The aim of this research was to understand the dependence of the mechanical behaviour of bitumen emulsion-cement composites on the structural transition of the co-binder system. The experimental programme mainly consisted of cyclic indirect tensile testing at a range of temperatures and frequencies, integrated by X-ray microtomography. The results showed the existence of a critical composition of the co-binder system at the bitumen to cement ratio between 1 and 2 at which the mechanical behaviour transformed from cement-dominated to bitumen-dominated. This was caused by both the percolation of the bituminous phase and the pore space, and the volume contraction of the co-binder system. However, this critical composition did not directly match with the percolation of the phases on the length scale of the mesostructure, but likely corresponded with the volumetric content of all elastic phases, with the aggregate playing a fundamental role.