[Editors] MIT: Nanoengineered concrete could cut CO2 emissions

[Elizabeth Thomson]

MIT News Office
Massachusetts Institute of Technology
Room 11-400
77 Massachusetts Avenue
Cambridge, MA  02139-4307
Phone: 617-253-2700
http://web.mit.edu/newsoffice/www

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MIT: Nanoengineered concrete could cut CO2  emissions
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For Immediate Release
MONDAY, JAN. 29, 2007
Contact: Elizabeth A. Thomson, MIT News Office
Phone: 617-258-5402
Email: thomson at mit.edu

PHOTO AVAILABLE

CAMBRIDGE, Mass.--While government leaders argue about the 
practicality of reducing world emissions of carbon dioxide, 
scientists and engineers are seeking ways to make it happen.

One group of engineers at MIT decided to focus its work on the 
nanostructure of concrete, the world's most widely used material. The 
production of cement, the primary component of concrete, accounts for 
5 to 10 percent of the world's total carbon dioxide emissions; the 
process is an important contributor to global warming.

In the January issue of the Journal of the Mechanics and Physics of 
Solids, the team reports that the source of concrete's strength and 
durability lies in the organization of its nanoparticles. The 
discovery could one day lead to a major reduction in carbon dioxide 
emissions during manufacturing.

"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.

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Written by Denise Brehm, MIT Department of Civil and Environmental Engineering