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Physicists ponder flowering masonry

Photograph of efflorescence

Efflorescence on a masonry wall (Courtesy: Mattes)

By Hamish Johnston

One thing you can say about most houses in the UK is that they are solid. All walls, including internal ones, tend to be made from masonry – bricks in Victorian and Edwardian homes, and cement blocks in more modern buildings. It’s rare to see the flimsy-looking wooden frames that miraculously become houses in North America, for example.

But there is a downside to this solid construction. Masonry – and older bricks in particular – tend to suck-up moisture from the ground. Indeed, this is such a common problem in the UK that they named a sitcom after it – Rising Damp.

One symptom of rising damp is efflorescence, which means “flowering out”. This refers to crystals of salts that grow out from the surface of masonry as the damp evaporates into the air. Efflorescence can be a real pain on interior walls because it can stain paintwork and push-off wallpaper. I should know, because we used to have it in the dining room!

Efflorescence has also fascinated physicists because, rather than emerging as a uniform coating of salt, the crystals tend to appear in clumps – but exactly why was a mystery.

But now, Marc Prat and colleagues at the University of Toulouse, France, have done experiments and computer simulations that suggest that several factors are involved in determining the locations of salt flowers.

One is that the rate of evaporation is often not uniform across a wall. Not surprisingly, moisture is drawn to regions of the surface where air currents or other factors boost evaporation, and this causes more efflorescence in these areas. The irony, of course, is that reducing humidity and increasing ventilation could actually encourage efflorescence!

Turning their attention to the network of tiny pores that exist in masonry, the team worked out that certain pathways are extremely efficient at transporting water to the surface, while others are not. The researchers concluded that the salt flowers form where these efficient pathways emerge at the surface. Once a crystal is established on the surface, its presence increases the flow of water through that particular pathway, further depriving surrounding less-efficient pathways of liquid. The result is regions with large crystals, and other regions with no salt.

Finally, they tried to explain why the salt crystals grow outwards from the surface, rather than spreading out. The reason, it seems, is that the moisture would rather travel through a salt crystal than along a masonry surface. Putting all of this together, the team believes that it has made a good first effort at understanding efflorescence.

The research is described in Phys. Rev. Lett. 108 054502 and you can read the paper here.

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