Bacterial slime creates water-repellent hybrid mortar


By Alice Richard
Monday, 08 August, 2016


Biopolymer-based hydrogels — slime formed by living organisms — sound like something out of Ghostbusters. They’re responsible for delightful things like dental plaque and the black slime that forms in sewage pipes. While you don’t want them in your mouth or in your plumbing, German researchers have found a place where they’re actually desirable: in mortar.

After learning about self-healing concrete that uses bacteria to repair cracks, Oliver Lieleg, a professor of biomechanics at the Technical University Munich (TUM), was inspired to use bacteria to prevent moisture from penetrating into mortar. When water invades mortar, it can cause significant problems, such as inducing the growth of mould or widening existing microcracks through freeze-thaw cycles.

Rather than creating self-healing mortar, Professor Lieleg harnessed the water-repellent properties of some bacterial films to prevent moisture from getting into the mortar in the first place.

He and his team added biofilm produced by the bacterium Bacillus subtilis to mortar powder to create a hybrid mortar. They then measured how much water was able to penetrate the mortar.

They found that water was significantly less able to wet the surface of the hybrid mortar, compared with untreated mortar. To evaluate this, they measured the contact angle between water droplets and the mortar’s surface. The steeper the angle, the more spherical the drops are and the less likely the liquid will soak into the material.

On untreated mortar, this angle was only 30° — but on the hybrid mortar, it was three times this. Water droplets on polytetrafluoroethylene (aka Teflon) have a similarly high contact angle.

The hybrid mortar is undergoing testing to see whether it’s resistant enough for use in construction applications.

“If the mortar is in fact suitable, there should be no problem for companies to produce it on a large scale,” Professor Lieleg said. Both the bacterial strain used and the culture media in which it is grown are standard and relatively inexpensive.

“We’ve also discovered in our experiments that freeze-dried biofilm can be used equally well. Then, in a powder form, the biological material can be stored, transported and added much more easily.”

The researchers will also investigate whether the biofilm can be used to protect concrete against water. The initial research was published in the journal Advanced Materials.

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