BCD Special Report on
Historic Churches
19
th annual edition
29
depending on the nature of their porosity
and permeability, and the chemistry of the
materials. These factors affect how easily water
(
containing dissolved salts) can penetrate into
the masonry, and how it may interact physically
and chemically with the building materials.
Building stones with a high percentage
of small pores (<0.5 microns) as well as
some larger pores, have been found to be
particularly susceptible to salt crystallisation
damage. These include many sedimentary
rocks (such as some limestones and
sandstones) as well as some granites.
Howdoes salt migrate
and crystallise?
One of the problems associated with
understanding the migration and crystallisation
of salts within porous materials is the difficulty
of seeing what is going on inside a wall. In
recent years a number of techniques have
been developed which permit visualisation
of moisture and salt movements within
porous materials, both in the laboratory
and in the field, such as nuclear magnetic
resonance methods (NMR) and resistivity-
based methods. Furthermore, microscope
techniques such as environmental scanning
electron microscopy (ESEM) allow real-time
monitoring of phase changes in salts as a
result of changing environmental conditions.
These methods have developed hand in hand
with new computer modelling techniques,
together providing excellent ways to test
and develop theoretical explanations.
Water laden with salts can move through
masonry in several different ways, depending
on the wetting and drying conditions. For
example, in unsaturated or partially saturated
conditions, salts migrate as a result of capillary
processes. Many old walls without damp-proof
courses experience clear capillary rise from
groundwater. Recent work by Professor Chris
Hall (University of Edinburgh) and colleagues,
as part of a Leverhulme Trust-funded project
on climate change and moisture regimes in
stone monoliths, has demonstrated that both
the height of capillary rise and the volume
of water moving through the wall are related
to the evaporation conditions that the wall
experiences. Higher capillary rise is found
in low evaporation situations, but higher
volumes of water are pumped through the
wall in higher evaporation environments.
In saturated conditions, salts may migrate
through masonry by ionic diffusion. Dr Stephen
McCabe (Queen’s University Belfast) has
recently been experimenting on this process as
part of the ‘Greening of walls’ research project
funded by the Engineering and Physical Sciences
Research Council (EPSRC). He has found that
halite moves more quickly than gypsum through
saturated sandstones and that rates of migration
vary when mixed salt solutions are involved.
Dissolution and crystallisation occur
when the amount of salt changes relative to
the capacity of the water to hold dissolved
salt. Solutions are described as ‘saturated’
when they contain the maximum possible
amount of dissolved salt for the environmental
conditions. Changing environmental factors
such as temperature and pressure can
create ‘super-saturated’ solutions which
can rapidly lead to salt crystallisation.
For example, evaporative drying reduces
the amount of water present meaning that
less salt can be kept in solution and some will
crystallise out at the point of evaporation,
usually at the surface of the masonry.
Cooling can also be important as sodium
sulphate, for example, becomes less soluble as
temperatures decrease, causing crystallisation
without any change in the volume of water
involved. In this case, crystallisation may
occur within the body of the masonry.
Howdoes salt damage masonry?
Over time, salts migrate into porous masonry
materials and start to clog pore spaces. Where
pores become filled with salt, repeated cycles
of dissolution/crystallisation and sometimes
also hydration/dehydration within pores
will lead to the imposition of considerable
stress on the surrounding pore walls.
Considerable scientific debate has raged
over how salts crystallise and how breakdown
of the surrounding material occurs. Recent
theory suggests that continued growth of salt
crystals in pores requires the presence of a
thin aqueous film between the salt crystal and
the pore wall. This film of liquid is caused by
what is known as ‘disjoining pressure’, or the
repulsive force between crystal and pore wall.
Different salt types have different crystal
forms (including cubic and needle-like)
and some may be more effective in causing
deterioration than others. For example,
experimental work by Professor Carlos
Rodriguez-Navarro and Dr Eric Doehne
published in 1999, showed that rapid
evaporation caused highly supersaturated
sodium sulphate solutions producing
irregular-sided (anhedral) crystals of mirabilite
which were found to be highly damaging.
Salts can also cause damage to masonry
through other processes, for example
corrosion of any iron cramps or bars within
walls, or chemical weathering of susceptible
minerals (as salt solutions are often highly
alkaline). Silica becomes highly mobile at
pH values greater than nine and thus many
sandstones may suffer partial dissolution of
silica cement under saturated and salt-rich
conditions where the salts are highly alkaline.
The presence of hygroscopic salts within
masonry (those which can absorb water vapour
from the atmosphere) can also accelerate
chemical and frost weathering processes by
increasing the amount of water present.
Diagnosis
How might one diagnose salt deterioration
problems in a historic building? Often,
the presence of salts within walls can be
detected through the occasional formation
of salt efflorescences on the surface.
Salt crystallising out as efflorescences on
the surface of walls causes no direct damage
itself (as it is not crystallising out in confined
pore spaces), but is symptomatic of the presence
of salts within the walls. Efflorescences may
2
D resistivity survey techniques in use at Byland
Abbey, Yorkshire provide non-destructive monitoring
of moisture regimes inside the masonry.
Salt efflorescence on replacement limestone block within a salt-affected wall in central Oxford
