Concrete forms through the solidification of a mixture of water, gravel, sand, and cement powder. Is the resulting glue material (known as cement hydrate, CSH) a continuous solid, like metal or stone, or is it an aggregate of small particles? As basic as that question is, it had never been definitively answered.

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 (together with other universities in the U.S., France, and U.K.) say they have solved that riddle and identified key factors in the structure of CSH that could help researchers work out better formulations for producing more durable concrete.

Researchers have addressed whether the solidified CSH material, which is composed of particles of many different sizes, should be considered a continuous matrix or an assembly of discrete particles.

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