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wrote:
Lime or cement mortar?

Phew, lot of feedback on this one! I guess there is a fair bit to
discuss. Maybe in time we could put together an FAQ on it, once we've
talked about the many points raised.

There is a whole lot to write about lime, and I'm not trying for a
book. Just wanted to put the hydrated lime mortar topic up here for us
to discuss, as its one thing I havent really seen addressed much here,
it is a question that comes up fairly regularly, and we dont seem to
have any real concensus so far, and no FAQ on it AFAIK.

So I can add a few more points to the discussion...

Re adding lime to cement mortars, this doesnt give any of the
properties of a lime mortar. Also most lime/cement mixes are liable to
fail prematurely, so adding a bit of lime to cement mortar or adding a
bit of cement to lime mortar are both not recommended. There has been
research done on this.

The chemical cycle of cement, I dont know that much about it, only that
its made from lime and clay, and takes a lot more heating energy to
produce than lime.

Re higher transport costs for lime putty, 2 points. First, transport
costs for ready mixed cement mortars will presumably be similar.
Second, I'm doubtful that transporting lime putty is likely to become a
significant part of building repair practice. As far as I can see it
seems to be an excessively priced product with no real need. Bagged
hydrated lime is a fraction of the price, and the logical option.

How lime mortar is fatter: Cement mortars are sand plus a thin liquid
with very little adhesion while wet. Lime has more stickiness, so the
mortar holds together better during working, and is easier to mould
into shapes that will keep their position. Hence less chance of snots
and waste.

Re lime and cement ceilings, there are plenty of cement ceilings
around, either cast in situ or fibre cement board. In time we'll see
how long they last, but possibly judging lime life by ceiling life is
unfair in that Victorian lime ceilings are normally supported by very
thin laths and nails and hung on flexible joists, quite unlike their
cast cement counterparts. And it is these lath supports failing that is
often the cause of lime ceilings starting to break, and that cant
fairly be blamed on the lime itself.

Re rain after application, last time I used lime outdoors it rained
when I was done and not a drop was washed away. (This was lime paint
not mortar.) I turned the hose on a little bit of it and found only a
pressurised jet (finger over hose end) would remove it, so its
vulnerable at first to heavy rain, but it was fine in the more common
light rain. I think the usual advice is check the forecast, and dont
use it before rain. One of its downsides, which I didnt really get into
the first time.

There are various lime mortar additives for use when the mortar
properties need modification. Examples are pozzolans, stone, hair &
other fibres, and linseed. There is also black mortar, and when my
brain returns I may remember whats in it. I think there are others that
get used too.

As usual the OP wasn't *discussing* anything, just delivering another sermon.

You can join in the discussion if you have something to add, or not,
upto you. Not much point just whimpering.

NT