Influence of fly ash and chlorides on the behavior of sulfate attack in blended cement pastes

Sulfate-induced concrete deterioration is associated with not only its material compositions but also environmental conditions. This study has performed some experimental investigations on the influence of fly ash (FA) and chlorides on the behavior of sulfate attack in blended cement pastes, and they were quantitatively characterized by the contents of bound sulfates and chlorides. Firstly, FA-blended cement paste particles (FCPs) with different FA content were prepared, and then they were performed a series of immersion experiments. Finally, free sulfate and chloride concentrations in the solutions were measured by using ultraviolet spectrophotometry and silver nitrate titration, respectively, while the change of phase and chemical compositions in FCPs were analyzed by quantitative X-ray diffraction (QXRD), scanning electron microscopy and energy spectrometry (SEMEDS). Results showed that, the addition of FA can not only increase the aluminum content in FCPs, improving the formation of sulfates-contained phases in Na2SO4 solution, but also influence the content and chemical compositions of C-(A)-S-H, resulting in the decrease of physical binding capacity of sulfates. The content of physically bound sulfates can be released by the sulfate-induced decalcification of C-(A)-S-H. After the sulfates-contained FCPs were immersed into NaCl solutions, the presence of chlorides can release the bound sulfates from the sulfates-contained phases, resulting in the formation of chlorides-contained phases. Additionally, the releasing effect of chlorides on the physically bound sulfates becomes more pronounced than that on the chemically bound sulfates, especially in the chloride solution with a high concentration. Eventually, these conclusions were consistent with the SEM-EDS results.