The　feasibility　of　using　steel　slag　and　bentonite　mixtures　to　construct　the　hydraulic　barrier　of　a　landfill　cover　was　explored　in　the　present　study.　Fine-grained　steel　slag　(SS;　particle　diameter　s)　was　measured　using　a　flexible-wall　permeameter.　Influential　factors　including　SACB　content　(BC),　SS　gradation,　water-washing　treatment　of　SS　and　compaction　water　content　(ωcomp)　were　investigated.　The　hydraulic　conductivity　results　were　interpreted　in　microscopic　scale　through　mercury　intrusion　porosimetry　(MIP)　and　scanning　electron　microscope　(SEM).　It　was　found　that　when　BC　was　below　10%,　the　ks　value　of　the　specimens　prepared　with　well　graded　SS　was　about　one　order　of　magnitude　lower　than　that　of　the　specimens　prepared　with　poorly　graded　SS.　This　was　due　to　less　macropores　caused　by　better　SS　gradation.　Yet,　the　effects　of　SS　gradation　on　ks　diminished　as　BC　further　increased　to　15%,　suggesting　the　dominant　role　of　BC　on　ks　at　high　BC.　Water-washing　treatment　of　SS　helped　reduce　ks　significantly　to　1.2　×　10−10　m/s　at　BC　of　10%,　owing　to　less　multivalent　cations　and　hence　lower　osmotic　swelling　reduction　caused　by　cations.　Controlling　ωcomp　1–2%　wetter　than　the　optimum　water　content　(ωopt)　also　helped　reduce　ks　significantly,　owing　to　the　reduction　of　macropores.　Accordingly,　it　is　suggested　to　use　well-graded　SS　mixed　with　10%　SACB　and　then　compact　at　ωcomp　slightly　wetter　than　ωopt　to　the　degree　of　compaction　greater　than　90%　in　engineering　practice.　©　2022　Elsevier　Ltd