What is Concrete?
[[File:Pantheon rome 2005may.jpg|thumb|[[Hadrian]]'s [[Pantheon, Rome|Pantheon]] in Rome is an example of Roman concrete construction.]] [[File:Pantheon dome.jpg|thumb|Outer view of the [[Roman Empire|Roman]] [[Pantheon, Rome|Pantheon]], still the largest (43.4m dia.) unreinforced solid concrete [[dome]].<ref>[http://www.romanconcrete.com/ The Roman Pantheon: the triumph of concrete]</ref>]] [[File:1981 BostonCityHall byLebovich8 HABS MA1176.jpg|thumb|A modern building: [[Boston City Hall]] (completed 1968) is constructed largely of concrete, both precast and poured in place.]] [[File:Concrete pouring 0020.jpg|thumb|right|Concrete used to make a building]] '''Concrete''' is an important [[material]] for making different [[building]]s and [[structure]]s. It is a [[Composite material|composite]] made of Portland [[cement]], [[sand]], [[gravel]] or [[rock aggregate]], and [[water]]. The amount of each can change depending upon what the concrete will be used for. Concrete is used more than any other man-made material in the world.<ref>{{cite book|title=The Skeptical Environmentalist: measuring the real state of the world|url=https://archive.org/details/skepticalenviron00lomb_0|authorlink=|last=Lomborg|first=Bjørn|year=2001|isbn=978-0-521-80447-9|page=[https://archive.org/details/skepticalenviron00lomb_0/page/138 138]}}</ref> As of 2006, about 7.5 billion cubic meters of concrete are made each year—more than one cubic meter for every person on Earth.<ref>{{cite web|title=Minerals commodity summary – cement – 2007|publisher=United States Geographic Service|date=1 June 2007|url=http://minerals.usgs.gov/minerals/pubs/commodity/cement/index.html|accessdate=16 January 2008|archive-date=13 December 2007|archive-url=https://web.archive.org/web/20071213052530/http://minerals.usgs.gov/minerals/pubs/commodity/cement/index.html|url-status=dead}}<!--Computed by taking 2007 figure for world concrete production and the mix at http://en.wikipedia.org/wiki/Concrete#Regular_concrete and computing the volume--></ref> The ingredien