Asphalt is a black-brown complex mixture of hydrocarbons with different molecular weights and their non-metallic derivatives. It is a kind of high viscosity organic liquids. It is liquid, black on the surface and soluble in carbon disulfide. Asphalt is an organic cementitious material for waterproofing, moisture-proof and anti-corrosion.
Asphalt can be divided into three types: coal tar pitch, petroleum pitch and natural asphalt.
Coal tar pitch
Coal tar pitch is a by-product of coking, that is, black substance remaining in distilling kettle after tar distillation. It is different from refined tar only in physical properties, and there is no obvious limit. The general division method is to stipulate that the softening point is tar below 26.7 C (cubic block method) and asphalt above 26.7 C. Coal tar pitch mainly contains volatile anthracene, phenanthrene, pyrene and so on. These substances are toxic, and the properties of coal tar pitches differ because of their different contents. The change of temperature has a great influence on coal tar pitch, which is easy to crack in winter and easy to soften in summer. It has a special odor when heated, and the anthracene, phenanthrene, pyrene and other components will evaporate when heated to 260 C for 5 hours.
Petroleum asphalt is the residue after distillation. Liquid, semisolid or solid at room temperature according to the degree of refinement. The oil is dark green and glossy with high temperature sensitivity. Because it has been distilled in the production process to more than 400 degrees Celsius, it contains very little volatile components, but there may be high molecular hydrocarbons have not been volatilized, these substances are more or less harmful to human health.
Natural bitumen is stored underground, forming some ore deposits or accumulating on the surface of the earth's crust. Most of the asphalt is naturally evaporated and oxidized, and generally does not contain any toxins.