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Material World
Element · Ca

Calcium

The metal inside limestone, cement, bone and shell — almost never seen as a metal.

Calcium is the fifth most abundant element in the crust and is encountered essentially always as a compound. Calcium carbonate is limestone, marble, chalk and most shells; calcium oxide is quicklime; calcium sulfate is gypsum and plaster.

The built environment is, to a first approximation, calcium chemistry. Burning limestone to make lime and letting it re-absorb carbon dioxide as it sets is a chemical cycle humans have run at scale for at least nine thousand years.

Uses

Calcium's uses are almost entirely as its compounds, and they are foundational rather than glamorous. Limestone and lime go into cement, mortar and plaster; calcium carbonate is the filler in paper, paint and plastics; and lime is the reagent that neutralises acidic soil, acidic water and acidic industrial waste.

In steelmaking, lime removes silicon, phosphorus and sulfur into the slag. The metal itself is used as a deoxidiser and as a reducing agent for other metals.

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 used as

  • Alloying — application · as a deoxidiser in steelmaking

is found in

  • Calcite — mineral Wikidata
  • Concrete — material
  • Gypsum — mineral Wikidata
  • Aragonite — mineral Wikidata
  • Anorthite — mineral Wikidata
  • Seawater — material · constantly removed by organisms building shells, which is why the concentration stays where it is rather than rising
  • Glass fibre — material · and aluminium and boron in E-glass, which is a composition chosen for drawability and electrical resistance rather than for strength
  • Gypsum plaster — material · the cation of the sulfate
  • Lime mortar — material · the whole of the binder, cycling from carbonate to oxide to hydroxide and back to carbonate
  • Nacre — material · as the carbonate that is nearly all of it
  • Precious coral — material · as the carbonate skeleton
  • Ivory — material · in the apatite
  • Bone — material · in the apatite, and the body's store of it
  • Chalk — rock · as the carbonate that is nearly the whole rock
  • Quicklime — compound PubChem
  • Apatite — mineral
  • Fluorite — mineral Wikidata
  • Plagioclase — mineral · substitutes within a solid solution — an individual specimen may hold little of it
  • Scheelite — mineral Wikidata

is produced by

is an input to

  • Metallothermic reduction — process · reserved for halides and the most stubborn oxides, where the cheaper metals will not do the job

Sources

  • Material World
    Our own writing
  • Wikidata
    Wikimedia Foundation · Creative Commons CC0 1.0 Universal (public domain dedication)
  • PubChem
    US National Library of Medicine · Mixed — aggregated third-party content, rights retained by depositors
  • NIST — National Institute of Standards and Technology
    United States Department of Commerce · US Government work — public information, credit requested

Questions this page answers

Where it comes from, and what it becomes

Follow Calcium 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

  • Calcium → is produced by (from molten calcium chloride; there is no smelting route) → Molten salt electrolysis → takes as input (after concentration, as one of the two routes to lithium chloride) → Lepidolite

Downstream — what it becomes

  • Calcium → is an input to (reserved for halides and the most stubborn oxides, where the cheaper metals will not do the job) → Metallothermic reduction → produces (magnesium reducing boron oxide, which gives an impure amorphous boron — the crystalline element needs a different route entirely) → Boron → is a component of (about one per cent, and structurally essential — the compound is Nd₂Fe₁₄B, and without the boron it does not form) → Neodymium magnet → is used as (the strongest in commercial use, and the reason a motor, a hard drive and an earbud can be small) → Permanent magnets
  • Calcium → is used as (as a deoxidiser in steelmaking) → Alloying
  • Calcium → is an input to (reserved for halides and the most stubborn oxides, where the cheaper metals will not do the job) → Metallothermic reduction → produces (magnesium reducing beryllium fluoride, and the reason beryllium is expensive before anyone accounts for how carefully it has to be handled) → Beryllium → is used as (copper-beryllium alloys, for non-sparking and spring applications) → Alloying
  • Calcium → is an input to (reserved for halides and the most stubborn oxides, where the cheaper metals will not do the job) → Metallothermic reduction → produces (aluminium reducing barium oxide under vacuum, the barium distilling off as vapour) → Barium → is used as (barium sulfate for gastrointestinal X-ray contrast) → Medical imaging
  • Calcium → is an input to (reserved for halides and the most stubborn oxides, where the cheaper metals will not do the job) → Metallothermic reduction → produces (the same aluminothermic route as barium, and in similarly small quantity) → Strontium → is used as → Pigment
  • Calcium → is an input to (reserved for halides and the most stubborn oxides, where the cheaper metals will not do the job) → Metallothermic reduction → produces (calcium or barium reducing caesium chloride under vacuum — caesium is too reactive to survive most alternatives) → Caesium → is used as (as a photocathode, exploiting the lowest ionisation energy of any stable element) → Electrical conduction

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