Bitumen emulsions are dispersions of bitumen in water. Hot bitumen, water and emulsifier are processed in a high-speed colloid mill that disperses the bitumen in the water in the form of small droplets. These droplets or particles of bitumen are normally in the 5-10 micrometer size range but may be even smaller. The emulsifier assists in forming and maintaining the dispersion of fine droplets of bitumen. Bitumen emulsions normally comprise between 30% and 80% bitumen by volume. If the bitumen starts to separate from the water solution in storage, the emulsion can usually be easily restored by gentle agitation to redisperse the droplets.

Bitumen emulsions have a low viscosity compared to the bitumen from which they are produced and can be workable at ambient temperatures. Their application requires controlled breaking and setting. The emulsion must not break before it is laid on the road surface but, once in place, it should break quickly so that the road can be in service again without delay.

The development of bitumen emulsions is an area where technological progress is still being made to meet current and emerging engineering demands. The first emulsions were the anionic types. They are currently less favored than the cationic types because the positively charged globules of bitumen in cationic emulsions better coat the majority of aggregate types and result in greater adhesion. Use of cationic emulsions is therefore preferred in most applications.