BCD SPECIAL REPORT ON
HERITAGE RETROFIT
FIRST ANNUAL EDITION
31
HOME AND DRY
Developing a non-hydraulic setting air lime for the insulation
and repair of traditional buildings
HARRY CURSHAM
A
LMOST ONE in four buildings
in the UK are traditionally
constructed using lime rather than
cement. If well maintained, solid walls of
brick or stone set in a lime mortar work
well, drying rapidly after a rain shower so
damp never penetrates far into the wall.
Condensation inside the building also
dries quickly, so the walls act as a buffer
for both humidity and heat, moderating
extremes in the building.
By the end of the first world war lime
technology had largely been abandoned
in favour of faster-setting mortars. It
was only in the late 20th century that
conservationists began to realise that
these cementitious mortars were actually
damaging traditionally constructed
buildings. Traditional mortars were softer
and tolerated the natural expansion and
contraction of solid masonry without
failing. And they were highly permeable.
It was discovered that problems occurred
when old masonry was repointed, as
this introduced just a thin layer of hard
cement at the surface alone. As the core
remained flexible, even modest thermal
movement could cause the surface to
spall, as pressure is exerted across the face
of the wall. Being relatively impermeable,
cement also prevented the mortar
from wicking moisture to the surface.
In particular, the cement renders used
tended to trap moisture, and if cracked,
more moisture is drawn in by capillarity,
making the walls cold and damp.
Over the course of 60 years or so prior
to the lime revival, traditional methods
of making and using mortars were
forgotten. Text book descriptions were
often ambiguous, and a new generation of
conservators had to rely on trial and error
and on the analysis of old mortars. Today,
new discoveries are still being made.
For the retrofit sector these
developments are important because
damp walls are known to leak up to
30 per cent more heat than dry walls,
and in some cases the actual figure can
be far higher. Simply by getting all our
traditionally constructed buildings up to
a sound condition would help reduce the
UK’s carbon emissions substantially.
The types of lime used generally fall
into two categories: non-hydraulic or ‘air’
limes which set very slowly by a chemical
reaction with carbon dioxide alone; and
hydraulic limes which stiffen more quickly
due to a partial crystallisation set. One
area of great interest is in the development
of non-hydraulic hot-mixed mortars,
because they seem to be producing
mortars which are much closer in nature
to those found historically and there is a
growing consensus that these mortars can
provide the best performance in use.
While their slow set means that
air limes require more care and skill in
use, they tend to be more permeable,
and some hydraulic limes have been
shown to become almost as hard and as
impermeable as cementitious mortars
when aged. Mortars made with air limes
also offer lower conductivity and are
therefore a good insulant, all of which
make these mortars ideally suitable
for older solid wall buildings. These
properties are also significantly enhanced
when used with appropriate aggregates,
and the final result is akin to most pre-
industrial mortars found in the UK and on
the continent.
This article looks at one proprietary
product which has recently been
developed. Although a form of non-
hydraulic calcium hydroxide, it is supplied
as a dry powder and when water is added
the mix stiffens to provide a preliminary
set without any addition of setting agent
or pozzolan. The set is referred to by
its manufacturer as a ‘Vivus’ set, which
results from the way that the quicklime
is manufactured and slaked. There are no
clay impurities in the limestone used to
make it, and none are added, so in essence
it remains a pure air-lime and within
the normal ‘lime cycle’. Carbonation is
a secondary setting process in that it
is unnecessary for construction work
to proceed, but adds strength in the
long term. Like a hydraulic lime, it
will continue to carbonate over the
following months and years, depending
on the depth of material. In the process
carbon dioxide is absorbed from the air,
completing the lime cycle (see Figure 1).
This non-hydraulic setting mortar is
able to perform in the application stage
A traditional roughcast lime render on a solid masonry wall