Concrete having high resistance to chloride attack has been demanded for coastal structures. The authors have developed a pozzolanic admixture that confers excellent resistance to chloride attack. The admixture can be used even in precast concrete factories without ample storage capacity such as extra silos. The study, which focuses on the properties of steam-cured concrete, considers the use of the admixture in precast concrete production. Concrete incorporating the admixture was investigated for chloride permeation resistance and its mechanism. The test results confirmed that the admixture (4% to 12% mass ratio mixture) improves chloride resistance of concrete. In addition, the results suggested that the admixture contributes to the increase in total volume of pores with diameter of 0.01 μm or less, and that chloride ions are immobilized by the generation of Friedel's salt.