How the World Really Works: The Science Behind How We Got Here and Where We're Going

by Vaclav Smil

Cement is the indispensable component of concrete, and it is produced by heating (to at least 1,450°C) ground limestone (a source of calcium) and clay, shale, or waste materials (sources of silicon, aluminum, and iron) in large kilns—long (100–220 meters) inclined metal cylinders.[79] This high-temperature sintering produces clinker (fused limestone and aluminosilicates) that is ground to yield fine, powdery cement. Concrete consists largely (65–85 percent) of aggregates and also water (15–20 percent).[80] Finer aggregates such as sand result in stronger concrete, but need more water in the mix than do coarser aggregates that use different sizes of gravel. The mixture is held together by cement—typically 10–15 percent of concrete’s final mass—whose reaction with water first sets the mixture and then hardens it.