How does 'delayed ettringite formation' (DEF) affect the durability of concrete structures?

Delayed ettringite formation (DEF) negatively affects the durability of concrete structures by causing internal expansion and cracking. DEF occurs when ettringite, a calcium sulfoaluminate hydrate, forms after the concrete has hardened. This delayed formation is typically caused by high curing temperatures or the presence of excessive sulfate in the concrete mix. As the ettringite crystals form, they expand, creating internal stresses that lead to cracking and disintegration of the concrete matrix. The cracking can reduce the concrete's strength, stiffness, and resistance to freeze-thaw damage. DEF can significantly reduce the service life of concrete structures, particularly in massive elements where heat buildup during hydration is more pronounced.