[Image above] Credit: Green Fire Productions; Flickr CC BY 2.0
Nature loves a good brick and mortar-style construction. And that’s for good reason—it’s one of the toughest designs you can get, combining both strength and flexibility.
For instance, nacre (aka mother of pearl) uses stacks of aragonite nanoplatelet bricks to provide strength to this super strong biomaterial.
The goal here is not necessarily to prevent cracking altogether—but rather to prevent a crack from catastrophically propagating through the entire material. Although the brick material in a brick-and-mortar design may crack, a softer in-between mortar material helps absorb crack energy, preventing in from ripping through the entire material.
Sea urchins use a similar strategy to build their spines, layering hard, crystalline calcite blocks with softer, amorphous calcium carbonate materials.
Researchers at the University of Konstanz in Germany have now shown that they can engineer stronger cement by giving the material a nano-level brick and mortar structure.
To do so, the researchers added polymer binders into cement to inhibit interactions between certain cement nanoparticles.
While it sounds like this strategy might make the material weaker, instead it allowed the scientists to create more ordered nanostructures within the mix. Without such interruptions, all the nanoparticles within cement bind to one another, creating an amorphous tangle.
Adding polymer binders to the mixture prevents specific particle interactions from happening, effectively controlling binding by occluding it—and building a more ordered cement structure.
The resulting nano-ordered cement, the authors report in an open-access Science Advances paper, is 40–100 times more fracture resistant than concrete.
“A pillar made of this cement could be built 8,000 meters high, or ten times as high as the current tallest building in the world, before the material at its base would be destroyed by its weight. Normal steel, which has a value of 250 megapascals, could only reach 3,000 meters in height,” according to a university press release.
To test their nanostructured cement formulation, the scientists ion beam-milled a thin pillar from the material, just 3 µm in size, and used a micromanipulator to test how well the pillar could recover after bending.
The design worked, at least at the nanoscale—when released, the pillar bounced back to its original position, rather than breaking. The scientists report an elastic deformation of 200 MPa, in comparison to 210 MPa for fracture-resistant mussel shells and just 2–5 MPa for standard concrete.
You can watch a video of the microscale bending test on the university’s website (links to the right of the news story).
Because nature is able to engineer achieve such strength with only calcite, the prospects for human engineering seem even better—although additional testing is needed to confirm that the material can hold up beyond these nanoscale tests.
“People have much better construction materials than calcite,” senior author Helmut Cölfen says in the release. “If we succeed in designing the structures of materials and reproduce nature’s blueprints, we will also be able to produce much more fracture-resistant materials—high-performance materials inspired by nature.”
The open-access paper, published in Science Advances, is “Mesocrystalline calcium silicate hydrate: A bioinspired route toward elastic concrete materials” (DOI: 10.1126/sciadv.1701216).
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