Impact of wetting and drying cycles on cement

Scientists shed light on the wetting-drying behavior of cement. The study was recently published in the Proceedings of the National Academy of Sciences.

Cement is widely used as a construction material and consists one of the key components of concrete. According to researchers from Massachusetts Institute of Technology (MIT), cement may behave in a similar manner as sponge when exposed to precipitation or humidity due to its porous network. Cement can absorb water and when it is dried, its shape is rearranged and thus a degradation of its mechanical strength occurs.

In order to better understand the phenomenon, the procedure of cement's production must be explained. Cement is produced via a controlled chemical combination of ingredients including calcium, silicon, aluminum, iron and others. These ingredients, when heated at high temperatures form a rock-like substance that is turned into fine powder. The powder is then mixed with water forming the cement paste and its porous network as microscopic compounds, that are also known as cement hydrates, are created

Cement hydrates aggregate to form an entire cohesive system that bonds all the mix together. During this procedure, empty spaces between the cement hydrates form a relatively large porous network. "You have numerous pores of variable sizes that are interconnected. It becomes very complex. And since they are so small, you don't even need rain to fill them with water. Even ambient humidity can fill these pores," Tingtao Zhou, lead author of the paper and a PhD student in the MIT Department of Physics, stated.

In their study, researchers tried to investigate the process of the network's drying. However, the attempt proved to be rather challenging. Zhou explained that due to the large number of calcium silicate hydrate grains, water distribution in a sample becomes very complex. This fact poses a problem when trying to study the wetting-drying behavior of a porous network. To address the issue, Zhou along with Katerina Ioannidou, a research scientist with the French National Center for Scientific Research (CNRS) and the MIT Energy Initiative and a corresponding author of the study, resolved 2 critical issues:

First of all, researchers made the pore water network less complex by using partial saturation. Secondly, they tried to investigate the water movement in multiple scales. As Zhou explains, "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. This means they lost a lot of information on the mesoscale — which is between the atomistic and continuum scales."

Computational modeling was used to derive the water distribution within the porous network and the water pressure. Then, the effect of drying on the mesoscale was simulated. The results showed that the hydrate grains were "irreversibly rearranged under mild drying" and despite the alteration appeared not to be extensive, the impact was not negligible. According to Zhou, the long term effects of drying cycles may radically change the cement's mechanical behavior. The issue will be further investigated in the future. "In this paper, we have dealt with different spatial scales. But we have yet to deal with 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 explains.