ettringite

Ettringite is a hydrous calcium aluminum sulfate mineral with the chemical composition Ca6Al2(SO4)3(OH)12·26H2O. In cement chemistry it is commonly represented as 3CaO·Al2O3·3CaSO4·32H2O, reflecting its role as a hydration product formed when aluminates in cement react with sulfates in the presence of water. The mineral typically forms as colorless to white, acicular (needle-like) crystals or fine fibrous aggregates.

Natural ettringite is rare. It occurs in hydrothermal veins and alteration zones in sulfide- and sulfate-rich rocks and is usually associated with gypsum, anhydrite, and other sulfate minerals. The name derives from Ettringen, a locality in the Eifel region of Germany, where the mineral was first described.

In Portland cement and concrete, ettringite forms rapidly in the early hydration process. It results from the reaction of calcium sulfate with tricalcium aluminate and water, producing a volume-expanding phase that contributes to early strength development but can also cause dimensional change if confinement limits crystal growth. If conditions promote prolonged or later formation of ettringite, for example at high curing temperatures or with excessive sulfate, delayed ettringite formation can lead to cracking and durability problems.

Ettringite is therefore both an important engineering phase in concrete and a mineral of interest in geology for its distinctive hydration-related chemistry. Its study helps explain early-age concrete behavior and the interactions between sulfate exposure and cementitious materials.