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Calcium as pure metal is unsuitable as building material for advanced atomically precise manufacturing since:

  • as metal it has meatllic interelemental bonds that is the bonds have have low directionality
  • it has weak interelemental bonds
  • => diffusion jumps at low twmperatures are likely
  • it is also highly electropositive and reacts strongly with oxygen and even water (this actually is less of a problem since parts can perfectly be sealed)

Just oxidizing calcium may solve the problem of weak undirected metallic bonds but the formed compound is still highly reactive. Calcium oxide CaO is commonly known as burnt lime. It strongly reacts with water to calcium hydroxide. The resulting compound calcium hydroxide is slightly water soluble (it is what makes water hard / slightly basic) and very soft. Hardly usable as building material.

Calcium oxide does also react with carbon dioxide to calcium carbonates (naturally occurring as: limestone, calcite, aragonite, ...) which (albeit still pretty soft) are better candidates for building materials. Only silicates and phosphates of calcium though form decently hard and more water resistant compounds.

Calclium oxoacid salts

Most simple stable common calcium oxoacid salts compounds are:

  • environmentally friendly
  • nontoxic when ingested
  • fireproof

(For oxoacid salts of other elements see the main article: Salts of oxoacids.)

Calcium silicates (decently hard)

  • Wollastonite CaSiO3 (Mohs 4.5-5 | Triclinic)
  • There are several other known stable stoichiometries of calcium silicate not found as natural mineral. See wikipedia: Calcium silicate
  • ...

Calcium phosphates (hard)

  • Hydroxylapatite Ca5(PO4)3(OH) (Mohs 5 | Hexagonal)
  • Fluorapatite Ca5(PO4)3F (Mohs 5 | Hexagonal)
  • Chloroappatite Ca5(PO4)3Cl (Mohs 5 | Hexagonal)

Polymorphs of calcium carbonate CaCO3 (soft)

Calcium sulfates (very soft)

  • Gypsum CaSO4·2H2O (Mohs 2 by definition | Monoclinic)
  • alpha & beta hemihydrate (CaSO4)2·H2O

Calcium nitrates (very soft and highly water soluble)

  • Nitrocalcit Ca(NO3)2•4(H2O) (Mohs 1-2)


  • Fluorite CaF2 (Mohs 4 by definition | Cubic) All other calcium halogenides are water soluble salts.
  • Antarcticite CaCl2·6H2O (Mohs 2-3 | Trigonal)
  • Calcium hydride CaH2 is reactive similar to calcium oxide.


Calcium carbide may be of interest for APM since on contact with water it releases ethyne C2H2. A compound which is due to its low hydrogen content (C:H = 1:1) a good resource molecule for mechanosynthesis of diamond (better than methane CH4 C:H = 4:1).
Mechanosynthetic means though will allow independence of crude chemical means of storage like in calcium carbide.

  • Calcium titanate: Perovskite CaTiO3 (Mohs 5-5.5 | Orthorhombic)


  • Calcium is one of the most abundant elements on earth.
  • The Mohs-scale levels of 2, 3 and 4 are all defined by calcium minerals.
    They are defined by (gypsum, calcite and fluorite respectively).


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