Salt damage is one of the most important threats to monuments worldwide, including in the Netherlands. Climate change will increase the risk of salt damage, according to Timo Nijland, natural stone specialist at TNO.
Nijland is a geologist by origin and involved in various restoration projects as a natural stone expert. He conducted research for the restoration of the Palace on Dam Square in Amsterdam and the St. John's Cathedral in Den Bosch. “We see damage from salt at many monuments. The salt is scattered in the building blocks themselves and in the mortar and becomes mobile as a result of moisture.”
Where it evaporates, it precipitates and the salt accumulates. An accumulation of salt crystals increases the pressure and breaks the rock. The culprits are water-soluble salts, in Dutch monuments mainly sodium chloride (NaCl) and sodium sulfate (Na2 SO4 ). We all know sodium chloride as the chemical formula of table salt; sodium sulfate is less well known; in nature it also occurs bound to water:Na2 SO4 .10H2 O.
Removal with hydrochloric acid
“The salt comes from various sources,” says Nijland. “Sodium sulphate can come from the brick, for example if it has not been baked hot; a considerable amount of sulphate remains behind. Sodium chloride can enter the building blocks from various sources, for example de-icing salts or salty sand that has been used as a raw material for mortar. In Amsterdam, for example, a lot of sand from the – formerly – salty IJ was used. Another source of chloride is hydrochloric acid, which was used to strip churches and other old buildings of stucco. This 'stripping' came into vogue in the 19th century."
Aerosols are also an important source of salt. They are solid (salt) particles that float in the air and fall down with rainfall. Coastal areas in the Netherlands are particularly affected by this. Last but not least we are increasingly dealing with 'saline seepage', which is moving deeper inland as a result of rising sea levels. The groundwater becomes saltier and often ends up as rising damp in the walls of monuments.
Salt seepage
Climate change will not only increase the spread of saline seepage – a phenomenon that, like aerosols, mainly occurs in coastal areas – it will also rain more heavily in some periods. According to the new climate scenarios published by the KNMI this month, we will have considerably wetter winters this century.
Nijland:“The rainwater will then penetrate deeper into the facades of a building. This moisture can mobilize more salts that are present in the building block. Also alternately moistening and drying up again under the influence of the air humidity causes more salt crystals to accumulate. The number of these cycles influences the occurrence of salt damage”.
Salt-resistant mortars
TNO is developing various new techniques to improve restorations of monuments with salt damage. “Here in Delft, for example, we are developing new mortars that are more salt-resistant,” says Nijland. “We do this by adding substances to the mortar that allow the salt to crystallize in a different way. The angle that the salt crystal makes with the surface influences the damage it causes. Yet another possibility is to ensure that the salt crystallizes more widely or is in solution longer, so that it only crystallizes on the surface and you can then remove it."
Even in special sculptures, whose rock is still well intact and the mortar does not need to be replaced, restorers may also remove the salt on the outside. For example, colleagues from Nijland have carried out research into removing salt from the master tests (special samples of masonry) in the Amsterdam Waag by applying compresses to the outside, in order to 'suck' the salt out, as it were.