Concrete nanopores: the hidden key to longer-lasting infrastructure

An ancient material has not necessarily revealed all its secrets, and concrete is proof of that: at the nanoscale, it remains remarkably elusive.

One of the world’s most widely used materials since the Romans refined it, concrete is both highly strong and durable, forming the backbone of all our infrastructure. Bridges, tunnels, roads, dams and foundations: every modern society is completely reliant on this material and its mechanical properties.

Yet, despite its apparent robustness, concrete is ultimately rather vulnerable under closer examination. It contains a network of nanometre-sized pores, whose arrangement determines how well it withstands the passage of time. These pores “control” the movement of ions that eventually corrode it, while such intrusions-water, ions and more-shorten its lifespan.

Worldwide, this premature deterioration comes at a major cost. Replacing a concrete structure means rebuilding it, which requires cement to be fired again, steel to be remelted, and millions of tonnes of greenhouse gases to be emitted once more. This weakness-no pun intended-is the focus of a Rice University study in Houston, Texas, published on 29 September in the Journal of Physical Chemistry.

Inside concrete at the nanoscale

Concrete is produced by combining cement, water and aggregates. When water hydrates cement-a blend of limestone and clay fired at very high temperatures-it triggers a series of chemical reactions that create a solid gel known as calcium silicate hydrate (CSH). This CSH is the “mineral glue” in which the pores form; it binds the grains together and gives concrete its strength.

Although it has long been known that these pores allow water and ions to pass through, precisely how this transport occurs within such minute cavities had remained entirely unclear. To investigate it more closely, the researchers behind the study examined concrete at the atomic scale, using simulations that enabled them to control different pore parameters.

“Until now, we lacked a truly localised view of how ions migrate through these nanopores,” says Kai Gong, the study’s lead author. By virtually recreating these nanopores at an extremely small scale, the researchers were able to track the movement of water and ions as though a microscopic camera had entered them. They found that pore walls behave like sticky surfaces, slowing molecules down sharply, whereas the middle of the pores speeds them up. Why does this matter so much?

How nanopores affect concrete corrosion

Concrete corrosion depends specifically on how quickly chloride ions reach the steel. When these ions move rapidly, they trigger far sooner the electrochemical reaction that eats away at reinforcement and cracks the concrete from within. Conversely, if the material slows their advance, the onset of damage can be significantly delayed.

This atomic mapping now allows engineers to identify which kinds of pores act as corrosion “accelerators” and which instead function as “slow-down mechanisms”.

More durable concrete with a lower climate impact?

In salt-rich coastal settings, for instance, concrete buildings are highly exposed to corrosion because of their proximity to the sea or ocean. Chloride ions, which are plentiful in these environments, gradually pass through the material’s microstructure, attack the steel it contains and speed up the deterioration of structures. Understanding how these ions travel through CSH nanopores means understanding how to design concrete that is stronger and more environmentally friendly.

The construction sector alone accounts for more than 40% of global greenhouse gas emissions, with concrete and steel making up a substantial share of that figure. If, before a project begins, it becomes possible to establish which nanopore matrix minimises ion mobility, then concrete formulations could be adapted to the conditions of their environment. Dry or humid climates, marine or inland locations, intense heat or freeze-thaw cycles: every one of these environmental factors affects ion behaviour. Rice University’s models would make it possible to incorporate these factors from the infrastructure design stage onwards. If this methodology were one day standardised worldwide, it could reduce concrete’s carbon footprint simply by optimising its microstructure.