This　paper　presents　experimental　investigations　into　the　effects　of　ultrafine　blast　furnace　slag　on　microstructure　improvements　against　chloride　penetration　in　saturated　and　unsaturated　cementitious　systems　exposed　to　cyclic　drying-wetting　conditions.　The　hydration　kinetics　of　ultrafine　slag　powders　and　pore　solution　chemistry　in　slag-blended　cementitious　systems　at　different　ages,　together　with　the　main　hydration　products　and　pore　structure　characteristics,　were　determined.　The　chloride　profiles　accounting　for　different　slag　contents　and　drying-wetting　cycles　were　measured.　The　results　reveal　that　the　reactivity　of　ultrafine　slag　can　be　well　described　with　Avrami＇s　equation.　The　dilution　effect　of　the　slag　predominated　the　pore　solution　chemistry,　and　the　pH　value　decreased　with　a　higher　inclusion　of　slag.　An　optimal　inclusion　of　65%　slag　by　mass　of　the　binder　corresponding　to　the　finest　pore　structure　and　highest　hydrotalcite　content　was　found,　which　provides　a　reasonable　basis　for　the　slow　chloride　diffusion　and　high　chloride　binding.　Under　drying-wetting　exposure,　the　specimen　with　a　lower　saturation　exhibited　a　higher　chloride　transport　caused　by　capillary　absorption　in　the　skin　layer.　The　chloride　transport　tended　to　be　diffusion　controlled　after　sufficient　drying-wetting　cycles.