A team of scientists at the Technical University of Munich (TUM) (Munich, Germany) is optimistic that they have found a novel way to protect mortar from moisture.
Led by Oliver Lieleg, a professor at TUM, the group has begun adding a biofilm—a soft, moist substance produced by bacteria—into a mixture with the mortar material. These bacterial biofilms can even include dental plaque and the black coating formed in sewage pipes, he says.
“Biofilms are generally considered undesirable and harmful,” Lieleg says. “I was therefore excited to find a beneficial use for them.”
Lieleg spoke with Christian Große, professor and chair of the school’s nondestructive testing (NDT) department and an investigator of self-healing concrete. One variant of the concrete being studied by Große contains added bacteria, which become activated by the uptake of moisture. When that occurs, the bacteria close the cracks in the concrete with metabolic products including calcium.
Based on that conversation, Lieleg came up with the idea of using biofilms to alter the properties of construction materials, starting with mortar. But rather than mending cracks after damage has occurred, as in Große’s study, Lieleg’s goal is to prevent moisture from penetrating into the material in the first place. To accomplish this, Lieleg set out to use water-repellent bacterial films.
Hybrid Material from Soil Bacteria
The bacterial strain Bacillus subtilis 3610 normally lives in soil and is a very common microorganism. When grown on enriched agar (a gel-like substance derived from algae), the strain typically forms biofilms with strong hydrophobic surface properties. The agar used for biofilm growth was enriched with 1% glycerol (C3H8O3) and 0.0001 mol of manganese sulfate (MnSO4), according to the technical report.1
The scientists sought to quantify the properties by observing the contact angle. In this case, the contact angle for water droplets on the Bacillus subtilis 3610 biofilms was about 110 degrees. Water droplets on polytetrafluoroethylene (PTFE), known by the trade name Teflon†, have a similarly high contact angle.
In contrast, the scientists noted that an unmodified mortar sample exhibited strongly hydrophilic behavior, with a contact angle of 30 degrees or lower.
As a result, the scientists sought to integrate the biofilm component into the mortar, thus creating a hybrid material with enhanced wetting resistance. They bred the bacterial film on standard culture media in their laboratory, and then added the moist biofilm to the mortar powder during the casting process.
“For our experiments, we used a simple laboratory strain that grows rapidly, forms plenty of biomass, and is completely harmless,” Lieleg says.
The TUM scientists observed that the hybrid mortar sample comprising 2% biofilm exhibited a strongly increased contact angle of ~90 degrees—or about three times as high as the contact angle for untreated mortar. As such, water was significantly less able to wet the surface, compared to the untreated mortar.
However, the TUM team knew that for industrial application, being able to produce the biofilm from simple agar—easier and cheaper to produce than the enriched model—would be advantageous. As a result, the scientists ran another experiment with biofilm from simple agar. They were not expecting the same results, since biofilms grown on simple agar tend to exhibit more hydrophilic behavior, with contact angles only slightly steeper than the standard 30 degrees for untreated mortar. In fact, only very small areas of the biofilm from simple agar exhibit strong hydrophobic properties, Lieleg says.
“Consequently, the hybrid mortar samples enriched with this biofilm would be reasonably expected to exhibit only weakly enhanced, if any, wetting resistance,” Lieleg says.
But surprisingly, this is not what occurred. The TUM scientists found contact angles on the hybrid mortar made from simple agar that were similar to or even higher than those observed when the enriched agar-grown biofilm was used in the mixture. This indicates that the increased wetting resistance of the mortar is not directly due to the hydrophobic properties of the biofilm.
Nanostructures in the Mortar
Since the wetting resistance of the material was not due to the material properties of the mixture, the scientists concluded that the addition of the biofilm altered the surface structure of the mortar.
Indeed, an explanation for the water-repellent properties of the hybrid mortar was found in the electron microscope images, which showed that the surface was covered in tiny crystalline spikes (Figure 1).
Those spikes result in what is known as the lotus effect, which also occurs on the leaves of the lotus plant. In the case of the plant, small uniform structures on the surface ensure that only a small part of a water droplet makes contact with the leaf surface. Therefore, the surface tension of the droplet is stronger than the forces making it adhere to the leaf, and the droplet easily rolls off the leaf when the leaf is tilted.
Similar to lotus leaves, TUM scientists say the hybrid mortar seems to possess high surface roughness. That, in turn, would substantially increase the contact angle, and thus the wetting resistance.
Lieleg explains that a cross-section of the hybrid mortar shows that the crystalline spikes are not only evenly distributed on the mortar surface, but they also can be found throughout the bulk volume of the mortar. This reduces the capillary forces that are normally responsible for the uptake of water in mortar when the material is immersed into liquid.
While similar spikes do also occur on untreated mortar, they are too long, rare, and scattered for the lotus effect to occur, Lieleg says. The researchers believe that the added biofilm stimulates uniform crystal growth throughout the hybrid material.
With the experiment complete, the mortar is now undergoing mechanical tests in Große’s NDT department (Figure 2). These tests will allow the researchers to determine if the hybrid mortar is resistant enough to actually be used in construction application.
“If the mortar is, in fact, suitable, there should be no problem for companies to produce it on a large scale,” Lieleg says, adding that both the bacterial strain used and the culture media are standard and relatively inexpensive.
“We’ve also discovered in our experiments that freeze-dried biofilm can be used equally well,” Lieleg adds. “Then, in a powder form, the biological material can be stored, transported, and added much more easily.”
Moving forward, the TUM scientists plan to examine whether the biofilm can also be used to protect concrete—in addition to mortar—against water.
1 S. Grumbein, D. Minev, M. Tallawi, K. Boettcher, F. Prade, F. Pfeiffer, C.U. Große, and O. Lieleg, “Hydrophobic Properties of Biofilm-Enriched Hybrid Mortar,” Advanced Materials