Sources: Massachusetts Institute of Technology, Cambridge; CP staff
In a project funded by outside parties and independent of the Concrete Sustainability Hub, MIT researchers are seeking to redesign concrete by following nature’s blueprints, contrasting cement paste with the structure and properties of bones, shells, and deep sea sponges. As they observe in a current Construction and Building Materials paper, such biological materials are exceptionally strong and durable, thanks in part to their precise assembly of structures at multiple length scales, from the molecular to the macro, or visible, level.
A team led by Department of Civil and Environmental Engineering (CEE) Professor Oral Buyukozturk proposes a new bio-inspired, “bottom-up” approach for designing cement paste. “These [natural] materials are assembled in a fascinating fashion, with simple constituents arranging in complex geometric configurations that are beautiful to observe,” he contends. “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.”
“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,” adds CEE Department head Markus Buehler. “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. Implementing nanotechnology in concrete is one powerful example [of how] to scale up the power of nanoscience to solve grand engineering challenges.”
Concrete’s strength and durability depends partly on its internal structure and configuration of pores; the more porous the material, the more vulnerable it is to cracking. However, there are no techniques available to precisely control concrete’s internal structure and overall properties, notes Buyukozturk, adding, “It’s mostly guesswork. We want to change the culture and start controlling the material at the mesoscale.”
The “mesoscale” represents the connection between microscale structures and macroscale properties, he observes. For instance, how does cement’s microscopic arrangement affect the overall strength and durability of a tall building or a long bridge? Understanding that connection would help engineers identify features at various length scales that would improve concrete’s overall performance.
“We’re dealing with molecules on the one hand, and building a structure that’s on the order of kilometers in length on the other,” Buyukozturk affirms. “How do we connect the information we develop at the very small scale, to the information at the large scale? This is the riddle.”
To start to understand the connection, he and colleagues looked to biological materials such as bone, deep sea sponges, and nacre (an inner shell layer of mollusks), which have all been studied extensively for their mechanical and microscopic properties. They looked through the scientific literature for information on each biomaterial, and compared their structures and behavior, at the nano, micro, and macro scales, with that of cement paste. They looked for connections between a material’s structure and its mechanical properties, observing how a deep sea sponge’s onion-like structure of silica layers provides a mechanism for preventing cracks, while nacre has a “brick-and-mortar” arrangement of minerals that generates a strong bond between the mineral layers, making the material extremely tough.
A framework for designing cement “from the bottom up” is essentially a set of guidelines that engineers can follow in order to determine how certain additives or ingredients of interest will impact cement’s overall strength and durability. For instance, in a related line of research, Buyukozturk is looking into volcanic ash as a cement additive or substitute. To see whether volcanic ash would improve cement paste’s properties, engineers, following the group’s framework, would first use existing experimental techniques, such as nuclear magnetic resonance, scanning electron microscopy, and X-ray diffraction to characterize volcanic ash’s solid and pore configurations over time.
Researchers could then plug these measurements into models that simulate concrete’s long-term evolution, to identify mesoscale relationships between, say, the properties of volcanic ash and the material’s contribution to the strength and durability of an ash-containing concrete bridge. These simulations can then be validated with conventional compression and nano indentation experiments, to test actual samples of volcanic ash-based concrete.
Ultimately, the researchers hope the framework will help engineers identify ingredients that are structured and evolve in a way, similar to biomaterials, that may improve concrete’s performance and longevity. “Hopefully this will lead us to some sort of recipe for more sustainable concrete,” Buyukozturk says. “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.”