Effect of carbonate ions on delayed ettringite formation

- The limestone powder in the cement can promote DEF.
- DEF expansion is greatly increased when carbonate ion is externally provided.
- The sulfate ions are released by the substitution reaction of carbonate ions with monosulfate in the pore solution.
- Fly ash can reduce the risk of expansion in the cement with limestone powder.

The effect of carbonate ions gradually dissolved from limestone powder on delayed ettringite formation (DEF) and the inhibition of DEF by fly ash were examined. To accelerate DEF expansion, 3% SO3 by cement weight was added and the specimens were exposed to a temperature of 90 °C at an early age (1 day) and then stored under tap water at 20 °C. In addition, the specimens submerged in a solution with a high carbonate ion concentration (0.15 mol/L) after exposure at 90 °C were also studied to explicitly investigate the effect of carbonate ions on DEF. The limestone powder in the cement accelerated DEF expansion in samples with 3% SO3 addition that were exposed to temperatures over 90 °C at an early age. Specimens in the solution containing carbonate ions showed greater swelling than those in tap water. This indicates that the carbonate ions in the pore solution can promote DEF. Fly ash inhibited DEF even in cement with 3% SO3 addition and during storage in the solution with a high carbonate ion concentration.