The industrial production of cement is currently responsible for 5-7% of global CO2 emissions. The manufacture of functionally graded concrete structures, where the use of material is optimized by varying the material composition over the volume, opens up vast opportunities to reduce the use of cement. In functionally graded concrete, horizontal gradation can be achieved by casting vertical layers having homogeneous properties. However, a major problem with this kind of application is the control of the fresh state deformations. This study investigates the fresh state stability of concrete prisms that consist of two vertical layers of different mixes. Experiments are performed to invoke stable or unstable behaviour. The results show that, for a given geometry, a relationship between the material parameters and system stability exists.