Soluble Salts in Masonry
|The face of this sculpture, carved in the 1830s, exhibits decay typical of magnesian limestone by salt crystallisation in a polluted urban environment.|
Soluble salts are a principal agent of decay
in porous building materials and a source of great frustration to those
involved in the conservation of historic buildings. The behaviour of salts
may seem unpredictable since they can remain dormant for long periods
and then suddenly become active causing damage and disfiguring historic
fabric. In other cases the action of salts is progressive, weakening surfaces
on a microscopic level over decades and centuries, causing natural erosion
of the kind that would occur to stone in a quarry face. In 1932 Schaffer
described the problem in his authoritative work The Weathering of Natural
Building Stones and his description remains the most comprehensive
source on the subject, outlining all the essential facts, the salts typically
found and the mechanisms of crystallisation and decay.
The damaging effects of soluble salts are intimately linked with wetting and drying cycles at the masonry face. Almost all historic building materials are porous to some degree. The network of pores in stone and brick contain water in which varying quantities and types of salts may be dissolved. As drying/evaporation occurs at the masonry face salts crystallise out of solution producing the white crystals known as efflorescence. While these fluffy white crystals can appear dramatic when projecting 10-20mm from surfaces, they may be relatively harmless compared to hidden salt crystallisation (cryptoflorescence) occurring within the pores below the masonry surface. Fine pores cannot accommodate the increasing accumulations of salts and are eventually broken apart by the expansive forces of the crystal growth, causing the surface to decay.
|'Tide line' of soluble salts in masonry at the Roman Baths, Bath caused by rising damp (Professor John Ashurst)|
Generally, limestone is considered the most susceptible of building stones to salt decay but all historic masonry is potentially at risk depending on the degree of salt contamination and on all the other factors that work in combination with salts to cause weathering and decay. Limestones are inherently susceptible because they contain calcium carbonate. In the external environment acidic compounds break down small quantities of carbonates converting them to sulphates, the most common salts found in masonry. Lime mortars and plasters, calcareous sandstones and other materials containing calcium carbonate are equally susceptible.
In addition to being harder, less porous and less permeable than stone, Portland cement is another potential source of sulphates. Sulphates contribute significantly to the decay of stonework and sculpture where cement has been used in past repairs, particularly for grouting.
Other sources of salt are external, such as
sea salt, which can cause dramatic decay depending on the masonry substrate.
Fertilisers, road salt, and acid gases from various atmospheric pollutants
are all sources of potentially damaging soluble salts. Salts in the soil
and ground water may cause problems by migrating through the masonry with
rising damp, typically visible as a 'tide mark' of staining above ground
Laboratory analysis of salt contaminated masonry samples typically yields a somewhat bewildering array of ions: sulphates, nitrates, chlorides, sodium, potassium, magnesium and calcium in varying concentrations. These ions which dissociate harmlessly in solution crystallise to form salts such as sodium sulphate and sodium chloride (table salt).
Chemicals in proprietary cleaning compounds are a potential source of soluble salts if not thoroughly rinsed from building facades. In particular care must be taken in using alkaline cleaners and paint strippers based on sodium hydroxide. Unusually high levels of salt may result from the building's function or from a single event in the building's history and in this case may cause problems in a specific area. For example storage of gunpowder salts above the south porch at Malmesbury Abbey in Wiltshire contributed to the decay of the Romanesque sculpture on the south portal.
The susceptibility of masonry substrates to salt crystallisation damage varies depending to a considerable extent on the size and distribution of pores. This is true in particular for limestone and lime mortar or plaster. It is difficult to generalise but micro-porous limestones, such as chalk which has many pores of relatively small size, are often of inferior durability. Open textured limestones with larger pores, such as Ketton, are generally more durable. Magnesian limestones are prone to salts particularly in polluted environments. Sandstones with a calcium carbonate binder (calcareous sandstones) tend to be more susceptible than other sandstones. More dense and less porous sandstones, such as York stone, are much less susceptible to salt related damage. High fired vitrified ceramic materials are more resistant than low fired porous ceramics.
|Dense cement based repair to sandstone at Trinity College, Toronto. De-icing salts and moisture have risen beyond the impermeable band of cement to create a new zone of decay just above it. (Professor John Ashurst)|
Although lime plasters and stucco will break down much more readily than
cement based plasters and renders when subject to wetting and drying cycles
and salt contamination, the relatively high density and impermeability
of cement binders makes them unsuitable for repairs to historic masonry,
particularly in cases of salt contamination (see illustration). Cement
based lasters encourage the failure of weak substrates.
