May 27 2016
MIT scientists are exploring new ways to make stronger and more durable concrete by studying the blueprints of nature. Concrete is considered to be the world’s most commonly used man-made material.
“If we can replace cement, partially or totally, with some other materials that may be readily and amply available in nature, we can meet our objectives for sustainability,” MIT Professor Oral Buyukozturk says. (Image: Christine Daniloff/MIT)
The group compared the binding ingredient of concrete called cement paste with the properties and structure of natural substances like shells, bones, and deep-sea sponges. The results of the study have been reported in the Construction and Building Materials journal. The scientists discovered that these bio-materials are extremely durable and strong, partly due to their accurate assembly of structures at various length scales, ranging from the molecular to the macro (visible) level.
The research group, headed by Oral Buyukozturk, a professor in MIT’s Department of Civil and Environmental Engineering (CEE), eventually put forward a novel bioinspired, “bottom-up” method to produce the cement paste.
These materials are assembled in a fascinating fashion, with simple constituents arranging in complex geometric configurations that are beautiful to observe. 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.
Oral Buyukozturk, Professor, CEE Department, MIT
The aim of this study is to discover natural materials that could act as durable and sustainable alternatives to Portland cement, which needs enormous amount of energy to develop.
“If we can replace cement, partially or totally, with some other materials that may be readily and amply available in nature, we can meet our objectives for sustainability,” says Buyukozturk.
The paper’s co-authors include Steven Palkovic, lead author and graduate student; Dieter Brommer, graduate student; Kunal Kupwade-Patil, research scientist; Admir Masic, CEE Assistant Professor and Markus Buehler, CEE department head, the McAfee Professor of Engineering.
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. 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.
Markus Buehler, Head of CEE Department, MIT
From molecules towards bridges
The concrete at present is an arbitrary collection of crushed stones and rocks that are adhered together by a cement paste. The durability and strength of concrete partly relies on its pore configuration and inner structure. For instance, as the material becomes more porous, it becomes more vulnerable to cracking. Conversely, no methods are available to accurately control the inner structure and general properties of concrete.
“It’s mostly guesswork,” says Buyukozturk. “We want to change the culture and start controlling the material at the mesoscale.”
According to Buyukozturk, the correlation between the microscale structures with its macroscale properties is represented by the “mesoscale”. For example, how does the microscopic arrangement of cement influence the strength and durability on the whole of a long bridge or a tall building? Comprehending this link would definitely enable engineers to point out the features that would enhance the overall performance of concrete at different length scales.
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. How do we connect the information we develop at the very small scale, to the information at the large scale? This is the riddle.
Oral Buyukozturk, Professor, CEE Department, MIT
Building upwards from the bottom
In order to interpret this relation, Buyukozturk and his co-workers examined biological materials like deep sea sponges, bone and nacre (internal shell layer of mollusks), all of which have been studied in depth for their microscopic and mechanical properties. The researchers skimmed through the scientific literature to obtain more data on individual biomaterials, and then contrasted their behavior and structures with cement paste at the macro, micro and nano scales.
The researchers then went on to study the relationship between the structure and the mechanical properties of a material. For example, it was discovered that the onion-like structure of silica layers in the deep sea sponge offers a mechanism by which cracks are prevented. Nacre is arranged in a “brick-and-mortar” manner, with minerals that create a strong bond amidst its layers, rendering the material very tough.
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.
Admir Masic, CEE Assistant Professor, MIT
The researchers then applied their new found knowledge of biological materials and also the information collected on present cement paste design devices to ultimately create a standard, bioinspired outline or method for engineers to create cement, “from the bottom up.”
The framework is a set of guidelines that can be followed by engineers to find out how specific target additives or compounds would affect the overall durability and strength of the cement.
For example, in an associated research, Buyukozturk is analyzing the use of volcanic ash as a substitute or additive for cement. In order to check whether volcanic ash could enhance the properties of cement paste, engineers would initially utilize existing experimental methods, like scanning electron microscopy, nuclear magnetic resonance, and X-ray diffraction in accordance to the group’s framework to characterize the solid and pore configurations of volcanic ash over a period of time.
Scientists can then convert the measurements into models that replicate the long-term progress of concrete to recognize mesoscale associations between, for example, the volcanic ash’s properties and the contribution of the material towards the durability and strength of an ash-containing concrete bridge. These simulations could be further validated with traditional nanoindentation and compression experiments to analyze the actual volcanic ash-based concrete samples.
The scientists are hoping that the framework would help engineers to recognize structured ingredients and progress in a way that might enhance the performance and durability of concrete.
Hopefully this will lead us to some sort of recipe for more sustainable concrete. 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.
Oral Buyukozturk, Professor, CEE Department, MIT
This study was funded partly by the Kuwait Foundation for the Advancement of Sciences through the Kuwait-MIT Center for Natural Resources and the Environment, Argonne National Laboratory, and the National Institute of Standards and Technology.