In the emulsification process, hot asphalt cement is mechanically separated into minute globules and is dispersed in water that is treated with a small quantity of an emulsifying agent, such as soap, colloidal clay, or one of numerous other organic agents. The rate at which the asphalt globules separate from the water is called the breaking or setting time. This rate is generally dependent upon the emulsifier used and the proportion of water to asphalt. Emulsions are also grouped according to their ability to mix with damp aggregate. Emulsions may be satisfactorily used as a tack coat for bituminous pavements.
Tars are products of the distillation of coal.
No natural sources of tar exist. Coal tar is a general term applied to all varieties of tar obtained from coal. It is produced by one of several methods, depending on the desired end product. When Table 13-7. Typical Uses of Bituminous Materials bituminous coal is destructively distilled, coke and gas are formed, and tar, ammonia, light oils, sulfur, and phenol may be recovered.