An international team of researchers from Rice University, the Massachusetts Institute of Technology and Marseille University has used sophisticated computer models to determine how the molecular structure of concrete results in its physical properties.

Rouzbeh Shahsavari, assistant professor of civil and environmental engineering at Rice University, said the computer models used enabled the scientists to perform detailed analyses of the basic molecular structure of concrete, enabling them to better create methods for altering its physical properties.

“The heart of concrete is C-S-H – that’s calcium, silicate and hydrate (water),” said Shahsavari. “There are impurities, but C-S-H is the key binder that holders everything together, so that’s what we focused on.”

“In a nutshell, we tried to decode the phases of C-S-H across different chemistries, thereby improving the mechanical properties of concrete in a material way.”

Rouzbeh Shahsavari

Rouzbeh Shahsavari

According to Shahsavari, the studies have successfully cracked the secrets to some of the material properties of concrete, with game-changing implication for the global construction industry and its impact on the environment.

“C-S-H is one of the most complex structured gels in nature, and the topology changes with different chemistries, from highly ordered layers to something like glass, which is highly disordered,” he said. “We came up with a comprehensive framework to decode it, a kind of genome for cement.”

“This is the first time we’ve been able to see new degrees of freedom in the formation of concrete based on the molecular topology…at any given calcium-silicon ratio there may be 10 to 20 different molecular shapes, and each has a distinct mechanical property.”

Studies conducted over a year-long period analysed the “defect attributes” of around 150 mixtures of concrete in order to determine how the material’s physical properties are created by its molecular structure, focusing on areas including the ratio of calcium to silicon, the topology of atomic-level structures, and the impact of the randomness or regimentation of molecules on strength and ductility.

molecular structure of concrete courtesy of Rice University

molecular structure of concrete courtesy of Rice University

The study concluded that the key to altering the properties of concrete is tweaking with its calcium to silicon ratio.

“For strength, a  lower calcium content is ideal,” said Shahsavari. “You get the same strength with less material because calcium is associated with the energy-intensive components of concrete, you use less rebar and you save energy in transporting the raw material. Also, it’s more environmentally friendly because you put less carbon dioxide into the atmosphere.”

Higher degrees of calcium, however, confer concrete with heightened fracture toughness, enabling it to better withstand the forces produced by strong winds or seismic events when used for as a building material for bridges or buildings.

Shahsavari considers the work to be “one of the most important discoveries in cement science this century,” since it will enable researchers to optimise the physical properties of concrete for specific applications at the molecular level.

The ability to reduce CO2 emissions associated with the manufacture of concrete could also have a dramatic impact on the sustainability of the construction industry, given just how much of the basic building material it uses.

According to the researchers 20 billion tons of concrete is produced globally each year, comprising five to 10 per cent of the world’s CO2 emissions, a percentage surpass solely by transportation and energy production.