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T W E N T Y T H I R D E D I T I O N
T H E B U I L D I N G C O N S E R VAT I O N D I R E C T O R Y 2 0 1 6
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SERV I CES & TREATMENT :
PROTEC T I ON & REMED I AL TREATMENT
4.1
materials. Successful repairs rely on matching
the properties of the repair material to those of
the existing historic fabric as closely as possible.
However, an additional set of requirements is
often imposed by the environment, adding a
degree of complexity to the process. Mortars
used in areas of high exposure are subjected
to more aggressive weathering agents such
as strong winds, wind-driven rain, frost
and percolating water. Specifying mortars
for pointing at high level (for example in
wall-heads, parapets and chimneys), where
these extreme conditions are encountered
more frequently, poses more challenges. The
problems posed by environmental conditions
should be considered at the specification stage
and, in most cases, the design of a mortar can
be modified to take account of the specific
challenges posed. There are a number of
features of mix design and preparation that can
be considered in tackling the specific problems
encountered in high exposure environments,
and these are outlined below.
AVOIDING FROST DAMAGE
Exposure of mortars to low temperatures and
frost can lead to freeze-thaw damage, causing
the mortar to fail or ‘burst’. Carbonation
of lime mortar is key in minimising frost
damage. Carbonation and drying reduce pore
water and promote the formation of a network
of pores which gives the mortar a level of
resistance or tolerance to the expansive forces
imposed during freezing conditions. Where
environmental conditions inhibit carbonation
(such as in very wet areas) the use of a
hydraulic lime is more appropriate. Hydraulic
mortars have a more rapid strength-gain
achieved via the hydration of calcium- and
aluminium-silicates.
Whether using hydraulic or non-hydraulic
limes, mortar resistance to frost damage can
be enhanced by several ways, such as the use
of a calcitic aggregate or a hot lime mix, or
simply by controlling the temperature while it
carbonates.
Calcitic aggregates
The use of calcitic aggregates such as
crushed limestone and oyster shell has been
shown to aid carbonation. These materials
act as seeding agents in the mix providing
nucleation sites for the precipitation of calcite
crystals, promoting the carbonation process.
The purity, crystallinity, particle size and
quantity of seeding agents all impact their
performance. Finely ground pure sources of
calcium carbonate (substituted for aggregate)
in concentrations above six per cent have been
found to positively impact carbonation and
strength development.
Hot lime mortars
Making a mortar by mixing water and
aggregate with quicklime, rather than with
previously slaked hydrated lime, produces
an exothermic reaction that generates heat.
The combination of heat and high alkalinity
experienced during the mixing of these
hot lime mortars is believed to produce
a more durable mortar. Microstructural
changes seen in these mortars are likely to
be beneficial, enhancing its durability. In
particular, mortars which have been laid
shortly after slaking have been shown to have
more interconnected voids (see petrographic
image above left), perhaps as a result of the
steam. This void structure would increase
the mortar’s tolerance to frost damage. It has
also been speculated that the combination
of heat and alkalinity leads to an ‘etching’ of
the surface of aggregate grains, creating a
stronger bond than would be achievable in
mortars mixed with previously slaked lime,
although this remains unverified.
Temperature control
Protection of mortar following application
can play an important role in preventing, or
at least minimising, freeze-thaw damage,
allowing the mortar to cure sufficiently before
it is exposed to harsh conditions. The use of
lime mortars in temperatures of 5ºC or below
is not recommended and following application
it is advisable to cover mortar with layers of
hessian and tarpaulin (or other protective
materials). Heated scaffold systems can provide
a solution to the problem of frost damage but
this is not always a practical or economically
viable approach. Where temperatures are not
controlled adequately, it could also lead to other
problems such as the mortar drying too rapidly.
EXPOSURE TO DRIVING RAIN
Mortars used for pointing at high level are
more exposed to rainfall than those at a
lower level which are better sheltered from
the elements, particularly where they are
protected by drip detailing. Furthermore,
projecting elements such as chimneys and
parapets have more than one face exposed.
The continuous percolation of water through
mortar will lead to the dissolution and
re-deposition (or ‘re-precipitation’) of lime
binder (illustrated above). Although this
usually occurs on a small scale it can result in
a material that lacks uniformity, being weak
and friable in some areas and much denser in
others. When the binder re-precipitates, this
can partially block pores (see petrographic
image above right) and is likely to reduce the
breathability of the mortar. On a larger scale
this can lead to the washing out of wall cores
and building collapse.
The susceptibility of a mortar to
binder leaching is dependent largely on its
composition. Calcium is present in lime
mortars in a number of forms: calcium
silicates (the primary hydraulic components
of natural hydraulic limes), calcium carbonate
(carbonated lime) and calcium hydroxide
(uncarbonated lime). The calcium in silicate
minerals is insoluble and is not affected by
leaching. Calcium carbonate and calcium
hydroxide, however, are both water-soluble
and as such can be leached (the lime ‘available’
for leaching from a binder is called free lime).
The leaching of calcium carbonate is a slow
process due to its relatively low solubility but,
where the right conditions exist, over time it
does occur. The solubility of calcium hydroxide
is much higher, which is why uncarbonated
mortars are more susceptible to leaching.
Where mortars are likely to be subjected
to percolation of water (for example in wall
heads and parapets), it is advisable to use a
binder with a low free lime content. However,
free lime does provide lime mortars with
positive performance attributes such as
flexibility and small-scale self-healing (on
very long timescales), so there is a balance to
be struck when selecting binders. Binders of
higher hydraulicity typically have lower free
lime contents, but variations do exist between
Petrographic thin section of a hot lime mortar with high porosity (indicated by
blue areas): note the partially slaked lime inclusion (left and centre)
Leaching of lime binder due to percolation of water, which has also resulted in
greening of masonry (Photo: Historic Environment Scotland)
Intra-inclusion
porosity
Lime
inclusion
Binder
Pore
Aggregate