Cement is a class of ground dry minerals that harden into stone when mixed with water. The type in common use is Portland cement. This is a mixture of about 65% calcium oxide (quicklime), 20% silicon dioxide (sand), 10% clay, and 5% sulfates from gypsum. It is a usually grey powder manufactured worldwide from common materials.
Cement mixed with water undergoes a chemical reaction. The substance takes up water and carbon dioxide, and it hardens into calcium carbonate (limestone). If the cement mixture is injected into an aperture between two stones or other irregular surfaces, it will fill the void and solidify into a binding agent.
Cements of various kinds were used in construction in Macedonia and elsewhere in the ancient world. On a large scale, the Roman Republic incorporated a cement into their aqueducts, roads (right), and other major structures.
Modern use of the Portland cement blend is an evolution of mixtures that were tried in England in the 18th Century and following. The civil engineer John Smeaton created a cement for the Eddystone Lighthouse that would set in the twelve hours between high tides. Joseph Aspdin in 1824 patented a substance that he called Portland cement, though it is now considered an ancestor of the modern version.
Improvements in cement over the next two-and-a-half centuries were driven by the increasing popularity of concrete over natural stone for building structures.
Manufacture of cement is energy intensive. The initial dry ingredients are mixed and baked in a kiln under high temperature (1300°C) where they transform into calcium oxide and other hydrophilic minerals. The key conversion of limestone to quicklime generates quantities of carbon dioxide. The next step involves considerable grinding and produces considerable dust. The final product is bagged and shipped. Cement is caustic and should not be inhaled.
The primary use of cement is as the binding agent in concrete, where it is incorporated into roads and large buildings everywhere.
Cement is also used as mortar for bricks and as a filler for natural or weathered cracks in surfaces.