Concrete is made of stone，sand，water and cement (水泥)．The materials in the cement hold the concrete together.Ancient Romans built with concrete.Yet strengthened concrete bridges did not appear until the late 1800s.People keep looking for new ways to improve concrete.Professor Tikalsky says it is one of the most complex of all chemical systems.

　　The new mixtures designed by his team contain industrial waste products.He says these make the concrete better able to resist damage from water and salt over time.One of the products is fly ash.This is released into the air as pollution when coal is burned.

　　Professor Tikalsky says particles (颗粒) of fly ash are almost exactly the same size and chemical structure as Portland cement.This is the most costly material in concrete.So using fly ash to replace some of it would save money.

　　Over the next two years，engineers will study ten bridges in Pennsylvania.These were built from the different cement mixtures designed by Professor Tikalsky’s team.He says longer－lasting bridges could save the state more than 35 million dollars a year.And he says the materials would be environmentally friendly.

　　The federal government is paying for part of the research.Engineers anywhere can use the technology.Professor Tikalsky says some of the ideas have already been put to use in China，the Philippines and other countries.

　　41．Why did the United Nations launch the campaign?

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