In this paper, the zeta potential of freshly mixed cement paste and hardened cement pastes, as well as the concentration index, was measured. The influences of chloride concentration in mixing water and slag content on zeta potential of freshly mixed pastes were studied. A proposed model was expressed to explain the relationship of zeta potential and concentration index of hardened cement pastes immersed in chloride solution. The results showed that the increase of chloride concentration in mixing water and slag replacement improved the zeta potential of freshly mixed cement, the hydration rate and concentration of ions in mixed water affects the zeta potential. With the increase of chloride concentration in soaking solution, the chloride concentration index and zeta potential of hardened cement paste all gradually decreased. The addition of slag gave some changes on chloride in concentration and zeta potential. The relationship among chloride concentration index, chloride concentration in soaking solution and slag replacement revealed by Gouy-Chapman model was in good agreement with the measured results
