During the hydration of cement, tricalcium aluminate (C3A) reacts with gypsum and forms ettringite (AFt). Once all gypsum is consumed, ettringite can further react with remaining C3A and form monosulfate (AFm) at 1-2 days. Normally, at early ages, ettringite all transfers to AFm phase. After several months or years, ettringite can form again if a new source of sulfate becomes available in the pore solution of the paste, viz. delayed ettringite formation. In previous study, it was found that delayed ettringite forms in Portland cement concrete when the concrete samples were cured under moist conditions, i.e., without external sulfate phase. This delayed ettringite formation may result in the decrease of resistance of Portland cement concrete to chloride penetration. After that, it was found that ettringite, viz. delayed ettringite formation, also generated in fly ash concrete. The formation of ettringite, however, has no obvious influence on the resistance of fly ash concrete to chloride penetration.@en