if it is good.

Are you talking about pure lime mortar, or one containing some cement as
well? My suspicion is that while simple lime/sand is fine for repointing
and filling cracks, you might want something a little bit stronger here.

It's the stuff Conserv supply ready mixed to repair this sort of stone -
by their website. In what little I've read they caution about adding
cement - ever.

More than a small proportion (exact figure unknown to me) of cement
turns it into relatively-weak well plasticised cement mortar.
Especially it gives it the undesirable property of being impervious.
But it is somewhat mechanically stronger.

that's true when the cement strength equals/exceeds lime strength. At 5% cement it's not, the cement just enables a quick (weak) set. The reason most cement/lime mixes are deprecated is that ratios other than 1:1 have been found to be at risk of premature failure in practice. The other reason is that as you say non-low cement content changes the properties of the mortar for the worse.

If adding 5% makes the difference between getting the job done or not I'd do it, and accept the degree of risk. If you can get it done a better way, great. Adding 5% is an often accepted tradeoff.


NT