When coal is burned in a boiler, two kinds of ash by-products are produced: a heavy bottom ash and a fine-as-powder fly ash. About 10 to 15 percent of the coal by-product is bottom ash which is used like sand on streets and highways covered with ice, and also on highways as paving material.
It is the fly ash, however, that is receiving much attention. Once considered a waste, fly ash is now regarded as a natural resource in the United States. It can be placed in some lower places, where it is compacted and covered with soil, for the seeds of various grasses and plants to be placed in the soil to make the land productive.
Fly ash may be added to concrete to build roads and other kinds of buildings, taking the place of the cement up to 20 percent. As a by-product of burned coal, fly ash requires no extra energy to be produced, while cement production requires great amounts of energy.
Besides, the technology is able to take out some useful minerals from the fly ash, which is still a very expensive process today. But the time may come when it is cheaper to take out these products from ash than to mine new supplies.
小题1: The bottom ash can be put on icy streets________.A．in order to rebuild themB． in order to make them colourfulC．so that people can walk on themD．so that cars can run faster小题2:The fly ash_______ bottom ash.A．has the same use asB． is more expensive thanC．has more uses thanD．is much cheaper than小题3:The fly ash can be used to________.A．grow various plantsB． take the place of soilC．cover productive landD．make more productive land小题4: Which of the following is right?A．To produce fly ash, some extra coal is to be burned.B． Fly ash is made while producing cement.C．It takes less energy to produce fly ash than cement.D．Fly ash is a costless by-product.小题5:The word “mine” in the last sentence of the passage means_______.A．to buy things from other countriesB． to search for useful materials in the mountainsC．to dig deeply into the ground to get useful materialsD．to take out certain materials from other things