A pair of researchers at a US university have published research aiming to help uncover new ways to fine-tune the chemistry of materials such as concrete to make them less prone to cracking and more suitable for specific applications.
Rice University, Houston, Texas researchers Rouzbeh Shahsavari and Saroosh Jalilvand’s study is the first to run sophisticated calculations that show how atomic-level forces affect the mechanical properties of a complex particle-based material like concrete.
The research appears in the American Chemical Society journal Applied Materials and Interfaces ("Molecular Mechanistic Origin of Nanoscale Contact, Friction and Scratch in Complex Particulate Systems").
The study used calcium-silicate-hydrate (C-S-H), aka cement, as a model particulate system. Shahsavari became quite familiar with C-S-H while participating in construction of the first atomic-scale models of the material.
C-S-H is the glue that binds the small rocks, gravel and sand in concrete. Though it looks like a paste before hardening, it consists of discrete nanoscale particles. The van der Waals and Coulombic forces that influence the interactions between the C-S-H and the larger particles are the key to the material’s overall strength and fracture properties, said Shahsavari. He decided to take a close look at those and other nanoscale mechanisms.
A calcium-silicate-hydrate (aka cement) tip hovers above a smooth tobermorite surface in a computer simulation by Rice University scientists. The researchers studied how atomic-level forces in particulate systems interact when friction is applied. Their calculations show such materials can be improved for specific applications by controlling the materials' chemical binding properties.
“Classical studies of friction on materials have been around for centuries,” said Shahsavari. “It is known that if you make a surface rough, friction is going to increase. That’s a common technique in industry to prevent sliding: Rough surfaces block each other.
“What we discovered is that, besides those common mechanical roughening techniques, modulation of surface chemistry, which is less intuitive, can significantly affect the friction and thus the mechanical properties of the particulate system.”
Shahsavari said it’s a misconception that the bulk amount of a single element — for example, calcium in C-S-H — directly controls the mechanical properties of a particulate system. “We found that what controls properties inside particles could be completely different from what controls their surface interactions,” he said. While more calcium content at the surface would improve friction and thus the strength of the assembly, lower calcium content would benefit the strength of individual particles.
“This may seem contradictory, but it suggests that to achieve optimum mechanical properties for a particle system, new synthetic and processing conditions must be devised to place the elements in the right places,” he said.
Shahsavari said atomic-level analysis could help improve a broad range of non-crystalline materials, including ceramics, sands, powders, grains and colloids.
