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?

【陕西省2017高考英语一轮复习 《Cloning》课时作业 外研版选修6】相关文章：

★ 2016年高考英语第二轮复习策略与技巧

★ 2015-2016高考英语一轮复习的四点建议

★ 2016高考英语二轮复习提高阅读能力要加强课外阅读

★ 四川省2016届高考英语一轮复习课时作业（39） Module 3 Literature 外研版选修8

★ 【人教版】2014届高考英语一轮复习指导考点针对练：必修2 unit2《The Olympic Games》

★ 【北京专用】2014届高考英语一轮复习方案课时作业：必修5 Unit 2 The United Kingdom

★ 【浙江专用】2014届高考英语一轮复习课时作业(十)必修2Unit 5《Music》

★ 湖南省汝城二中2014届高考英语一轮复习阅读训练 (82)

★ 【人教版】2014届高考英语一轮复习指导考点针对练：选修6 unit4《Global warming》

★ 天津市武清区杨村第四中学2016届高考英语一轮复习语法专题十《虚拟语气》课件外研版