After concrete is formed 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 the Massachusetts Institute of Technology (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. (*The MIT Concrete Sustainability Hub is sponsored by the RMC Research & Education Foundation and Portland Cement Association.)