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Figure
1: Depositional routes to material surface. Dry deposition on a sheltered
surface forms a deposition crust. It can also occur on a surface that is exposed
to rainfall and run-off, but here the products of reaction are removed once rainfall
and run-off reaches the surface and move across it and into the material. In the
case of limestone both wet and dry deposition produce the same alteration; limestone
is converted to calcium sulphate (gypsum). The reaction through dry deposition
(direct gas to solid reaction) is about five times slower in producing this decay
product than the reaction via wet deposition (reaction in solution).
|
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| Cavernous
weathering in sandstone in a wall in Durham. Small particles at the base of each
cavern are eroded small flakes of stone mixed with salts. Repointing has isolated
each block and focused weathering processes to produce these forms. |
John
Evelyn noted that the noxious air of London was causing the discolouration and
degradation of buildings as early as the 1660s. Throughout the late 18th and 19th
century, painters and writers used the smog and squalid conditions of urban areas
to dramatic effect, providing an evocative image of the deleterious impact of
urban atmospheres. More recently, there have been attempts to understand the nature
of atmospheric pollution and its impacts on buildings in a more systematic and
scientific manner.
POLLUTANTS
‘Atmospheric
pollution’ usually refers to the element of the atmosphere which has been altered
by human activities. Changes in natural and polluted atmospheres can be highly
variable both across space and through time. In coastal environments, for example,
there is naturally more salt in the atmosphere than in inland environments. Salting
activities in cold spells, however, create artificially elevated levels of salt
in inland locations.
Some
of the major pollutants that effect stonework are carbon dioxide, sulphur dioxide,
nitrogen oxides and particulates such as smoke. Carbon dioxide is probably most
familiar as a ‘greenhouse gas’, contributing to global warming, but it also combines
with water in the atmosphere to produce carbonic acid. This means that natural
rainfall is a weak solution of carbonic acid with a pH of about 5.6 (pH 7 being
neutral and pH 0 being the most acidic). Even if carbon dioxide levels rise by
as much as the most pessimistic predictions, the increase in rainfall acidity
this gas will cause is relatively minor. It is the reactions of other pollutant
gases, such as sulphur dioxide and nitrogen oxides, that produce the more acidic
rainfall. Sulphur dioxide reacts with water in the atmosphere to produce sulphurous
acid (H2SO3). This is what is popularly referred to as ‘acid rain’. Acidities
of rainfall can commonly be reduced to pH 4.5 or, in the extreme, to pH 3.5 by
this pollutant. Nitrogen oxides can produce a similar reduction in pH by the formation
of nitric acid, but these oxides have tended to be harder to detect, especially
in terms of their impact on stonework.
The
above gases are produced by industrial, commercial and residential activities
in urban, and increasingly, in rural areas. Carbon dioxide, as has been well documented,
is a by-product of much industrial activity and is almost uniform in its distribution
around the UK. Sulphur dioxide was a major by-product of the electricity production
and domestic heating using coal. Up to the 1950s and the great London smogs, domestic
use of coal for heating produced a very concentrated and confined blanket of sulphur
dioxide and smoke over major urban areas. One of the effects of this was sulphation,
a chemical reaction between the masonry surface and the atmospheric sulphur dioxide,
evident on limestone (calcium carbonate) as a black crust of dirt, sulphates and
stone which, in the worst cases, cause the surface of the stone to peel away.
Increasingly stringent regulations, such as the Clean Air Acts of the 1950s, have
greatly reduced the amount of sulphur dioxide and smoke in the urban atmospheres
of the UK. The effects of a long history of high sulphur dioxide and smoke levels
are, however, less easy to remove from the stonework of the UK.
Nitrogen
oxides, like sulphur dioxide, are produced by exhaust fumes and include a family
of pollutants, including ozone, resulting from the reaction of nitrogen oxides
in urban atmospheres.
There
can be a great deal of variation in the amount of pollution found in any particular
part of an urban area depending on the local industries, other pollutant sources,
topography and climate. This means that the contemporary concentration of pollutants
can vary greatly over an urban area. Likewise, transportation of pollution beyond
the area of its production can increase pollution levels in seemingly clean areas.
In the UK this has meant that rural areas do not represent a pristine environment
and cannot be used to gauge the natural, background level of pollution.
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| Figure
2: Example of factors influencing the interactions between pollutants and
materials. |
CLIMATE
CHANGE
Pollutants
such as carbon dioxide are also having an impact on the general climate. Although
a 2-5°C rise in temperature and a rise in sea-level are predicted over the next
50 years, these general figures hide a great deal of regional variation in climate
change. It is likely that southern Britain will experience a more Mediterranean-like
climate, with increasing periods of drought and dry weather, whilst the distribution
of rainfall is likely to become more seasonal. Similarly, the frequency of ‘extreme’
climatic events, be they droughts or storms, is likely to increase. Such changes
are likely to affect building degradation, but in ways currently difficult to
predict.
