Material · Engineered

Concrete

Cement, aggregate and water — the most-used manufactured material on Earth, and a significant share of global carbon emissions.

Construction material

Concrete is cement paste binding an aggregate of sand and stone. The cement is the expensive, energy-intensive, chemically active part; the aggregate is most of the volume and is essentially inert filler that happens to be strong.

It is enormously strong in compression and weak in tension, in roughly a ten-to-one ratio. Every structural use of concrete is organised around that asymmetry: steel reinforcement is placed exactly where the tension will be, and a reinforced concrete beam is really a composite in which each material does the thing the other cannot.

Processing

Concrete does not dry, it cures. The cement reacts chemically with water — hydration — growing interlocking crystals that bind the aggregate. Water is consumed by the reaction rather than evaporating, which is why concrete sets under water and why curing slabs are kept damp rather than allowed to dry out.

The water-to-cement ratio governs the outcome more than any other variable. More water makes the mix easier to place and the finished concrete weaker and more porous, and managing that trade-off is most of the practical craft of concrete work.

Uses

Foundations, frames, floors, walls, dams, bridges, roads, tunnels and pipes. By mass, more concrete is used than any other manufactured material, and its consumption tracks urbanisation closely.

It is used because it is cheap, made from locally abundant raw materials, poured into any shape, fire-resistant, and durable for decades with no maintenance. No proposed substitute matches that combination at anything like the scale required.

Environmental impact

Cement manufacture is responsible for a substantial share of global carbon dioxide emissions, and the problem is not only fuel. Calcining limestone releases carbon dioxide as a direct chemical product of the reaction, so even a kiln heated entirely by renewable energy would still emit it.

That is what makes cement one of the harder decarbonisation problems. Approaches under development include supplementary cementitious materials that replace part of the clinker, alternative chemistries, and carbon capture at the kiln. Concrete does reabsorb some carbon dioxide over its life as it carbonates, but far less than was released making it.

Medium confidence Weak evidence

How we know: checked recently · only one source, so there is nothing to cross-check it against · stated directly by the source.

How this connects

Where a connection has been confirmed by an outside reference, that reference is named beside it.

is composed of

Portland cement — material · the binder — the expensive, energy-intensive, chemically active part
Aggregate — material · 70 to 80 per cent by volume — the cement is the expensive part and the stone is most of the material
Water — compound · 14–18% · and it is consumed rather than dried off: cement sets by hydration, which is a chemical reaction with this water, and concrete cured under water is stronger than concrete left in the air
Sand — material · 20–30% · the fine aggregate, filling the space between the gravel — roughly a quarter of the mix by volume, and it must be angular river or marine sand rather than rounded desert sand

contains

Calcium — element

is used in

Construction — industry · foundations, frames, floors and civil structures

is an alternative to

Steel — alloy · as a structural frame: concrete is cheap, fireproof and slow to build with, steel is fast and needs protecting from fire, and tall buildings routinely use both
Wood — material · in building frames, where the argument has moved from cost and fire to embodied carbon — and engineered timber has taken back mid-rise on that ground
Aggregate — material · not a substitute — recorded because recycled concrete is crushed back into aggregate, which is the closest thing construction has to a loop at this scale

is used as

Structural engineering — application · by mass, the most used structural material there is, and the reason cement production alone accounts for a large share of industrial carbon emissions

is a component of

Reinforced concrete — material · with steel put where the tension is

is commonly confused with

Asphalt concrete — material · one is stone in bitumen and flexes, the other is stone in cement and cracks, and 'concrete' in the American sense means only the second

is produced by

Hydration — process · it does not dry — the calcium silicates react with the mix water and grow an interlocking hydrate gel, which is why it sets under water and must be kept wet to reach strength

Sources

Material World
Our own writing

Where it comes from, and what it becomes

Follow Concrete back to what it starts as, and forward into what it becomes. Each step is a documented one — a real route material takes, not a chain of inference.

Upstream — what it comes from

Concrete → is composed of (the binder — the expensive, energy-intensive, chemically active part) → Portland cement → is composed of (as calcium oxide within the clinker phases, not as free lime) → Quicklime → is produced by (the solid residue once carbon dioxide has been driven off) → Calcination → takes as input (heated until the carbonate decomposes to lime and carbon dioxide) → Limestone → is composed of → Calcite
Concrete → is composed of (70 to 80 per cent by volume — the cement is the expensive part and the stone is most of the material) → Aggregate → is produced by (crushed and graded from quarried rock, and the grading is the product — a concrete mix specifies a size distribution, not a stone) → Quarrying → takes as input (the most quarried rock in the world, for building stone, aggregate and cement alike) → Limestone → is sourced from (precipitated from it, mostly by organisms building shells and skeletons out of dissolved calcium and carbonate) → Seawater → is composed of (about 96.5 per cent by mass — seawater is 3.5 per cent dissolved solids and the rest of it is this) → Water
Concrete → is produced by (it does not dry — the calcium silicates react with the mix water and grow an interlocking hydrate gel, which is why it sets under water and must be kept wet to reach strength) → Hydration → takes as input (slaked with water to make the lime putty that mortar and plaster are mixed from) → Quicklime → is produced by (the solid residue once carbon dioxide has been driven off) → Calcination → takes as input (heated until the carbonate decomposes to lime and carbon dioxide) → Limestone → is composed of → Calcite
Concrete → is composed of (the fine aggregate, filling the space between the gravel — roughly a quarter of the mix by volume, and it must be angular river or marine sand rather than rounded desert sand) → Sand → is sourced from (the ultimate source of most of it — granite weathers, the feldspar becomes clay and washes out, and the quartz survives as grains) → Granite → is composed of (by volume, alongside feldspar and lesser mica) → Quartz
Concrete → is composed of (and it is consumed rather than dried off: cement sets by hydration, which is a chemical reaction with this water, and concrete cured under water is stronger than concrete left in the air) → Water → is sourced from (by desalination, where energy is cheap and coastline available — the only route that adds fresh water rather than moving it) → Seawater
Concrete → is composed of (the fine aggregate, filling the space between the gravel — roughly a quarter of the mix by volume, and it must be angular river or marine sand rather than rounded desert sand) → Sand → is composed of (almost everywhere, because quartz is what is left when everything else in a rock has weathered away — though a volcanic beach is basalt grains and a tropical one is broken shell) → Quartz

These are the most distinct paths back. Concrete can be traced through others besides.

Downstream — what it becomes

Concrete → is a component of (with steel put where the tension is) → Reinforced concrete → is associated with (after it rather than during — Portland cement is 1824 and reinforcement is the 1860s onward, and the Romans had concrete with no reinforcement at all) → Industrial Revolution complete chain
Concrete → is used in (foundations, frames, floors and civil structures) → Construction
Concrete → is used as (by mass, the most used structural material there is, and the reason cement production alone accounts for a large share of industrial carbon emissions) → Structural engineering
Concrete → is a component of (with steel put where the tension is) → Reinforced concrete → is used as (more of the built environment than any other structural material) → Structural engineering
Concrete → is a component of (with steel put where the tension is) → Reinforced concrete → is used in (frames, floors, foundations, bridges and dams — and its eventual corrosion is a materials failure rather than a design one) → Construction