Secondary Ettringite Formation - Mechanism

Secondary ettringite formation (SEF) is a problem that adversely affects the durability of Portland cement concrete. Secondary ettringite formation in concrete is a relatively recent phenomenon. SEF was discovered within the past decade and extensive research into the subject has only been performed in recent years. Due to the limited research into SEF and its mysterious nature, SEF happens to be the most non-understood concrete durability problem today. Most researchers agree on both the evidence of an SEF occurrence in concrete and the manifestation of SEF that causes damage. However, many researchers do not agree upon conclusions to explain the trigger mechanism to generate SEF damage or the primary conditions necessary for SEF to occur.

The destructive manifestation of secondary ettringite formation (SEF) is the expansive pressure caused by the reaction of monosulfoaluminate to ettringite in hardened concrete. This reaction occurs in the presence of sulfates originally in the cement. Delayed expansion occurs due to the postponed formation of ettringite from monosulfoaluminate within the hydrated paste. This reaction is delayed since high curing temperatures destroy the ettringite that should have formed during initial hydration. After heat curing, the concrete drops to service temperature and the monosulfoaluminate may revert to ettringite in the presence of an internal sulfate source. This reaction produces damaging expansion since the product (ettringite) is several times larger in volume than the reactants. Expansion will lead to cracking of the hardened concrete. In addition to cracking, concrete will suffer from a large loss of dynamic modulus, loss of bonding with reinforcing steel, and air voids (to prohibit freeze-thaw damage) may fill up with ettringite.

The exact mechanism that generates this manifestation is under considerable debate among researchers. Two different theories exist that attempt to explain the expansive mechanism. The first mechanism theory is homogenous paste expansion. The second theory is crystal pressure. The practicing engineer need not concern his or herself with the exact trigger mechanism, for they are very similar in nature and the results are the same. The exact conditions to trigger SEF reaction to occur in concrete are also the topic of heated debate among researchers. Three differing theories exist; instability of ettringite at an elevated curing temperatures, the postponed availability of SO₃ from clinker, and the presence of reduced alkali hydroxyl concentration in the concrete pore solution.