A bitumen emulsion is an intimate mixture of two or more substances which normally do not mix together. Bitumen emulsion consists of a dispersion of tiny globules (4 to 10 microns in diameter) or bitumen in water, stabilized by the addition of an emulsifying agent.
The nature and quantity of the emulsifying agent controls the type and stability of the emulsion. Emulsions are manufactured by forcing the required proportion of bitumen, water and emulsifying agent into an homogenized, whereby the bitumen is broken up into the globules, dispersed and held in suspension by the emulsifying agent. The bitumen content of an emulsion can vary from 30% to 70% (usually 55-70%) by volume depending on the grade of emulsion required.
Emulsions are used primarily on maintenance work, surface enrichment, dust laying processes, slurry sealing, and in projects where weather conditions may prevent the use of hot
bitumen or cutbacks.
“Breaking” of an emulsion is the term applied when the bitumen globule suspension breaks down resulting in complete separation of the bitumen and water components. Breaking
normally occurs on the pavement surface after spraying whereby the water from the emulsion drains off or evaporates leaving behind a bitumen residue.
The surfaces of the suspended bitumen globules develop an ionic (electrical) charge. The type of charge depends on the emulsifying agent employed. The type of charge depends on the emulsifying agent employed. The presence of this charge is used to advantage to secure greater adhesion of the bitumen to the aggregate.
The basic types of bitumen emulsion are available :-
• Anionic emulsions
• Cationic emulsions.
The two types of bitumen emulsions are incompatible and should not be mixed under any circumstances otherwise premature breaking will occur. Some aggregates, particularly acidic types such as granite and quartzite, develop a negative ionic surface charge when wetted. If a cationic emulsion is used with such aggregate, the bitumen globules, which have a positive ionic surface charge, will be attracted electrochemically to the aggregate and a better bond will be established between the bitumen and the aggregate.
Similarly other aggregates, such as limestone, can develop positive surface charges and under these conditions an anionic emulsion containing bitumen globules with a negative ionic
surface charge is best used.