Investigation into effect of mineral admixture on delayed ettringite formation

Abstract

Recently, severe map cracking of concrete in pile caps of bridge piers in Southern highway project in Sri Lanka was observed about four years after the construction. These pile caps are thick enough to cause the excessive heat generation at early age and continuously exposed to moisture. The map cracking under such an environmental condition was suggested to be due to Delayed Ettingite Formation (DEFV\ The swelling and cracking bv the DEF have been also observed in the precast concrete with hot steam curing in Japan" In this studv, the effect of mineral admixture such as limestone powder and fly ash on the DEF is focused on because the cement used in the above pile contained limestone powder and fly ash is expected to inhibit the DEF according to the previous literature. Mortar and cement Jjaste specimens with and without limestone powder or fly ash were used in the experiment. The replacement ratios of limestone and fly ash with early-high strength cement were 15 % and 25 %. respectively. To accelerate the DEF expansion. 3 % of SO* for cement weight was added in the cement and the specimens were exposed to high temperature of 90 °C and then stored in the tap water at 20 °C. In addition, the specimens submerged in the solution with high concentration of carbonate ion were also studied in order to investigate the influence of carbonate ion on the DEF clearly. It was found that the limestone power is likely to promote the DEF while fly ash is effective to inhibit the DEF even though the content of SO* in cement is high. It is also suggested that the carbonate ion may gradually increase the DEF and the expansion.