BCD Special Report on
Historic Churches
19
th annual edition
23
MatchingMortars
for Pointing
Claire Davies
W
ith so
many uses as bedding,
pointing and facing material,
mortar plays a critical role in the
structure and survival of historic buildings
and their component materials and features.
A pointing mortar fills the gaps between
building materials, providing support at the
surface and forming a continuous face to
help protect against the weather. It needs to
be strong enough to withstand weathering
but its role is ultimately sacrificial – that is
to say that, in the event of a problem such
as movement occurring, it is the mortar
that disintegrates, not the host material.
So, in the vast majority of cases the mortar
should be weaker than the host material.
Pointing needs to be repaired only
when its condition or absence means it
is failing to fulfil its protective function,
and replaced where its presence is actively
damaging surrounding materials.
Replacement and repair mortars should be
as similar as possible to existing or surrounding
mortars in chemical composition and physical
appearance; this ensures the best material
compatibility and visual continuity, and it
encourages similar weathering. However,
exceptions must be made where that means
reinstating a poor or unsuitable mortar. It is
therefore important to identify and understand
why a mortar has failed. Mortar analysis can
help establish the components of successful
mortars and help understand the reasons
for the failure of unsuccessful mortars so
that inherent faults are not replicated.
A mortar can fail to perform its function
for various reasons, and material composition is
just one of a number of factors to be considered.
A mortar which was originally well formulated
and applied can still struggle if it is not able to
fulfil what is now being asked of it; coping with
the effects of poor maintenance, such as ivy
growth or blocked and overflowing rainwater
systems, or when its requirements have changed
over the life of the building. When looking to
devise a replacement or replicate mortar, it is
important to look closely at a mortar’s suitability
for its present role, the current condition of the
building and its constituent materials. Further
to this, the quality and longevity of a mortar
is equally dependent upon the use of good
quality materials, correct specification, skilled
workmanship and sustained maintenance.
Centuries of practical evolution have advanced
the understanding of mortar technology and
all surviving historic pointing is the product
of the successful combination of all these
elements: as the saying goes: ‘it’s not good
because it’s old; it’s old because it’s good’.
AGGREGATES
A mortar is essentially composed of aggregate
particles bound together with a binder which
sets or hardens to create a sound matrix.
The main bulk of a mortar is generally
made up by the aggregate which is usually
an inert component (although particles of
some igneous rocks and certain ceramic
materials may encourage a slightly faster,
harder set due to their weak ‘pozzolanic’
effect). Aggregates found in historic mortars
include pit, river and sea sand, grit, subsoil,
crushed stone and stone dust, crushed brick,
clinker, sea shells, kiln slag and old mortar.
Constituting the majority volume, the
choice of aggregate not only provides much
of the visual character of a mortar but is also
essential to its performance, almost equal to
that of the binder, adding strength, assisting
carbonation (setting) and reducing shrinkage.
When looking to replicate a historic mortar,
it is often necessary to use a mixture of
aggregates to achieve a really good aesthetic
match and a good range of particle sizes
to ensure the mortar is strong. Although
washed sand has not been widely available
until recent times, it is generally preferable as
its use ensures that impurities such as salts,
clay, silt and loam (which inhibit the adhesion
between aggregate and binder) are removed.
Locally sourced aggregates often provide
the best geological and visual match but
substitutes from elsewhere can usually be
found for any aggregates which are no longer
available from the original source. For any
identified components which are unavailable,
hard to reproduce or are likely to be detrimental
to the performance of a mortar, an inert,
visually similar substitute can be used.
In the past, the limited availability of good
quality materials could be compensated for by
clever workmanship. Galleting for example,
which involves pushing shards of stone into
joints to improve the durability of the mortar
surface, is usually found in areas where the
local lime is weak. Through identifying and
understanding the available materials and
their component role and effects it was also
possible to make skilled adjustments to the
binder/aggregate ratio, and with the addition
of other additives, allowed effective mortars
to be created with very limited resources.
BINDERS
Portland cement, which was first patented in
1824
by Joseph Aspdin and improved by his
Mortar plays a key role in the appearance and durability of masonry, and must itself withstand extreme
weathering: correct specification is essential.