The
duration and intensity of driving rain, for example, is likely to increase as
extreme wind speeds increase along with storminess. As well as increasing the
penetration of rainwater into a building, there may also be an increase in the
amount and location of runoff generated during rain events. Increased driving
rain may mean that previously sheltered areas experience runoff for the first
time, removing any sulphation crusts. Likewise, increased penetration of moisture
may activate salts passively accumulated and stored in the building material.
Once activated, the salts cause harm by migrating, in solution, to the point where
the moisture evaporates. Here they accumulate and crystallise. If the crystallisation
takes place within the pores of the masonry, crystal growth can exert enormous
pressure on the walls of the pores, causing the stone to crumble. Thus the ‘memory
effect’ of salts acquired either in the formation of the stone or subsequently,
can produce accelerated rates of weathering without any change, or even with a
decrease, in the pollutants in the atmosphere. Contemporary weathering is thus
dependent on the constituents of the building materials and the past weathering
history of the material, as well as on current pollution.
POLLUTANTS
AND MATERIALS
Materials
and pollutants interact in a complicated manner. Dry deposition, the direct deposition
of gaseous pollutants onto a surface, varies with factors such as wind speed,
building orientation and relative humidity. For sulphur dioxide, for example,
the reaction of the gas with limestone is enhanced if relative humidity is above
80 per cent. Wet deposition, the delivery of gaseous pollutants to a surface via
their incorporation into water, varies with factors such as the geometry of the
surface. Where a surface is exposed to rainfall and runoff, then wet deposition
can occur and chemical reactions such as dissolution can take place. The interaction
between pollutants and materials highlights the importance of specific physical
properties for the vulnerability of materials. A matrix of calcium carbonate or
a calcium carbonate rock is highly susceptible to reactions with acid solutions.
Porous materials, whatever their chemical composition, are likely to be vulnerable
to degradation by acid solutions, as their large pore volume provides a large
surface area for chemical reactions. Similarly porous or fractured material will
also be susceptible to the actions of salt, as salts can penetrate into confined
spaces where their expansion can exert great stress upon the material. Lastly,
the presence and movement of moisture within a material, facilitated by high porosity,
can enhance and alter the concentration of weathering agents and aid their damaging
activity.
Glass
can undergo chemical alteration in polluted environments, although this may not
be visually obvious at first on modern glass. Sodium can be leached out of the
surface layers of modern glass in atmospheres rich in sulphur and carbon dioxide.
Medieval glass can be leached of its potassium and calcium content, weakening
the structure and durability.
Metals
can also be chemically altered within a polluted atmosphere. The degree of alteration
depends to a great extent on the presence of moisture (and how long the metal
remains wet), the concentration of pollutants present and the supply of oxygen
to the reacting surface. If the supply of oxygen is cut off by the formation of
a layer of weathering products, the reactions can slow down dramatically. Oxidised
layers may actually form a protective barrier to the further alteration of the
metal surface. If these layers become wetted, however, there may be a rapid electro-chemical
reaction, enhancing alteration.
FORMS
OF WEATHERING
Three
sets of interrelated factors determine how materials interact with atmospheric
pollutants and the form of decay; material, environment and process. Unfortunately,
this complex set of relationships means that it is often difficult to establish
a clear and distinct one-to-one relationship between pollutants and decay forms.
It may be that the same pollutant can produce different decay forms depending
on the specific circumstances of interactions, or it may be that the same form
can be produced by a set of different pollutants. It is also important to note
that some reactions will only occur if the level of pollutants or an environmental
parameter crosses a critical threshold. Salt weathering, for example, may not
occur unless pore spaces are filled and stress can be exerted on the pore walls.
Classification
schemes for the various forms of decay are varied and often assume that forms
are produced by specific processes. Visually, a simple distinction can be made
between areas of a building washed and unwashed by rainfall and runoff. Washed
areas can be dissolved by rainfall and runoff, and localised dissolution can be
produced where small gullies or runnels form. Rain-washed surfaces often show the
effects of dissolution by having a roughened surface, in some cases such as marble,
a sugary feel to the surface. Sheltered surfaces, particularly where calcium carbonate
is present, can produce sulphation crusts, blackened by the incorporation of carbon
particles into the crust. The location and extent of these forms depends upon
the configuration of the building and how heavy the rain is and how often it falls.
As climate alters, the location of these sheltered areas could change.
In addition to simple dissolution surface and crusts, a further series of weathering
forms are those produced by the action of salt within the material. Efflorescence
of crystals on a surface may indicate the presence of salts, such as sodium chloride
or calcium sulphate, but need not indicate that these salts are causing damage
to the material. If salts move rapidly through a material to the surface they
may not exert sufficient stress to cause damage. Salt crystallisation within constrained
volumes, such as pores, can exert great stresses within the material and result
in its breakdown. The form of this breakdown can vary and produce forms such as
flaking, blisters and honeycomb weathering.
Weathering
forms, as noted above, need not be dependent upon current pollutants for their
development. Once a blister is produced on a surface, for example, that surface
is weakened and salt migration enhanced within the blister. The weathering form
may follow a sequence of development independent of any change in atmospheric
pollution levels. Identifying these weathering pathways is still very much a case
of experience rather than systematic analysis.
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