How cement is made

Making cement gives off nearly its own weight in carbon dioxide, partly because limestone itself releases CO2 when baked.

stations: 7
things upstream: 3
raw sources: 3
step from the ground: 1

The short version

Portland cement is made by grinding limestone and clay, heating the mix to about 1,450 °C in a long rotating kiln until it fuses into marble-sized lumps called clinker, then grinding the clinker with a few percent gypsum. The gypsum controls how fast the powder sets once water is added.

Dig limestone and clay

Limestone is the main ingredient. It is usually impure, with clay mixed in as a source of aluminum and silicon. Shale, sand, iron ore and bauxite can fill in what the local rock lacks.

Goes in
Limestone Clay
Grind and blend the mix

The rock is ground to a fine powder and blended to a precise chemical recipe. Hot exhaust from the kiln is typically used to dry the raw materials in the mill.
Preheat in the cyclone tower

The powder falls through a stack of cyclones, where rising kiln gas heats it. Limestone begins to break down into lime and carbon dioxide above 600 °C. Between 1 and 6 cyclone stages are used.
Burn it in the rotary kiln

A steel tube lined with firebrick slopes 1–4° and turns between 30 and 250 times an hour. Feed creeps downhill into a flame fired by coal, oil or gas. At about 1,450 °C the material partly melts and gathers into nodules 1–10 mm across, called clinker.

about 1,450 °C
Cool the clinker

The hot clinker drops into a cooler, where air blows through it. That air comes out hot and returns to the kiln as combustion air.
Grind it with gypsum

Between 2 and 8% gypsum, typically 5%, is ground in with the clinker in a cement mill. Without gypsum the powder would set almost instantly when wetted. Typically 15% of the finished powder is finer than 5 μm.

typically 5% gypsum

Goes in
Gypsum
Store and ship

The finished powder is pumped to silos and shipped in bags or in bulk by truck, rail or ship.

End of the lineCement Next it goes into washing machine, concrete.

Worth repeating

More than 95% of the world's cement comes from rotary kilns.
Joseph Aspdin named it Portland cement in his 1824 patent because the hardened material resembled Portland stone.
Making a tonne of Portland cement releases nearly a tonne of carbon dioxide.