In general it is better to encourage soluble salts to migrate out of the contaminated masonry substrate rather than to try to contain salts behind an impermeable plaster or render based on cement or other impervious material. Salts find the path of least resistance through softer, more permeable and weaker materials, bypassing the dense impermeable materials. Inevitably the salts will find routes through the masonry to a drying face/zone, either at a gap in the impervious layer or behind it, where they will continue causing damage undetected. In such a situation lime based plaster or render is always preferable. Although the lime plaster may eventually fail locally with salt efflorescence and need to be renewed, it will protect the underlying masonry, behaving sacrificially to the masonry in the same way that soft lime based mortar repairs applied to sculpture behave sacrificially to the limestone surface.
Good pointing mortar in historic masonry buildings is typically more permeable and porous than the masonry units it bonds together and consequently often functions sacrificially to the masonry. Mortar joints exhibiting salt related decay should not necessarily be considered to have failed but may be protecting the masonry by absorbing salts and breaking down preferentially to the masonry.
|Terracotta pilaster dating to 1872 suffering from salt and moisture penetration exacerbated by the application of paint. White salt efflorescence is visible on the decayed terracotta surface which is pitted and friable.|
Soluble salt crystallisation causes characteristic
pitting and powdering of surfaces. This is unsightly and destructive in
any situation but is particularly problematic for decorated masonry surfaces
and for sculpture on buildings. Painted plaster surfaces are at risk in
a salt contaminated environment. In the past it was common practice internationally
to remove valuable wall paintings from archaeological sites where salt
damage was evident. The method originated with Italian wall painting conservators
and is known as the strappo technique. The first century BC wall paintings
at the site of Masada in Israel are an example of this practice. Pitting
in the painted surfaces began to appear subsequent to the Masada excavations
in the 1970s. The environmental change resulting from uncovering the paintings
greatly accelerated decay; salt concentrations are inevitably high on
Masada due to its proximity to the Dead Sea and relative humidity changes
occur even in this desert environment. All wall paintings on the site
have been detached from the masonry backing and the thin surface layer
of paint given a new synthetic backing mounted on metal frames. This sort
of drastic intervention is no longer recommended. Not only is the process
completely irreversible but the new backing systems and frames have proved
to be incompatible with painted lime surfaces in a hot climate where thermal
movements of metal are inevitable.
An alternative approach to detachment of Masada's wall paintings would have been local desalination, to poultice out the salts, drawing them in solution through the masonry face into another absorbent drying zone applied to the painting surface, composed, for example, of paper fibres, cotton fibres or clay. Salt crystals can form harmlessly in the poultice and be removed. This sort of treatment for important surfaces, such as wall paintings or sculpture, should be viewed as a maintenance treatment used as necessary to extract salts, rather than as a single treatment, never to be repeated.
Masonry may be stable for a long period and then suddenly exhibit salt related decay, typically due to a change in environment. A typical situation is for saturated masonry in a humid environment such as a basement to remain stable as long as the dampness in the walls is accompanied by dampness in the air. If this equilibrium is altered by heating of the internal environment drying at the masonry face can induce salt crystallisation. Conversely masonry surfaces may suffer a short but intense period of salt efflorescence and then stabilise once the immediate supply of salts has been exhausted.
If masonry is known to suffer salt contamination any remedial work must be designed to accommodate these salts. For example the pre-wetting required for grouting may mobilise salts and cause them to migrate to the masonry face. Repair and pointing mortars can be designed to perform sacrificially to the masonry substrate. Correct mortar specification and an open textured mortar face to increase evaporation will assist salt movement through repairs and repointing. It is normally impractical to extract salts from masonry, even in limited areas. It is easier to use traditional masonry repair techniques including, if appropriate, plasters to protect masonry surfaces and to plan for ongoing maintenance of locally salt contaminated areas.
- Building Research Establishment, The Selection of Natural Building
Stone, BRE Digest 269, Garston, Watford, 1983
- E Leary, The Building Limestones of the British Isles, Building
Research Establishment, Garston, Watford, 1983
- RJ Schaffer, The Weathering of Natural Building Stones, Special
Report No18, (1932), Building Research Establishment, Garston, Watford, 1972
- G Torraca, Porous Building Materials, ICCROM, Rome, 1982