Bentonite　was　used　to　modify　steel　slag　to　improve　impermeability　of　steel　slag　in　cover　liner　of　solid　waste　landfills.　The　saturated　hydraulic　conductivity　of　the　compacted　steel　slag-bentonite　mixtures　ks　was　investigated　through　the　flexible-wall　permeation　tests.　Then,　the　pore　size　distributions,　interlayer　spacing　of　montmorillonite　and　mineral　morphology　were　investigated　through　mercury　intrusion　porosimetry　tests,　X-ray　diffraction　tests　and　scanning　electron　microscopy　tests,　in　order　to　analyse　the　impact　mechanism　of　the　ks　at　the　microscopic　scale.　Thereafter,　a　formula　and　a　preparation　method　were　proposed,　which　meets　the　permeability　requirement　of　the　hydraulic　barrier　(ks≤10−9　m/s).　Results　showed　that　the　ks　of　the　specimen,　treated　by　prehydration　of　bentonite,　was　reduced　to　1/2　of　that　of　the　untreated　one.　After　improving　the　gradation　of　steel　slag,　the　content　of　macropores　between　the　steel　slag　particles　was　decreased,　reducing　the　ks　of　the　specimens　to　1/5　of　that　of　the　specimen　with　poorly　graded　steel　slag.　The　ks　and　pore　size　distribution　of　the　specimens　mixed　steel　slag　with　Wyoming　were　similar　to　those　of　sodium-activated　calcium　bentonite.　The　high　salt　content　leachate　of　steel　slag　results　in　the　increase　of　the　interlayer　spacing　and　the　decrease　of　the　bound　water　of　montmorillonite.　Subsequently,　the　osmotic　swelling　of　bentonite　is　suppressed.　Water-washing　treatment　of　steel　slag　can　alleviate　the　suppression　effect　and　the　formation　of　montmorillonite　clusters,　and　reduce　the　ks　of　specimens　to　1/10　of　that　of　the　untreated　one.　©　2022　Zhejiang　University.　All　rights　reserved.