"If everything depends on the organizational
structure of the nanoparticles that make up concrete, rather than on
the material itself, we can conceivably replace it with a material
that has concrete's other characteristics-strength, durability, mass
availability and low cost-but does not release so much CO2
into the atmosphere during manufacture," said Franz-Josef Ulm, the
Esther and Harold E. Edgerton Professor of Civil and Environmental
Engineering.
The work also shows that the study of very common
materials at the nano scale has great potential for improving
materials in ways we might not have conceived. Ulm refers to this work
as the "identification of the geogenomic code of materials, the
blueprint of a material's nanomechanical behavior."
Cement is manufactured at the rate of 2.35 billion
tons per year, enough to produce 1 cubic meter of concrete for every
person in the world. If engineers can reduce carbon dioxide emissions
in the world's cement manufacturing by even 10 percent, that would
accomplish one-fifth of the Kyoto Protocol goal of a 5.2 percent
reduction in total carbon dioxide emissions.
Ulm considers this a very real possibility.
He and Georgios Constantinides, a postdoctoral
researcher in materials science and engineering, studied the behavior
of the nanostructure of cement. They found that at the nano level,
cement particles organize naturally into the most densely packed
structure possible for spherical objects, which is similar to a
pyramid-shaped pile of oranges.
Cement, the oldest engineered construction
material, dating back to the Roman Empire, starts out as limestone and
clay that are crushed to a powder and heated to a very high
temperature (1500 degrees Celsius) in a kiln. At this high temperature,
the mineral undergoes a transformation, storing energy in the powder.
When the powder is mixed with water, the energy is released into
chemical bonds to form the elementary building block of cement,
calcium-silicate-hydrate (C-S-H). At the micro level, C-S-H acts as a
glue to bind sand and gravel together to create concrete. Most of the
carbon dioxide emissions in this manufacturing process result from
heating the kiln to a temperature high enough to transfer energy into
the powder.
Ulm and Constantinides gathered a wide range of
cement pastes from around the world, and, using a novel
nano-indentation technique, poked and prodded the hardened cement
paste with a nano-sized needle. An atomic force microscope allowed
them to see the nanostructure and judge the strength of each paste by
measuring indentations created by the needle, a technique that had
been used before on homogenous materials, but not on a heterogeneous
material like cement.
To their surprise, they discovered that the C-S-H
behavior in all of the different cement pastes consistently displays a
unique nanosignature, which they call the material's genomic code.
This indicates that the strength of cement paste, and thus of concrete,
does not lie in the specific mineral, but in the organization of that
mineral as packed nanoparticles.
The C-S-H particles (each about five nanometers, or
billionths of a meter, in diameter) have only two packing densities,
one for particles placed randomly, say in a box, and another for those
stacked symmetrically in a pyramid shape (like a grocer's pile of
fruit). These correspond exactly to the mathematically proved highest
packing densities allowed by nature for spherical objects: 63 and 74
percent, respectively. In other words, the MIT research shows that
materials pack similarly even at the nano scale.
"The construction industry relies heavily on
empirical data, but the physics and structure of cement were not well
understood," said Constantinides. "Now that the nano-indentation
equipment is becoming more widely available-in the late 1990s, there
were only four or five machines in the world and now there are five at
MIT alone-we can go from studying the mechanics of structures to the
mechanics of material at this very small scale."
If the researchers can find-or nanoengineer-a
different mineral to use in cement paste, one that has the same
packing density but does not require the high temperatures during
production, they could conceivably cut world carbon dioxide emissions
by up to 10 percent.
This aspect of the work is just beginning. Ulm estimates that it will take about five years, and says he's presently
looking at magnesium as a possible replacement for the calcium in
cement powder. "Magnesium is an earth metal, like calcium, but it is a
waste material that people must pay to dispose of," he said.
He recently formed a research team with colleagues
in physics, materials science and nuclear engineering to perform
atomistic simulations, taking the work a step deeper into the
structure of this ubiquitous material.
The research was funded in part by the Lafarge
Group. |