Bitumen emulsion is a combination of water and bitumen. As the mixture of water and oily products quickly separate, a third component is normally added to the mixture, in order to make it more stable.
Known as emulsifier, this third component is used for making a mixture of water and oil dispersed in each other. In addition to their ability to reduce the tension between oil and water molecules, emulsifiers have an electronic charge that affect the molecules of bitumen emulsion and lead to the categorization of it into Cationic and Anionic.
Cationic emulsifiers, for example, form a layer of positive charge around the bitumen droplets that prevent them from joining each other. This makes bitumen dispersion in water more durable.
The electronic charge of bitumen emulsion decides on what kind of surface it should be used. Some aggregates (such as marble aggregates) mix better with anionic bitumen emulsion and some others are a better solution to cationic bitumen emulsion (such as granite aggregates).