Delayed ettringite formation (DEF) is a concrete pathology phenomenon where ettringite, a mineral composed of calcium, aluminum, and sulfate, forms within hardened concrete long after the concrete has set. This formation can occur when concrete is subjected to high temperatures during its early curing process, typically above 70 °C. DEF is particularly relevant in scenarios where steam curing is used to accelerate strength development. The ettringite that forms during this delayed phase can create expansive forces within the concrete, leading to cracking, spalling, and overall reduction in structural integrity.