What Is Asphalt?
Asphalt is a black material composed primarily of hydrocarbons obtained by refining crude oil.
It is also known as soil bitumen or ground bitumen. It is solid or semi-solid at room temperature, but melts readily at high temperatures and becomes a liquid.
Most of the asphalt used today is derived from petroleum. However, naturally occurring asphalt is also available. Asphalt is sometimes distinguished from sand or stone, which are processed products made by compositing asphalt with sand or stone, which are called asphalt mixtures.
Similar to asphalt is concrete. Concrete is a very different substance because it is a powder that is solidified by adding sand and water to cement and using a polymerization reaction.
Uses of Asphalt
The main uses of asphalt are for road paving, fuel, and building materials.
1. Road Paving
Asphalt is used for paving roads. It is useful for several reasons, one of which is its low price.
Compared to concrete, asphalt can be paved at a lower unit cost. Asphalt is also used for road paving because it can be constructed in a short period of time and its permeability can be controlled.
Recently, asphalt coatings with water retention and thermal barrier properties, as well as coatings that emphasize quietness, have been developed to combat the heat island effect.
2. Construction Materials
Asphalt is also used as a building material. Roofing tarps, for example, are sometimes made of asphalt and are called asphalt roofing. Asphalt roofing is placed under roofing components to prevent rainwater from entering the roof.
In recent years, modified rubber asphalt roofing, in which asphalt is mixed with rubber or synthetic resin, has been developed.
3. Fuel
Asphalt is solid at room temperature, making it a difficult material to handle as a fuel. However, it is used as a fuel for boilers used in power generation because of its low cost and supply compared to heavy oil.
The naturally occurring, less viscous product is ground to a fine powder and used in combustion furnaces by blowing it into the furnace with a fine powder burner. Heavy residues generated from the distillation of heavy oil are highly viscous, so they are heated to make them fluid during transportation. When used, it is heated using excess steam from the boiler to maintain fluidity, and then blown into the combustion furnace using a burner.
In both cases, the residual carbon and metal ash content after combustion is higher than that of heavy oil, so it is necessary to take measures to deal with the residuals after combustion.
Principle of Asphalt
Asphalt is obtained by distilling crude oil, and its main components are paraffins, naphthenes, and aromatic hydrocarbons. The product is then mixed with sand and aggregates. These mixtures are heated and used in liquid form, but they solidify at room temperature, making construction difficult.
Emulsifiers are used to reduce viscosity. Asphalt emulsifiers disperse asphalt in particulate form, which reduces viscosity and makes it possible to apply the emulsion at room temperature. There are three types of emulsifiers: cationic emulsions, anionic emulsions, and nonionic emulsions, but cationic emulsions are currently the most commonly used.
The properties of emulsions vary depending on the emulsifier used, but the filler mixed with the asphalt also affects the properties. Filler is a pulverized material such as limestone. Mixing filler is said to improve viscosity.
A dense-graded asphalt mixture used for a typical road has a composition of 55% coarse aggregate, 35% fine aggregate, 5% filler, and 5% asphalt. More coarse aggregate is used for higher permeability, and more fine aggregate for lower permeability. The desired properties can be achieved by adjusting the composition.
Other Information on Asphalt
Problems as a Fuel
There are two main problems in using asphalt as a fuel. One is wear and pollution of combustion system equipment, and the other is air pollution by combustion exhaust gas. As for impurities in asphalt, these are caused by the various heavy metals and sulfur content in asphalt.
Among the various heavy metals, vanadium compounds have a lower melting point than other metal oxides, so they form a mist in the furnace and adhere and deposit. The molten deposits induce corrosion and inhibit heat transfer, resulting in reduced boiler performance. Therefore, periodic cleaning is necessary.
Most of the sulfur content is gasified as sulfur oxides. If left untreated, this can cause air pollution, so flue gas desulfurization equipment or flue gas treatment facilities are required. Depending on the volume of flue gas, large-scale facilities may be required. In recent years, the use of sulfur as a fuel has become increasingly difficult due to stricter environmental regulations and the increased cost of maintaining facilities.