Briefly describe the process of distillation by which asphalt cement is produced from crude petroleum. Also describe in detail how to obtain asphalt binders that can be used to coat highly siliceous aggregates.
What will be an ideal response?
Asphalt cement is obtained from the distillation of crude petroleum by
either fractional or destructive distillation. Fractional distillation processes
involve the separation of the different materials in the crude petroleum without
significant changes in the chemical composition of each material. The different
volatile materials are removed at higher and higher temperatures until the
petroleum asphalt is left as residue. Either steam or vacuum is used to increase the
temperature. Destructive distillation, or cracking, involves the application of high
temperatures and pressure and results in chemical changes. Cracking processes
are used when larger amounts of the light fractions (e.g. motor oil) are needed.
The asphaltic material obtained from cracking is not widely used in paving
because it is more susceptible to weather changes.
Asphalt emulsions are produced by breaking asphalt cement into minute
particles and dispersing them in water with an emulsifier. The particles are of like
electrical charge and therefore do not coalesce. Aggregates containing a high
percentage of siliceous materials are electronegative and therefore cationic
emulsions are more effective.
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