The　fire-retardant　lightweight　mortar　spalling　from　the　tunnel　lining　substrates　can　threaten　the　safety　of　vehicles.　The　use　of　steel　slag　(SS)　in　alkali-activated　cement　can　reduce　the　carbon　emission　and　cost　of　lightweight　mortar.　The　effects　of　SS　on　the　bond　strength　and　the　microstructure　of　the　interface　between　alkali-activated　SS/fly　ash　(FA)　lightweight　mortar　(ASFm)　and　concrete　substrates　was　studied.　The　SS　contents,　namely,　the　mass　ratio　of　SS　to　the　summation　of　FA　and　SS,　were　0　%,　20　%,　40　%,　60　%,　and　80　%.　The　properties　of　ASFm,　including　compressive　strength,　fire　resistance　and　bond　strength,　were　studied.　Results　demonstrated　that　the　bond　strength　of　ASFm　increases　and　then　decreases　with　the　increase　in　SS　content.　The　highest　bond　strength　of　0.65　MPa　was　obtained　at　40　%　SS　content.　Bond　strength　was　determined　by　the　pore　structure　and　micro-　mechanical　properties　of　the　matrix.　When　the　SS　content　was　increased　from　0　%　to　40　%,　the　content　of　C-A-S-　H　gel　and　C-N-A-S-H　gel　contents　increased,　and　the　porosity　was　reduced;　hence,　the　bond　strength　of　ASFm　became　larger.　When　the　SS　content　increased　from　40　%　to　80　%,　SS　acted　as　an　inhibitor,　which　reduced　the　micromechanical　properties　and　increased　the　porosity　of　ASFm;　therefore,　the　bond　strength　of　ASFm　decreased.　This　achievement　provides　an　alternative　way　for　the　application　of　SS　in　tunnels.