Utilization of pretreated fly ash to enhance the chloride binding capacity of cement-based material

Formation of Friedel's salt (FS) and Kuzel's salt (KS) can chemically bind chloride in cement-based materials, and in cement-fly ash system, this can be facilitated by the pozzolanic reaction of fly ash (FA), with obvious contribution to the chloride binding capacity. In this study, FA was pretreated with alkali solution in order to depolymerize the surface structure and hasten ions dissolution to accelerate its pozzolanic reaction at the early age, with the intention to enhance its chloride binding capacity. Difference in chloride binding capacity between cement-FA system and cement-pretreated FA system was investigated, and the mechanism behind this difference was discussed. Formations of FS and KS in hydrates were verified with XRD, TGA and SEM; and the hydration process of cement-FA system was analyzed with NMR and SEM. The results show that pretreatment can depolymerize the surface structure and benefit the pozzolanic reaction of FA. In comparison with cement-untreated fly ash (C-UFA), more amounts of FS can be produced in cement-pretreated fly ash (C-PFA) at 3 d and 28 d age, and the reason is that the accelerated pozzolanic reaction can hasten the dissolution of aluminum from FA into liquid and participate in hydration. Additionally, the chloride binding capacity of C-PFA system is much stronger than that of C-UFA, and the main reasons can be revealed: on the one hand, more amounts of chloride can be chemically bound in the process of forming FS and KS; on the other hand, more amounts of C-S-H gel in hydrates, resulting from the accelerated pozzolanic reaction, can also physically adsorb the chloride. The findings would be expected to provide guidance on improving the chloride binding capacity of cement-fly ash system.