Nanoengineered concrete cuts CO2

Engineers at MIT are studying the nanostructure of concrete with a view to making a replacement product that would not generate the same high carbon dioxide emissions.

The production of cement, the primary component of concrete, accounts for five to 10 per cent of the world's total carbon dioxide emissions, making it an important contributor to global warming.

The team found that the source of concrete's strength and durability lies in the organisation of its nanoparticles, a discovery that could one day lead to a major reduction in carbon dioxide emissions during manufacturing.

‘If everything depends on the organisational 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, Professor of Civil and Environmental Engineering.

If engineers could reduce carbon dioxide emissions in the world's cement manufacturing by even 10 per cent, that would accomplish one-fifth of the Kyoto Protocol goal of a 5.2 percent reduction in total carbon dioxide emissions.

Ulm 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 organise naturally into the most densely packed structure possible for spherical objects, which is similar to a pyramid-shaped pile of oranges.

Cement starts out as limestone and clay that are crushed to a powder and heated to 1500 degrees Celsius in a kiln. 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 used a nano-indentation technique to examine various hardened cement pastes 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.

They discovered that the calcium-silicate-hydrate (C-S-H) chemical bonding behaviour in concrete 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 organisation of that mineral as packed nanoparticles.

If the researchers can find or nanoengineer a different mineral to use in cement paste 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 per cent.

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.