Researchers at the Massachusetts Institute of Technology (MIT) have proposed a new bio-inspired, “bottom-up” approach for designing cement paste – concrete’s binding ingredient. Led by Oral Buyukozturk, a professor in MIT’s Department of Civil and Environmental Engineering (CEE), the team compared cement paste to natural materials such as bones, deep sea sponges, and nacre, an inner shell layer of mollusks.
“These materials are assembled in a fascinating fashion, with simple constituents arranging in complex geometric configurations that are beautiful to observe,” Buyukozturk said. “We want to see what kinds of micromechanisms exist within them that provide such superior properties, and how we can adopt a similar building-block-based approach for concrete.”
These biological materials have long been studied for their microscopic arrangements that allow for mechanical properties such as exceptional strength and durability, and some of the learnings have already been applied in corporate solutions - Airbus, for example, looked to bones to help optimize strength in a lightweight galley partition.
By studying the existing literature on these natural structures and their behavior, Buyukozturk aims to better understand the connections between these features at the nano-, micro-, and macroscales in hopes of applying their findings to the cement industry.
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Based on what they discovered – such as that a deep sea sponge’s onion-like structure of silica layers provides a mechanism for preventing cracks and that nacre has a “brick-and-mortar” arrangement of minerals that generates a strong bond between the mineral layers and makes the material extremely tough – the team was able to develop a general, bio-inspired framework for engineers to design cement “from the bottom up.”
The framework, recently released in a paper published online in the journal Construction and Building Materials, provides a guideline that engineers can follow to determine how certain additives or ingredients of interest will impact the cement’s overall strength and durability.
“In this context, there is a wide range of multiscale characterization and computational modeling techniques that are well established for studying the complexities of biological and biomimetic materials, which can be easily translated into the cement community,” said CEE assistant professor Admir Masic, who was a co-author of the paper.
To use the framework, engineers would first use existing experimental techniques (such as magnetic resonance or X-ray diffraction) to characterise a material’s solid and pore configurations, then would plug their measurements into models that simulate concrete’s long-term evolution and determine the relationships between its microscale structure and macroscale properties. The simulations can then be validated with conventional compression and nano-indication experiments to test actual samples of the concrete. For example, one of Buyukozturk’s other projects involves whether volcanic ash would improve cement paste’s properties. Using this methodology, he could evaluate the properties of the volcanic ash and its contribution to the strength and durability of an ash-containing concrete bridge.
“The merger of theory, computation, new synthesis, and characterization methods have enabled a paradigm shift that will likely change the way we produce this ubiquitous material, forever,” said CEE department head Markus Buehler, another of the paper’s co-authors. “It could lead to more durable roads, bridges, structures, reduce the carbon and energy footprint, and even enable us to sequester carbon dioxide as the material is made.”
Some building designers are already pioneering such work. For example, the entire outdoor surface of Nemesi & Partners' Italy Pavilion at the Milan Expo 2015 was made of biodynamic concrete panels that, when in contact with sunlight, allows it to absorb pollutants present in the air.
“Hopefully this will lead us to some sort of recipe for more sustainable concrete,” Buyukozturk said. “Typically, buildings and bridges are given a certain design life. Can we extend that design life maybe twice or three times? That’s what we aim for. Our framework puts it all on paper, in a very concrete way, for engineers to use.”