Concrete is the most widely used man-made material, commonly used in buildings, roads, bridges and industrial plants. But producing the Portland cement needed to make concrete accounts for 5-8% of all global greenhouse emissions. There is a more environmentally friendly cement known as MOC (magnesium oxychloride cement), but its poor water resistance has limited its use—until now. We have developed a water-resistant MOC, a "green" cement that could go a long way to cutting the construction industry's emissions and making it more sustainable.