Researchers have found that damage caused by salt to buildings can be predicted.
Historic stone buildings suffer from weathering caused by salts. These crystallise inside the porous building materials and generate enough force for the stone to break or crumble.
Researchers at the Institute for Building Materials at ETH Zurich and Princeton University have conducted an experiment to test the effect of salts under controlled conditions.
Salt can enter building materials in a variety of ways, said Francesco Caruso, a post-doctoral researcher in the group of Robert Flatt, Professor of Building Materials.
Cement, for example, a component of concrete, always contains gypsum (calcium sulfate) and alkali sulfates, both of which are salts.
Building materials can also be infiltrated by salt from the environment, such as through mineralised ground water close to the surface, which permeates porous building materials through capillary force, or via the atmospheric pollutant sulfur dioxide, which reacts with the calcium carbonate in limestone to form gypsum.
Damage can also be caused by de-icing salt and seawater spray that accumulates on the surface of buildings.
"If these salts are dissolved by rain, the saline liquid can enter the building material through pores and cracks," said Caruso.
The salts crystallise as the liquid dries out and evaporates, causing parts of the stonework to crumble away.
For their laboratory experiment, the ETH researchers used sodium sulfate, the most destructive salt known and which exists in two forms: anhydrous and hydrated.
Researchers placed limestone cubes with a side length of two centimetres into a sodium sulfate salt bath, allowing the salt solution to permeate the pores of the limestone.
They then dried the stones at high temperature before placing them in the salt bath again at a lower temperature for the next cycle. During the drying phases, the salt crystallised in the stone's pores in anhydrous form.
In the salt bath phases, the salt solution permeated the pores again and the crystallised salt turned back into a liquid solution.
With this controlled cyclical process, the scientists managed to accumulate a large amount of salt within the stone and create a supersaturated salt solution with respect to the hydrated form.
A supersaturated salt solution is a liquid in which, because of special circumstances, more salt is dissolved than would be possible under normal circumstances.
The experiment showed that the greater the supersaturation, the greater the salt's destructive potential.
Results showed that if environmental conditions are such that a salt solution repeatedly infiltrates porous stone and the fluid can then evaporate again (eg due to strong sunlight or wind), the salt in the building material can become supersaturated.
