Solid building materials are facing a turning point. As one of the largest emitters of CO2, they are now heavily criticized: In building research, however, there is potential to turn them in the direction of CO2 negativity and thus into a beacon of hope. The lecture will report on building material innovations from geopolymers to carbon black as a CO2 sink. A special highlight is Roman concrete, a cement-free building material from a time 2000 years ago, which can lead us into a more sustainable future of construction.