When the bitumen emulsion is applied on the aggregate for the road works the water evaporates leaving behind the bitumen droplets. These droplets spread on the aggregate and bind with each other and gains  strength eventually. Based upon the time taken by the emulsion to evaporate the water, bitumen emulsion is classified into 3 types based on setting time. 

Medium setting bitumen does not break as when applied on aggregate. the process of evaporation starts when the fine dust of mineral is mixed with  the aggregate emulsion mix.