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Material World
Compound · CaO

Quicklime

Calcium oxide, produced by burning limestone — the reactive intermediate behind mortar, plaster and cement.

Construction material Agricultural input

Quicklime is calcium oxide, made by heating limestone until carbon dioxide is driven off. It is strongly reactive: adding water produces slaked lime and releases enough heat to boil the water, which is where the name comes from — 'quick' in the old sense of living.

It is the pivot of one of the oldest chemical processes humans operate. Limestone is stable and useless as a binder; burn it and it becomes reactive; slake it and it sets, slowly reabsorbing carbon dioxide from the air and returning to calcium carbonate. Mortar completes a chemical cycle back to the rock it started as.

Uses

Steelmaking is the largest consumer. Lime added to the furnace forms a slag that carries off silicon, phosphorus and sulfur, and without it the steel would be unusably brittle.

Construction takes the next largest share, as lime mortar and plaster and as a soil stabiliser that turns wet clay into a workable base for roads. Lime neutralises acid — in drinking water treatment, in flue gas scrubbing to capture sulfur dioxide, in acidic soils and in mine drainage. It is also used in sugar refining, papermaking and the manufacture of calcium carbide and precipitated calcium carbonate.

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 a component of

  • Portland cement — material · as calcium oxide within the clinker phases, not as free lime

is produced by

  • Calcination — process · the solid residue once carbon dioxide has been driven off

is an input to

  • Basic oxygen steelmaking — process · forms a basic slag that strips phosphorus and sulfur out of the melt
  • Hydration — process · slaked with water to make the lime putty that mortar and plaster are mixed from

is an alternative to

  • Gypsum — mineral · as plaster. Lime plaster sets by absorbing carbon dioxide over months and stays permeable and repairable; gypsum sets in an hour and does not — which is why historic masonry must be repaired with the slow one
  • Portland cement — material · 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

is a source for

  • Lime mortar — material · slaked to hydroxide, then setting by reabsorbing carbon dioxide from the air

contains

Sources

  • Material World
    Our own writing
  • PubChem
    US National Library of Medicine · Mixed — aggregated third-party content, rights retained by depositors
  • Wikidata
    Wikimedia Foundation · Creative Commons CC0 1.0 Universal (public domain dedication)

Questions this page answers

Where it comes from, and what it becomes

Follow Quicklime 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

  • 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 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
  • Quicklime → is produced by (the solid residue once carbon dioxide has been driven off) → Calcination → takes as input (supplies the silica and alumina that combine with lime in the cement kiln) → Clay → is sourced from (clay is chiefly what feldspar becomes when it weathers) → Orthoclase
  • Quicklime → is produced by (the solid residue once carbon dioxide has been driven off) → Calcination → takes as input (at around 150 °C, which is driving off water rather than decomposing a carbonate — so no carbon dioxide comes out of the rock) → Gypsum plaster → is sourced from (calcined to drive off three quarters of the water, and it takes it back when mixed) → Gypsum
  • Quicklime → is produced by (the solid residue once carbon dioxide has been driven off) → Calcination → takes as input (gently calcined to plaster of Paris, a far lower temperature than lime burning) → Gypsum
  • 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
  • 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 (in young limestone, before conversion to calcite is complete) → Aragonite

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

Downstream — what it becomes

  • Quicklime → is a component of (as calcium oxide within the clinker phases, not as free lime) → Portland cement → is a component of (the binder — the expensive, energy-intensive, chemically active part) → 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
  • Quicklime → is an input to (slaked with water to make the lime putty that mortar and plaster are mixed from) → Hydration → produces (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) → 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
  • Quicklime → is an input to (forms a basic slag that strips phosphorus and sulfur out of the melt) → Basic oxygen steelmaking → produces (most of the world's primary steel) → Steel → is associated with (produced in small quantities long before it could be made reliably) → Iron Age complete chain
  • Quicklime → is a source for (slaked to hydroxide, then setting by reabsorbing carbon dioxide from the air) → Lime mortar → is an input to (at around 900 °C, which does decompose the carbonate, and the mortar reabsorbs a large part of it over the following years) → Calcination → produces (as clinker, ground with gypsum to retard the set) → Portland cement → is a component of (the binder — the expensive, energy-intensive, chemically active part) → Concrete → is used in (foundations, frames, floors and civil structures) → Construction
  • Quicklime → is an input to (slaked with water to make the lime putty that mortar and plaster are mixed from) → Hydration → produces (the same reaction around a steel cage, and the concrete's alkalinity is what passivates the steel and stops it rusting — until carbonation or chloride reaches it) → 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
  • Quicklime → is a component of (as calcium oxide within the clinker phases, not as free lime) → Portland cement → is an input to (the reactive component — everything else in a concrete mix is aggregate, water, or an admixture adjusting how this reaction runs) → Hydration → produces (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) → Concrete → is used in (foundations, frames, floors and civil structures) → Construction

These are the most distinct paths onward. Quicklime ends up in others besides.