cement
Cement is the grey powder that, mixed with water, sets into artificial stone — the glue that binds sand and gravel into concrete, the most-used manufactured material on Earth by a wide margin. Pour it, and a few hours later it is holding a footprint; a month later it is a sidewalk, a bridge, a dam. What makes cement remarkable is that it does not dry hard like mud — it chemically reacts with water and grows interlocking crystals, so it can even set underwater. It is, by tonnage, by far the biggest ceramic humanity makes.
Ordinary Portland cement is made by grinding limestone (calcium carbonate) and clay and firing them in a rotary kiln to about 1450 degrees C. At that heat the mix partly melts and reacts to form hard nodules called clinker, whose main phases are calcium silicates, chiefly tricalcium silicate (3CaO times SiO2) and dicalcium silicate. Grinding the clinker to a fine powder gives cement. When you add water, those calcium-silicate grains hydrate — they react with water to precipitate a nanoscale, gel-like calcium-silicate-hydrate (C-S-H) that grows through the paste and knits the particles together, plus calcium hydroxide. This hydration is why concrete gains strength over days and weeks, not minutes, and why it needs to stay moist (be cured) to develop full strength. Concrete itself is cement paste plus sand and stone aggregate; the cement is only the binder, roughly 10 to 15 percent of the mix.
Concrete behaves like a ceramic where it counts: it is superb in compression — a column carries enormous downward load — but weak in tension, which is exactly why it is cast around steel reinforcing bars that take the pulling loads it cannot. The honest cost is climate: making cement releases enormous carbon dioxide, both from burning fuel to reach 1450 degrees C and, chemically, from driving CO2 out of the limestone. Cement manufacture alone is responsible for roughly 7 to 8 percent of global human CO2 emissions, which is why low-carbon cements and geopolymer binders are an active field. And a common misconception worth correcting: concrete does not set by drying out — it sets by hydration, and letting it dry too soon actually weakens it.
Cast a concrete beam and let it dry in the sun and it may crack and stay weak; keep it damp for a week (curing) and the calcium-silicate grains keep hydrating, growing C-S-H gel that raises the strength for weeks — proof that concrete sets by chemical reaction with water, not by drying.
Cement is fired calcium silicate that hydrates into interlocking gel; concrete is that binder plus aggregate — compression-strong, tension-weak, and hydration-set.
Concrete does not harden by drying — it hardens by hydration, a chemical reaction with water that continues for weeks and even works underwater. Letting fresh concrete dry out too fast starves that reaction and leaves it weak and crack-prone.