Portlandite

Portlandite is the mineral form of calcium hydroxide, with the chemical formula Ca(OH)2. It is a relatively rare mineral that occurs in hydrothermal veins and as an alteration product of calcium-rich rocks. In addition to its natural occurrence, portlandite is a major hydration product formed when Portland cement hardens with water.

In cement chemistry, portlandite forms as cement hydrates and contributes to the highly alkaline environment of cement pore water. It can appear as small crystals or as part of microcrystalline masses within the cement paste. In natural settings, portlandite is often found alongside minerals such as calcite, dolomite, and serpentine, typically in calcium-rich environments.

Physical properties of portlandite include a color range from colorless to white and a translucent appearance. It commonly forms as prismatic or needle-like crystals or as powdery masses. The mineral has a Mohs hardness of about 2.5 and a approximate specific gravity of 2.2. Portlandite is relatively soft and is reactive with moisture and carbon dioxide in air, slowly converting to calcium carbonate over time.

Chemically, portlandite readily reacts with carbon dioxide to form calcium carbonate and water: Ca(OH)2 + CO2 → CaCO3 + H2O. It is soluble in water to a limited extent and reacts with acids to form calcium salts. The name portlandite derives from Portland stone, a limestone from England, reflecting its early association with calcium-rich rocks and building materials.

See also: calcium hydroxide, Portland cement, lime.