Bitumen emulsions are usually dispersions of minute droplets of bitumen in water and are examples of oil-in-water emulsions. The bitumen content can be varied to suit different requirements and is typically between 30% and 70%. The primary objective of emulsifying bitumen is to obtain a product that can be used without the heating normally required when using cutbacks and paving grade bitumen. In the manufacture of bitumen emulsions, hot bitumen is sheared rapidly in water containing an emulsifying chemical (emulsifier). This produces very small particles of bitumen (the dispersed phase) dispersed in water (the continuous phase). The bitumen particles are stabilized in suspension and do not readily coalesce due to the presence of the emulsifier, which is concentrated on the surface of the bitumen particles. During application, the water in a bitumen emulsion is either lost by evaporation, or it may separate from the bitumen because of the chemical nature of the surface to which the emulsion is applied. This process is referred to as breaking. Because bitumen has a density only slightly higher than water, sedimentation of the bitumen droplets in an emulsion during storage is very slow. Emulsions can usually be regenerated after long storage times by gentle stirring to re-disperse the bitumen droplets.
