Abstract: Effect of different cations on the chloride ion binding capability of Portland cement was studied.The chloride ion binding rate of cement pastes were obtained through free chloride ions concentrations testing and analyzed by XRD and pH testing techniques .The results show that the chloride ion binding rate of cement pastes reaches 50, 70, 80 and 90 percent respectively of 28 d at the age of 6 h, 12 h, 1 d and 3 d.The formation of Friedel's salt and binding of chloride ions take place in the early stage of hydration while the chloride binding capability is mainly C-S-H gel dependent and smaller at the later stage ..Different cations cause the difference of amount of formation of Friedel 's salt in cement pastes, with the chloride ion binding capability of Ca 2+>Mg2+>K+>Na+which is negatively related to its influence on the pH value .Na +and K +lead to the rise of the pH value and reduce the chloride ion binding capability, while Ca2+and Mg2+are just the opposite.The increase of the Mg2+concentration promotes the formation of Friedel's salt and chloride ion binding capability , while Na + concentration shows little effect .

Key words: portland cement;cation;chloride binding;age