Delayed ettringite formation (DEF) is a chemical reaction in hardened concrete that leads to internal expansion and cracking due to the late formation of ettringite crystals.
Explanation
Ettringite normally forms during the early hydration of cement. However, when concrete is exposed to high curing temperatures—typically above 70°C—early ettringite formation is suppressed. As the concrete cools and moisture becomes available, ettringite can form later within the hardened matrix.
This delayed formation creates expansive forces within the concrete, leading to internal microcracking, map-like cracking patterns, and gradual loss of strength. DEF is most commonly associated with mass concrete elements such as foundations, bridge components, and precast units subjected to heat curing.
The risk is influenced by cement composition, curing temperature, moisture availability, and long-term environmental exposure.
Diagnosis often requires petrographic analysis and laboratory testing, as symptoms can resemble other forms of deterioration such as sulphate attack.
Why It Matters
Delayed ettringite formation is a serious durability issue that can lead to progressive internal damage over time. Unlike surface defects, it originates within the concrete and can be difficult to detect in early stages.
In structural repair, identifying DEF is critical, as conventional surface treatments are ineffective. Long-term management may require monitoring, load assessment, or partial replacement depending on severity.