Portland cement
The hydraulic binder in almost all modern concrete, made by burning limestone with clay.
Portland cement is made by burning limestone with clay or shale at around 1,450 °C, producing nodules called clinker which are ground with a little gypsum. Mixed with water it sets by hydration rather than by drying, which means it sets underwater — the property that makes it hydraulic and the reason it displaced lime mortar.
It is the most-manufactured material on Earth by mass after water-based products, and among the largest single industrial sources of carbon dioxide. Roughly half those emissions are chemical rather than from fuel: driving CO₂ out of limestone is the reaction, not a side effect, which is why cement is so difficult to decarbonise.
History
Patented by Joseph Aspdin in 1824, who named it for its resemblance to Portland stone. The material as used today owes more to later refinements, particularly the higher burning temperatures that produce true clinker.
Environmental impact
Cement production accounts for a substantial share of global industrial carbon dioxide emissions. The chemical portion, released as limestone decomposes, cannot be eliminated by changing fuel — only by changing the chemistry or capturing the gas. This is why cement is classed among the hard-to-abate sectors.
How it forms
Portland cement is made, not found, and the making is the point. Limestone and clay or shale are ground together and fired in a rotary kiln to around fourteen hundred degrees, hot enough that the mixture partially fuses into nodules called clinker.
What happens chemically is that calcium carbonate decomposes to lime and carbon dioxide, and the lime then combines with silica, alumina and iron oxide to form calcium silicates. Those silicates are the compounds that react with water and give the set. The clinker is ground with a few per cent gypsum, which retards the reaction enough to make the cement workable.
Uses
Almost all cement goes into concrete and mortar, and concrete goes into everything: foundations, frames, floors, pavements, dams, pipes and the reinforced structures that make up most of the modern built environment.
A smaller share is used in grouts, renders and screeds, in soil stabilisation, and in oil well cementing, where cement is pumped down the borehole to seal the casing against the rock. Blended cements — where some clinker is replaced with ground slag, fly ash or limestone — are increasingly standard, because the replacement lowers both cost and the carbon released in manufacture.
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
- Quicklime — compound · as calcium oxide within the clinker phases, not as free lime
is sourced from
- Limestone — rock · the calcium source, fired with clay to make clinker
is used in
- Construction — industry
- Energy generation — industry · wind turbine foundations and dam construction, and it is a larger share of a wind farm's material mass than the turbine is
is a component of
- Concrete — material · the binder — the expensive, energy-intensive, chemically active part
is produced by
- Calcination — process · as clinker, ground with gypsum to retard the set
is an alternative to
- Quicklime — compound · as a mortar and render binder. Cement is stronger and faster and traps moisture in soft masonry, and lime is the reason a conservation specification will refuse it
- Epoxy resin — material · as a repair and anchoring material in concrete construction, where epoxy bonds to the existing structure and cementitious grout mostly does not
- Lime mortar — material · an order of magnitude weaker, and in masonry that is the advantage — the joint should be the sacrificial element, so movement cracks the mortar rather than the brick
succeeded
- Lime mortar — material · within about a century, on speed and strength — and the rediscovery of lime for conservation followed the damage cement repairs did to historic fabric
is an input to
- Hydration — process · the reactive component — everything else in a concrete mix is aggregate, water, or an admixture adjusting how this reaction runs
Sources
- Material WorldOur own writing