combination of silica sand, limestone, clay, chalk and other minerals that is baked around 2,000C and crushed into fine powder – and pieces of rock or sand called aggregate. Mixing the rocky aggregate, which varies in size from sand to gravel to small chunks of stone, with the cement makes the resulting concrete stronger and saves cement. Finally, adding water to the concrete mix sets off a chemical reaction in the cement that binds these elements together. For the most part, aggregate in modern concrete is carefully chosen to be as chemically inert as possible. The idea is to avoid any unwanted chemistry once this initial reaction concludes, since any additional reactions down the road usually crack or otherwise weaken the concrete.

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