The Efficiency of Fly Ash Concrete in the Context of the Combined Action between Chlorides and Carbonation

Abstract

The fly ash concrete (FAC) has often been used in chloride contaminate environments. The aluminates present in fly ash (FA) react with free chlorides binding them and reducing the amount of chlorides able to penetrate into concrete. However, Ca(OH)2 present in FAC is easily consumed by pozzolanic reactions, making easier the advance of carbonation. In this context, it is important to know the FAC behaviour in environments subjected simultaneously to chlorides and carbonation. Chloride diffusion coefficient (D) increases in environments containing chlorides and CO2 when compared to one containing only chlorides. This increase is related to the effects of carbonation on concrete. Therefore, this paper investigates the efficiency of FAC in context of the combined action of chlorides and carbonation. Two different concretes were cast, a FAC (40% FA) and a reference concrete (RC), that is a concrete with similar properties to FAC but without FA. After 90 days humid curing, half of specimens were subjected to accelerated carbonation (20ºC, 55% RH and 4% CO2) during 1, 2 and 7 months. The other half was protected and remained in laboratory during the same period. Afterwards, non-steady-state migration tests were performed. The results show that, under the studied conditions, the efficiency of the FAC against chloride penetration is not affected when compared to the results obtained for RC. The carbonation depth in FAC is higher than in RC, but the D in FAC remain lower than in RC. This behaviour can be attributed to the higher capacity of FAC binding chlorides.

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