For researchers looking for ways to reduce emissions, cement is not just a commonplace, high-volume commodity; it is also one of the most complex substances known in materials science. From its structure and composition to the reactions that ensue when it is mixed with water and poured into a mould to set, "we still have some of the most basic questions about cement unanswered," says Hamlin Jennings, director of the Concrete Sustainability Hub at the Massachusetts Institute of Technology in Cambridge.