ettringiet

Ettringiet, known in English as ettringite, is a hydrated calcium aluminium sulfate mineral with the chemical formula Ca6Al2(SO4)3(OH)12·26H2O. It is named after Ettringen, a locality in the Eifel region of Germany, where it was first described in the 19th century. Crystallizing in the hexagonal system, ettringite commonly forms white to colorless prismatic or needle-like crystals, though impurities may tint specimens.

In nature, ettringiet occurs in sulfate-rich environments, particularly in hydrothermal veins and other low-temperature alteration zones. It is also prominent in cement and concrete chemistry, where it forms as an early hydration product when calcium sulfate (gypsum) and aluminum-bearing compounds react with water.

Formation and stability: Ettringiet forms rapidly during the initial hydration of Portland cement, contributing to early strength gain. In cement systems, continued hydration can transform ettringite to monosulfate under certain conditions, or additional ettringite can form if sulfate supply remains high. Excessive water exposure or improper curing can lead to delayed ettringite formation, which may cause expansion and cracking in hardened concrete.

Significance: In mineralogy, ettringiet helps identify sulfate-rich, aluminous, low-temperature environments. In construction materials science, it is a key phase in cement hydration, and its controlled formation is essential for durability and dimensional stability.