28
BCD Special Report on
Historic Churches
19
th annual edition
SaltCrystallisation
inMasonry
Heather Viles
W
hile salt
crystallisation in
porous building materials has long
been seen as an important cause
of deterioration problems, much remains
to be understood about how it works, why
it causes damage and how best to prevent
and treat salt problems. This article reviews
the current state of scientific understanding
on the problem and potential solutions.
Within the last 15 to 20 years numerous
research projects have investigated salt
crystallisation problems from theoretical
perspectives, as well as based on laboratory
experimentation and field monitoring and
observations. Only now are we starting
to be able to bring findings together
from disparate research projects, and to
provide a more complete explanation
of salt deterioration problems.
Salts and masonry
Some of the world’s most important historic
monuments, such as the Nabatean rock-hewn
temples at Petra in Jordan, the Harappan
remains at Mohenjo-Daro in Pakistan,
the Alhambra in Granada, Spain, and the
Sphinx in Egypt, are seriously affected by
salt crystallisation. Indeed, the problem
affects buildings and monuments around the
world, from hyper-arid desert environments
to Mediterranean climates and the cooler
and wetter conditions found in the UK.
In order to understand the problems
of salt crystallisation in masonry, it is
important to consider the nature of salts,
how they are delivered into the masonry,
and the vulnerability of the masonry itself.
Salts are ionic compounds formed
between positively charged ions (sodium,
for example; Na
+
)
but excluding hydrogen
(
H
+
)
and the negatively charged ions of acids
(
chloride, for example; Cl ‐ ). In the natural
and built environments many different
types of salts are found, including chlorides,
sulphates, nitrates and carbonates. Commonly
occurring salts are sodium chloride (table salt
or ‘halite’; NaCl), gypsum (calcium sulphate;
CaSO4) and sodium sulphate (Na2SO4).
The ions making up these salts may be of
purely natural origin or may be sourced from
de-icing salts and other compounds deliberately
applied to roads or walls, or they may come
from pollutants in air or water. Alternatively,
salts can come from within the masonry itself –
they are found, for example, in some mortars.
The reason salts pose a problem to masonry
is because they are soluble and can dissolve
and recrystallise, often within the pores of the
stone at the point of evaporation. Each salt has
different solubility characteristics, and some
are more problematic than others. Some are
able to take up water from the air, and some
can change structure as a consequence of their
hydration and dehydration. (For example,
sodium sulphate can occur as thenardite or
mirabilite depending on its hydration state,
and the metastable heptahydrate has also been
recently observed.) Some salts can dissolve
in the water they have taken up from the
air, through deliquescence (as is the case for
halite). In many situations salts are present in
complex mixtures and their behaviour in damp
masonry is, as a result, quite complicated.
Salts can be transported onto and into
masonry in many ways, such as by capillary rise
from groundwater and soil water, splashed from
run-off on nearby roads and other impermeable
surfaces, in rainfall and driving rain, in fog
and dew, and as sea spray. Many buildings will
receive salts from multiple sources. Whether
or not these salts pose a problem will depend
upon the aggressiveness of the environmental
conditions and the vulnerability of the masonry.
The nature of the environmental conditions
surrounding a building or monument, and those
inside a building, are crucial to influencing
how far moisture and salts penetrate and how
often dissolution/crystallisation and hydration/
dehydration reactions take place. Relative
humidity and temperature, and variations in
these over daily and annual timescales, are
particularly key environmental factors.
Notwithstanding the environmental
conditions surrounding and inside a building,
different materials will have different levels of
vulnerability to such environmental conditions,
Historic buildings and monuments affected by salt crystallisation include the megalithic temples of Hagar Qim, Malta (left) and Whitby Abbey, Yorkshire (right).
