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This was an experiment in which chloride was externally permeated into cement paste. The influence of Cl- on the content and distribution of free-form NO2- in the cement paste was researched using the chemical quantitative analysis method. The action mechanism was investigated by the micro-means of X-ray diffraction (XRD) and scanning electron microscopy (SEM). The results show that the physical competitive adsorption of Cl- with NO2- on C-S-H and the chemical substitution of Cl- to NO2-AFm caused more free-form NO2- in the cement paste. In the cement paste with chloride salt erosion, the NO2- concentration in the erosion surface was the lowest, and the NO2- concentration reached the highest value at 10mm from the erosion surface. The concentration decreased gradually with the depth from the erosion surface.