How concrete is made

Concrete is the second most-used substance on Earth after water, and it hardens by a chemical reaction, not by drying out.

stations: 6
things upstream: 44
raw sources: 18
steps from the ground: 8

The short version

Concrete is cement, water, sand and crushed stone mixed into a pourable slurry. The cement reacts with the water in a process called hydration, which hardens the mix into stone over several hours. Steel rebar is usually cast in, because concrete is strong when squeezed but weak when stretched.

Quarry sand and stone

Aggregate is the bulk of any mix. Sand, natural gravel and crushed stone such as limestone or granite are used, and recycled demolition rubble increasingly stands in for part of it.

Goes in
Gravel and crushed stone Silica sand
Weigh every ingredient

At a batching plant, hoppers feed aggregate, cement and water to scales that weigh each ingredient precisely. Less water gives stronger, more durable concrete, and more water makes it flow more freely.

Goes in
Cement Water
Add the admixtures

Small doses of additives tune the mix. Plasticizers let it flow with less water, retarders slow hydration on hot days or big pours, and accelerators speed it up. Fly ash or blast furnace slag can replace part of the cement.
Keep it turning on the truck

Once the ingredients are combined, workers must place the concrete before it hardens, so mixer trucks keep it moving on the road. Hydration begins as soon as water meets cement.
Pour it around the steel

The wet mix is poured into forms around a cage of steel rebar. Concrete can be formulated for very high compressive strength, but its tensile strength is always lower, so the steel carries the stretching loads.

Goes in
Rebar
Let it cure

Hydration is irreversible. The cement dissolves into the water, and interlocking crystals grow through the mix and bind it into one mass. Strength keeps building for weeks.

End of the lineConcrete Next it goes into skyscraper.

Worth repeating

Roman concrete, made with volcanic ash, had a compressive strength similar to modern Portland-cement concrete, around 20 MPa.
Smeaton's Tower, the third Eddystone Lighthouse, built between 1756 and 1759, pioneered the use of hydraulic lime in concrete.
Cement makes up only about 15% of concrete, yet it is the most energy-hungry ingredient.