Hydroxide

Many hydroxides are rather soft but there are a few exceptions that might be pretty useful as structural building materials.

Hydroxides of aluminum

• α-AlO(OH) Diaspore Mohs 6.5-7 (pretty hard!)
• γ-AlO(OH) Beohmite Mohs 3.5
• γ-Al(OH)₃ Gibbsite (Hydrargillite/Bayerite) Mohs 2.5-3.5

Decently hard iron and manganese hydroxides

• α-FeO(OH) Goethite Mohs 5-5.5
• β-FeO(OH) Hydroxy-Akaganeite Mohs ??
• γ-FeO(OH) Lepidocrocite Mohs 5
• δ-FeO(OH) Feroxyhyte Mohs ??

• α-MnO(OH) Groutite Mohs 3.5-4
• β-MnO(OH) Feitknechtite (webminerals) Mohs ??
• γ-MnO(OH) Manganite Mohs 4

Soft hydroxides of earth alkali metals

• Mg(OH)₂ Brucite Mohs 2.5-3
• Ca(OH)₂ Portlandite Mohs 2

The hydroxides of alkali metals (sodium and potassium - NaOH & KOH) are highly water soluble and form highly basic solutions aggressive to human skin and dangerous to the eyes. They are not suitable for surface exposed building materials.

Hydroxides of more rare elements

• CrOOH Guyanaite Bracewellite Grimaldiite -- Mohs ??
• GaO(OH) Tsumgallit Mohs 1-2 (very soft and containing rare gallium)
• Vanadium hydroxides

Notes

Many hydroxides can be found in the Bauxite Laterite mineral group. Bauxite is today (2016) the primary aluminium ore. With todays non AP technology it's not economically possible to extract aluminium from rocks containing silicon which is the second most common element in earths crust after oxygen. With advanced atomically precise gem-gum-technology red mud could become a better usable resource.