The architecture of sea urchin spines — and the efforts of researchers from the University of Konstanz, Germany — has made a major contribution to the development of stronger concrete.
Despite being composed primarily of calcite, which is usually brittle and fragile, sea urchin spines are very strong. The nanoscale “brick and mortar” arrangement of these structures impart mechanical strength: layers of brittle crystalline calcite blocks alternate with softer disordered layers. When force is applied to the brittle calcite, its crystalline block will crack but the energy is then transferred to the soft disordered calcium carbonate layer. Since this material has no cleavage planes to tear, it prevents further cracking.
The researchers applied the same design principle to the synthesis of durable cement. “Our cement, which is significantly more fracture-resistant than anything that has been developed thus far, provides us with completely new construction possibilities,” says Professor Helmut Cölfen. 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. Conventional steel could only reach 3,000 meters in height.
Cölfen describes the process as “encoding fracture-resistance at the nano-level." The technique entailed identifying a material that bonds only with cement nanoparticles and nothing else in the cement. About 10 negatively charged peptide combinations were identified that both adhere to and bond materials well.
The researchers next collaborated with the University of Stuttgart in using an ion beam under an electron microscope to cut a bar-shaped micro-structure out of the nanostructured cement that was three micrometers in size.
A micro-manipulator was applied to bend the micro-structure. Upon its release, the micro-structure returned to its original position. Mechanical values calculated based on the elastic deformation of the micro-structure reveal that the optimized cement achieved a value of 200 megapascals, denoting high tensile and compressive strength. Mussel shells, deemed the gold standard in fracture-resistance, reach a value only slightly higher at 210 megapascals. Commonly used concrete has a value of two to five megapascals.