Lime or cement mortar?


This is specific to old buildings, and walls of similar construction.
These houses typically use soft bricks that break easily, often have
little in the way of foundations, sometimes none, and most have no dpc.



Movement
--------

Skimpy foundations mean minor movement is a normal event for many old
houses.

Cement is not movement friendly, and with any wall movement cement
typically makes a single clean break. It has no self healing ability.
Normal wall movement thus results in broken walls, which compromise the
overall house structure, and require extra repair work.

Cement mortar is stronger than soft brick, so when movement occurs it
is the bricks that will break rather than the cement. Broken bricks
have their core exposed, and without the protection of the fireskin
these bricks will usually begin to slowly deteriorate due to wet freeze
cycles. Gradual erosion of the brick leaves the wall in need of many
bricks being replaced. Its a shame to see walls like this, knowing that
just a little more knowledge and no damage would have occurred. Theres
a building near here that has about half the bricks near ground level
badly decayed, and is now in need of large numbers of bricks replaced.

Lime mortar is weaker than the soft bricks, so when movement occurs it
is the lime that cracks, not the bricks. This is the better option. No
bricks need replacement.

When moved to breaking point, instead of forming a single break, lime
tends to form lots of microcracks. Lime then reacts with the CO2 in the
air to grow hard crystals across these microcracks, and thus rebonds
itself. It self-heals. Lime mortar is not flexible, but it behaves as
if it were in this way. Lime accomodates normal minor movement without
incident.


Damp
----

Old houses handle damp differently to new builds. Soft bricks are
porous, and rain hitting them soaks in a little. Interior condensation
can also be a source of dampness in walls. Old buildings handle these
sources of damp by evaporation, which, if all is well, keeps the level
of damp below the point where any problems can occur.

Impurities or salts sometimes have to be got rid of as well as water.

Cement does not give much evaporation, and does not draw water from the
bricks, so with cement mortar almost all evaporation occurs through the
brick face. The problem is with salts: salt deposit crystals break the
faces of soft bricks. When this happens, the brick loses its protective
fireskin and begins to slowly disintegrate due to wet freeze cycles.

Lime mortar evporates water, and draws water from the bricks. This
saves the bricks from damage. When salt depositing occurs, the lime
mortar sacrifices itself instead, and any mortar damage is dealt with
by repointing on a normal timescale. No extra work is required and no
damage occurs.

The greater evaporation of lime also means a drier wall, and less
chance of damp problems.


Brick damage
------------

As well as the causes of brick damage above, cement adds one more. When
cement eventually breaks up and detaches from soft brick (or stone) at
the end of its life, it frequently pulls the skin off the brick with it
along the edges. This causes rounding of the brick edges, with the red
core exposed. It doesnt look good, and it is the start of slow
deterioration of the bricks due to wet freeze cycles. Over decades
cement work can do great damage to such walls. In the worst cases this
eventually leads to the need to rebuild with new bricks.

Lime does not cause this problem, and the wall stays in good health.


Environment
-----------

Lime requires lower temperatures and less energy to produce than
cement.

Used lime is chalk, a normal component of soil, thus use of lime leaves
no ecological footprint behind, causes no environmental waste problems,
and needs no disposal. Cement OTOH is a harmless bulk that needs to be
disposed of. Landfill is in limited supply, and causes its own eco
issues.

Lime sets by reacting with CO2 in the air, and is manufactured by
reversing this reaction using heat. The chemical cycle of lime is thus
CO2 neutral.


Workability
-----------

Lime is slow setting, meaning a single batch can be used as long as you
like. If you need to store it until tomorrow, this can be done by
putting it in an airtight container. In centuries past all the lime
used in a house may have been made in a single mix, and buried on site.
It stayed there throughout the construction, being used as needed. Lime
only sets when exposed to air.

Lime is much fatter than cement, making it nicer/easier to work with,
and reducing waste snots.


Cost
----

Hydrated bagged lime is around £6 a bag, cement around £3 a bag. The
lime bag however is larger as its lower density.

Lime is usually used as 3:1 mix by volume of sand to lime putty. (Lime
putty is lime mixed with water to a paste and stored.) It can also be
used straight from the bag as a 3:1 mix.

Bagged lime expands when water is added to make it putty, but by how
much I dont remember. Due to the various uncertain factors I dont know
which is cheaper, but I expect the initial cost difference is either
none or small.

Victorian properties with their original lime ceilings are still the
norm. They may be getting tatty but the fact that theyre usually still
there after 100-200 years says a lot. I dont have any hard data on life
expectancy of lime vs cement, but given this I expect lime may last
longer, and thus work out cheaper and less hassle in the long term.


Appearance
----------

Cement is a nice ugly heavy grey.

Lime is white, and lime mortar takes on the colour of the sand used to
some extent. Golden sand makes white mortar, red sand makes pale pink
mortar. In its most basic form lime mortar looks prettier, but it also
gives scope for further decoration. It is simple to include aggregate
to get black flecks in white mortar, or any other colour stone fleck.
Also lime mortar can be dyed any colour if wanted, since it starts out
white.

FWIW there are also various other materials that can be included in
lime mortar such as crushed bricks, tiles, cinders, shingle and so on,
all of which produce a decorative effect. While these can be included
in cement mortars as well, the heavy cement colour covers them and
makes them non-attractive.

Lime weathers by slow wear. This means it very gradually sheds its
surface over the years. This means surface dirt is carried away over
time. This self cleaning effect is very slow, but in any given
situation lime will end up less dirty than cement after many years.


g'nite

NT