Study Results Help Develop Better Formulations to Make More Durable Concrete

The most widely used construction material is concrete, and its production is a leading source of greenhouse gas emissions. However, some basic questions on this common construction material relating to its microscopic structure and behavior have remained unanswered.

When a mixture containing gravel, water, cement powder, and sand solidifies, concrete is formed. The question is whether the resultant glue material (cement hydrate or (CSH)) is a continuous solid resembling stone or metal, or a collection of small particles.

This basic question has never received a definitive answer. In a paper published this week in the Proceedings of the National Academy of Sciences, a team of researchers at MIT, Georgetown University, and France’s CNRS (along with other universities in the U.S., France, and the U.K.) claim that they have resolved the riddle, and noted the key factors in the CSH structure. This could aid researchers for working out better formulations to produce more durable concrete.

Roland Pellenq, a senior research scientist in MIT’s department of civil and environmental engineering, director of the MIT-CNRS lab <MSE>2 hosted by the MIT Energy Initiative, said that the new work is based on an earlier research that he conducted along with others at the Concrete Sustainability Hub (CSHub), which was a collaborative effort between MIT and the CNRS. He is also a co-author on the paper.

Although the first atomic-scale model of the concrete structure was accomplished, questions remained about the larger, mesoscale structure, which were of the size of a few hundred nanometers. The new work provides answers to some of the remaining uncertainties, he added.

The solidified CSH material comprises particles of varying different sizes, and the key question is whether the material should be taken as a continuous solid or a collection of discrete particles. The answer is that it is a mixture of both. The particles are distributed such that almost every in-between space among the grains is packed by still smaller grains, so that it can be considered as a continuous solid. “Those grains are in a very strong interaction at the mesoscale,” he says.

“You can always find a smaller grain to fit in between the larger grains,” Pellenq says, and thus “you can see it as a continuous material.” But the grains present inside the CSH “are not able to get to equilibrium,” or a minimum energy state, over length scales involving several grains, with the effect that the material is vulnerable to changes over a period of time, he says. This situation can lead to “creeping” effect of the solid concrete, finally resulting in degradation and cracking. “So both views are correct, in some sense,” he explains.

The investigation of hardened concrete structure showed that pores of varying sizes play key roles in forming the material’s characteristics. While the nanoscale pores have been researched before, the characteristics of mesoscale pores with sizes of 15-20nm have not been well-known, informed Pellenq. These pore gaps can play an important role in finding out the susceptibility of the material to water, which can seep into the CSH material and cause cracks, ultimately resulting in structural failure. However, the cracking is not related to the water expansion during freezing.

According to Pellenq, these mesoscale simulations are the foremost to sufficiently match with the confusing and conflicting results that are noticed in experiments evaluating the CSH texture.

The new simulations help matching the key parameters such as elasticity, stiffness, and hardness, which are noted in actual concrete samples. This proves that the modeling is useful, he says, and might be of help for guiding research on the development of improved formulas, such as those that reduce the quantity of water when mixing with cement powder. The cement powder manufacture is a process that involves cooking limestone (with clays) at elevated temperatures, thus making concrete production a leading source of man-made greenhouse gas emissions.

The researchers realized that precise managing of the quantity of water required for a particular use could improve the material’s durability. The quantity of water used for mixing it with cement powder can impact concrete’s longevity, even when most of the water gets evaporated during the setting process. The team concluded that while water is required for making the slurry flow and pour into a place, excess water results in larger pore spaces and more loose “fluffy” regions in the set concrete. Such regions can make the material more vulnerable to subsequent degradation, or alternatively, can also be engineered to improve its durability.

This is a quintessential step towards the provision of a seamless atom-to-structure understanding of concrete, with huge mid-term practical impact in terms of material design and optimization, this research helps to promote concrete research as a cutting-edge scientific discipline, where the cooperation of engineers and physicists emerges as a driving force for the reunification of natural sciences across the often too-tightly set boundaries of sub-disciplines.

Christian Hellmich, director of the Institute for Mechanics of Materials and Structures at the Vienna University of Technology

The first contributor of this new work is MIT postdoc Katerina Ioannidou. The team also consisted of other researchers at MIT; the University of California at Los Angeles; Newcastle University in the U.K.; and Sorbonne University, Aix-Marseille University, and CNRS, in France. The work received supported from Schlumberger, the French National Science Foundation (ANR) through the Labex ICoME, and the CSHub at MIT.