Ettringit

Ettringite is a hydrated calcium aluminum sulfate mineral and the common early hydration product formed when Portland cement reacts with water in the presence of sulfate. Its chemical formula is Ca6Al2(SO4)3(OH)12·26H2O. In nature, ettringite forms under low-temperature, sulfate-rich hydrothermal conditions and appears as colorless to white prismatic crystals. It is typically found in veins and alteration zones and is often associated with gypsum and other calcium sulfates.

In cement chemistry, ettringite forms rapidly during the initial hydration of cement when tricalcium aluminate (C3A) reacts with calcium sulfate and water. This reaction contributes to early strength development and helps control the setting process. However, excessive or late formation of ettringite can cause expansion and cracking in concrete if not managed. The phenomenon known as delayed ettringite formation (DEI) can occur when hydration proceeds at elevated temperatures, leading to long-term durability issues. Over time, ettringite can transform to monosulfate hydrates as sulfate supply diminishes, influencing the long-term microstructure and dimensional stability of the cement paste.

The mineral is named after Ettringen, a town in Germany, where it was first described. Ettringite thus has significance both as a natural mineral in sulfate-rich geological environments and as a key phase in the early hardening of cement-based materials.