In a National Academy of Sciences paper, researchers at the Massachusetts Institute of Technology-hosted Concrete Sustainability Hub (CSHub) discuss how the porous network of portland cement absorbs water, and propose how drying permanently rearranges the material while leading to potential structural damage.
|Paper co-author Katerina Ioannidou and peers offer a close-up view of a cement sample’s pore network.|
“Cement hydrates are small, on the nanoscale scale,” says lead author Tingtao Zhou, a PhD student in the Department of Physics. During cement hydration, nanograins aggregate with each other, forming a network that glues all constituents together, he adds. While the process gives concrete its strength, spaces between the hydrates create an extensive pore network in the cement paste.
This poses a problem when trying to study pore network drying. “Let’s say you only have two grains of calcium silicate hydrate; you can imagine there is some water condensation between them,” notes Zhou. “It is easy to measure the water in the pore space and capillary pressure. But when you have a massive number of grains the water distribution becomes really complicated—the geometry becomes a mess.”
To deal with water in cement’s messy pore network, Zhou and co-author Katerina Ioannidou wrestled with two issues. The first was partial saturation. Since the pore network is so complex, water becomes unevenly distributed, which makes it difficult to calculate its distribution. The second issue is that of multiple scales. “In the past, researchers would study the movement of water in pores at either the scale of the atom or on the continuum, or visible, scale,” Zhou explains. “They lost a lot of information on the mesoscale, which is between the atomistic and continuum scales.”
Using computational modeling techniques, Zhou and Ioannidou calculated how water distributes within a pore and then determined the force it exerted on the pore wall. Once complete, they grouped pores together and simulated the effect of drying on the mesoscale. “We found irreversible structural changes on the mesoscale,” Zhou affirms. “It’s not propagating to a larger scale yet. But what happens when we have many of these drying cycles over many years?”
Though it is too early to know how exactly this kind of structural change affects concrete structures, he hopes to develop a model to study the long-term consequences of drying. “We have dealt with different spatial scales but [not] different time scales. These changes occur in a period of nanoseconds and we would like to see their influence over the typical lifetime of concrete structures,” Zhou concludes.