What could graphene make waterproof?

Bacteria make mortar waterproof

Biofilm particles mixed with dry mortar cause innumerable tiny spines to grow. They carry water that wants to reach the surface on their hands, so to speak: It cannot penetrate and destroy the material. Instead, it just rolls off.

The surface structure of the mortar mixed with biofilm (left) creates a lotus effect: water droplets have significantly less contact with the surface than on untreated mortar (right).

Photo: Stefan Grumbein / TUM

Oliver Lieleg's area of ​​expertise is hydrogels. These are plastics that contain water. You are flexible and adaptable. An example: soft contact lenses that nestle against the eyeball.

Lieleg, Professor of Biomechanics at the Central Institute for Medical Technology and the Faculty of Mechanical Engineering at the Technical University of Munich, has now implemented his know-how in this area in an area that seems to be far removed from his research activities: with the help of special hydrogels, which are produced by bacteria and are called biofilms, refined mortar, as it is applied by craftsmen on facades or bricklayers use.

Initially, mortar is usually waterproof. However, if it gets fine cracks, moisture penetrates. At sub-zero temperatures, it turns into ice, which has a larger volume than water. It bursts the mortar, the cracks get bigger and more water penetrates.

The building materials industry turns to bacteria

Not so with Lieleg mortar. He vaccinated it with biofilm material produced by a bacterium called Bacillus subtilis. It occurs in the earth and is widespread. "We used a simple laboratory strain for our experiments that can be reproduced well, produces a lot of biomass and is completely harmless," says Lieleg.

In the lotus effect, a thin layer of air fills the microscopic roughness of the surface. This means that water cannot penetrate and runs off.

Source: Jutta Wolf / MPG

He mixes the bacterial product with the dry mortar mass. Once this has set, i.e. it has become solid, its surface is hydrophobic, i.e. water-repellent. It does not wet the surface over a large area, but rather forms spheres due to its surface tension, which roll off like a lotus leaf (lotus effect).

Brakes rising water

What causes the miraculous change in the mortar could only be seen under the electron microscope at high magnification. The surface of the mortar is prickly like the skin of the hedgehog. Except that these spines are tiny. They stretch themselves like arms towards the water and, again because of its surface tension, prevent the liquid from reaching the mortar. The drops simply roll down.

The researchers also found such spikes inside the mortar. There they prevent water from rising against gravity. Capillary action. The spines are not a unique selling point for Lieleg plaster. They can also be found on non-impregnated material, but in such a low density that the water can reach the surface. The researchers suspect that the biofilm stimulates the growth of the sting.

Now concrete should also be waterproof

The mortar is currently being tested for its usability in practice. Professor Christian Große from the Chair for Non-Destructive Materials Testing has taken on this. If the mortar is actually suitable, I see little problem with producing it on a large scale, ”says Lieleg. Because the biofilm can also be freeze-dried. The powder is easy to transport and mix with the dry mortar. Lieleg is now examining whether the method is also suitable for concrete.

Freshly pressed manhole rings made of concrete: TUM scientists now want to test whether they can make concrete waterproof with the help of a biofilm made from bacteria.

Source: Jan-Peter Kasper / dpa

2000 years ago, ancient builders used a special mortar that is more stable than some modern concrete. They mixed in volcanic ash, which prevents microcracks.

A contribution by:

  • Wolfgang Kempkens

    Wolfgang Kempkens studied electrical engineering at RWTH Aachen University and graduated with a diploma. He worked for a daily newspaper and magazine before establishing himself as a freelance journalist. He mainly deals with environmental, energy and technology issues.