Utilizations　of　industrial　byproducts　and　wastes　as　much　as　possible,　together　with　desirable　durability,　are　essential　to　sustainable　development　of　building　materials.　In　this　work,　the　chloride　diffusion　behavior　of　alkali　-activated　steel　slag　(SS)　and　ultrafine　blast　furnace　slag　(UFS)　is　studied,　towards　deeper　insights　into　the　effect　of　electrical　double　layer　on　the　resistance　to　chloride　penetration.　The　microstructure　improvement　of　alkali　-activated　SS　with　increasing　addition　of　UFS　was　examined　by　means　of　X-ray　diffraction,　Fourier　transform　infrared　spectroscopy,　thermogravimetric　analysis,　mercury　intrusion　porosimetry　and　nitrogen　sorption.　The　zeta　potentials　above　the　pore　surface　of　various　alkali　-activated　SS-UFS　systems　were　measured　and　compared.　The　electrostatic　resistance　of　the　electrical　double　layer　to　chloride　diffusion　was　analyzed　and　discussed.　Results　indicate　that　UFS　dosages　above　40%　substantially　refines　the　pore　structure　of　alkali　-activated　SS　and　then　the　mechanism　governing　chloride　penetration　shifts　from　capillary　transport　to　gel　transport.　The　exposure　of　higher　chloride　sodium　concentration　results　in　the　pore　surface　of　alkali　-activated　SS-UFS　systems　to　be　less　negatively　charged　and　the　zeta　potential　is　reversed　to　be　positive　value　after　continuous　ion　exchange　between　sodium　and　calcium.　The　presence　of　UFS　can　remarkably　increase　the　proportion　of　physically　bound　chloride　that　attains　approximately　80%　of　the　total　binding　capacity.　The　impediment　of　electrical　double　layer　to　chloride　diffusivity　becomes　increasingly　pronounced　with　pore　structure　refinement,　especially　for　mixtures　with　diffusion　coefficients　below　5　x　10　-12　m　2　/s　obtained　based　on　Fick　＇　s　law　of　diffusion.