Calcium

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.

• Portlandite Ca(OH)₂ (Mohs 2 soft| Trigonal)

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)

• Aragonite (Mohs 3.5-4 | Orthorhombic)
• Calcite (Mohs 3 by definition | Trigonal)
• Vaterite μ-CaCO₃ (Mohs 3 | Hexagonal)

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• Monohydrocalcite CaCO₃·H₂O
• Ikaite CaCO₃·6H₂O (Mohs 3 | Monoclinic)

Calcium sulfates (very soft)

• Anhydrite γ-CaSO₄ (Mohs 3 | Orthorhombic)

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• 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)

Other tertiary compounds

Aluminates?

• CaAl₂O₄ krotite (monoclinic almost orthorhombic, Mohs 6.5, 2.944 g/cm³) – https://www.mindat.org/min-40468.html
• CaAl₁₂O₁₉ hibonite (hexagonal,Mohs 7.5 to 8.0, 3.83 to 3.85 g/cm³) – https://www.mindat.org/min-1897.html

Aluminates:

• Calcium aluminates < interesting
  e.g. Dodecacalcium hepta-aluminate - cubic, big unit cell

Halogenides

• 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.

Misc

• Calcium hexaboride CaB₆ is a good ceramic
• calcium silicide CaSi₂ is pretty stable
• Calcium carbide CaC₂ strongly reacts with water (it's commonly known as carbide)

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)

Trivia

• 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).

Related

• Alkali and earth alkali metals: S-block metals
• Chemical element
• Concrete

External Links

Wikipedia:

• Calcium minerals
• Calcium compounds