How salt can make buildings age faster

High salt is not only detrimental to health but salt crystals can also push historical buildings and wall paintings to age faster. How?

Salt can enter building materials in a variety of ways. 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 or via the atmospheric pollutant sulfur dioxide which reacts with the calcium carbonate in limestone to form gypsum," explained

Francesco Caruso, a researcher from the Institute for Building Materials at ETH Zurich, Germany.

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," Caruso added.

The salts crystallise as the liquid dries out and evaporates, causing parts of the stonework to crumble away.

During the study, ETH researchers used sodium sulfate - the most destructive salt known.

In several cycles, they 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.

"The experiment showed that the greater the supersaturation, the greater the salt's destructive potential," Caruso added.

The experiments may help conservation scientists decide how much salt needs to be removed from a building to avoid damage or - if the salt cannot be removed - to predict when a building might be damaged, researchers concluded.