Evolution of aluminate hydrate phases in fly ash-cement system under the sulfate conditions

Abstract The aluminate hydrate phases play an important role in the sulfate attack of concrete. In this paper, the aluminum hydrate phases evolution of fly ash blended cement paste powders immersed in different sodium sulfate solution is studied. In order to accelerate the reaction, the fly ash blended cement paste powders are immersed in different sulfate solution at ambient temperature up to 28 d. The results show that the hydration of fly ash and the distribution of aluminum hydrates phases in the hydration products of fly ash-cement blends are significantly influenced by the sulfate. The addition of fly ash into the cements introduces the reactive Al3+ into the systems, leading to an increase of Al(IV)/Si and mean chain length of C-A-S-H. Furthermore, although the pozzolanic reaction of fly ash consumes the calcium hydroxide, where the calcium hydroxide is required for the formation of ettringite, the addition of fly ash is favorable for the formation of ettringite.