Mother Nature as inspiration for a new cement composite

Princeton engineers have created a new composite cement that is 17 times more resistant to cracking and 19 times more flexible than standard cement. This innovation is inspired by the natural substances present in oyster and abalone shells. The results, published in the journal Advanced functional materials, suggest that this development could improve the durability of various fragile ceramic materials, ranging from porcelain to concrete, by making them less susceptible to cracking.

If we can design concrete to resist crack propagation, we can make it stronger, safer and more durable.

Shashank Gupta, graduate student, Department of Civil and Environmental Engineering, Princeton University

In a recent study led by Reza Moini, assistant professor of civil and environmental engineering, a new cement composite mimicking the structure of nacre, or nacre, was developed. Published on June 10^th2024, research highlights that alternating layers of cement paste and thin polymers significantly improves the crack resistance and ductility of the material.

Mother-of-pearl, mainly made of hexagonal aragonite tablets linked by flexible biopolymers, inspires this design. These aragonite tablets build strength, while biopolymers provide flexibility and resilience. Under stress, the aragonite tablets slide, a movement which, combined with the deflection of the fracture and the deformation of the biopolymer, helps to disperse the energy. This allows the material to withstand high mechanical stress while maintaining its structural integrity, providing both strength and resilience.

This synergy between hard and soft components is crucial for the remarkable mechanical properties of nacre.

Shashank Gupta, graduate student, Department of Civil and Environmental Engineering, Princeton University

The Princeton research team, inspired by the structure of nacre, created innovative composites by fusing traditional Portland cement paste with a small amount of polyvinylsiloxane, a highly expandable polymer. They constructed small multi-layer beams by alternating sheets of cement paste with this polymer, which were then subjected to a three-point notched bending test to assess their resistance to cracking and fracture toughness.

The study focused on three different beam designs:

1. Regular alternating layers: These beams alternated between sheets of cement paste and thin layers of polymer.
2. Grooved layers: For this type, hexagonal grooves are laser etched into the cement paste sheets, which were then coated with polymer.
3. Fully Separated Hexagonal Tablets: This design most closely imitates mother-of-pearl, with fully cut cement paste forming individual hexagonal tablets, held together and separated by layers of polymer.

The experimental comparison included these three designs against a reference beam made of solid, monolithic cement paste. The reference beam exhibited typical brittle fracture, breaking abruptly and completely when it reached its failure threshold. In contrast, regular and grooved layered beams demonstrated improved ductility and crack resistance.

Specifically, beams designed with fully separated hexagonal shelves showed the best performance. While maintaining comparable strength to solid cement paste beams, these beams exhibited 19 times greater ductility and 17 times greater toughness, significantly outperforming the standard model in terms of resilience and flexibility.

Our bio-inspired approach is not simply about mimicking the microstructure of nature, but about learning underlying principles and using them to inform the engineering of human-made materials. One of the key mechanisms that makes a pearly shell strong is the sliding of the tablet at the nanometer level. Here, we focus on the tablet sliding mechanism by designing the integrated tabular structure of cement paste in balance with the polymer properties and the interface between them. In other words, we intentionally design defects into fragile materials to make them stronger by design.

Reza Moini, Assistant Professor, Department of Civil and Environmental Engineering, Princeton University

The researchers cautioned that the results are based on laboratory circumstances and that developing the approaches for field application will require additional work and study. They are studying whether the fracture toughness and ductility of structures apply to ceramic materials other than cement paste, such as concrete.

Moini added: “We are only scratching the surface; There will be many design opportunities to explore and design the constituent properties of hard and soft materials, interfaces and geometric aspects that play into fundamental size effects in building materials.»

Hadi S. Esmaeeli, a research associate at Princeton, co-authored the study. The study was partially supported by CAREER Award #2238992 from the National Science Foundation through the Engineering for Civil Infrastructure program.

Journal reference:

Gupta, S., And. al. (2024) Nacre-type cementitious composites, robust and ductile architecture. Advanced functional materials. doi:10.1002/adfm.202313516