How can cities be built to be more climate-friendly? Construction activities and buildings cause almost 40% of global CO2 emissions. In order to counteract this trend, new construction concepts are required: Flexible construction, renovation, expansion and continued use of buildings require more flexible building permits. A promising solution is hybrid constructions such as the combination of wood and concrete. By using new recycled building materials such as more climate-friendly cement, ash, slag and R-concrete, CO2 emissions can be significantly reduced. The EU project CIRCUIT provides important insights for circular cities by integrating circular economy principles into urban planning and construction processes. When cold ash becomes a hot pavement: As the end product of thermal waste recycling, the replacement building material “emvau slag” is created via a refining process. The result is a high-quality and certified replacement building material that is versatile and can be used as a base layer under waterproof surface layers such as concrete and asphalt. “emvau-schlacke” is used particularly in road and path construction as well as for paved commercial and industrial areas such as parking lots and storage areas. This replacement building material can therefore make a sustainable contribution to conserving resources, especially in urban environments.