What Is Delayed Ettringite Formation (DEF) in Concrete?
Delayed ettringite formation is an internal sulfate attack: concrete cured at high temperatures (above ~158°F/70°C, typical in precast steam curing) forms ettringite months or years later, expanding and cracking the concrete from within. Unlike external sulfate attack, DEF needs no outside sulfate source — the ingredients are already in the cement. It is prevented by controlling curing temperatures, not by anything done after the concrete hardens.
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How it happens
High early curing temperatures decompose the normal ettringite formed during hydration and trap sulfates in the calcium silicate hydrate. Over months to years, moisture allows the ettringite to reform — but now it forms in hardened concrete with no room to expand, generating internal pressure that cracks the paste. The damage pattern (map cracking, expansion) resembles ASR, and the two are sometimes confused in forensic investigations.
Where it is a risk
Precast elements steam-cured above the temperature threshold, mass concrete where internal temperatures spike from hydration heat, and any concrete with high cement content cured hot. Railroad ties, precast bridge members, and steam-cured pipe have documented DEF cases. The risk factors compound: high-alkali cement, high sulfate content, and high curing temperature together are the dangerous combination.
Prevention in specifications
Limit maximum concrete temperature during curing (typically 158°F/70°C per precast standards), use cements with controlled sulfate and alkali content, incorporate SCMs (slag, fly ash) that suppress the reaction, and monitor internal temperatures in mass placements. For precast, the curing cycle records are the quality evidence — specifications should require them. Once DEF damage appears, there is no repair that stops the reaction; management is structural assessment and eventual replacement.
The bottom line
DEF is internal sulfate expansion from high-temperature curing — prevent by limiting curing temperatures and using SCMs. It cannot be repaired once started; the curing records are the prevention evidence.
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