Influence of fly ash on the properties of Recycled Aggregate Concrete

This paper discussed the influence of the Recycled concrete Aggregate (RA), water/binder ratio and fly ash on the workability, compressive strength and SEM morphology of Recycled Aggregate Concrete (RAC). The first batch specimens were prepared with the same water/cement ratio of 0.55 but with different replacements of RA, i.e. 0, 20%, 30%, 40%, 50%, 60%, 70%, 80% and 100%, respectively. It presents the fluidity and compressive strength of RAC decreases with the increasing RA, with the exception of the group with 30% RA, whose compressive strength is even higher than the control concrete. Another three groups were prepared with 100% RA and with different water/binder ratios of 0.50, 0.55 and 0.6, each using 0%, 10%, 20% and 30% fly ash, respectively, as cement replacement. It shows the compressive strength of RAC decreases after the addition of fly ash, but the group with 20% fly ash keeps the level very close to that with 10% fly ash. The SEM tests imply that the fly ash can improve the interface between the cement stone and the aggregate, while at 28 days, the compressive strength of RAC with fly ash is still lower than the control specimens due to its slow hydration speed.